Table of contents

We put limits on the time variation of the electron mass in the early universe using observational primordial abundances of D , ⁴He , and ⁷Li , recent data from the cosmic microwave background, and the 2dFGRS power spectrum. Furthermore, we use these constraints together with other astronomical and geophysical bounds from the late universe to test the Barrow-Magueijo model for the variation in m_e. From our analysis we obtain –0.615 < Gω/c⁴ < − 0.045 (3 σ interval), in disagreement with the result obtained in the original paper.

We report the results of an extragalactic point-source search using the 61 and 94 GHz (V- and W-band) temperature maps from the Wilkinson Microwave Anisotropy Probe (WMAP). Applying a method that cancels the "noise" due to the CMB anisotropy signal, we find in the |b| > 10° region 31 sources in the first-year maps and 64 sources in the three-year co-added maps at a 5 σ level. The 1 σ position uncertainties are 1.6^' and 1.4^', respectively. The increased detections and improved positional accuracy are expected from the higher signal-to-noise ratio of the WMAP three-year data. All sources detected in the first-year maps are repeatedly detected in the three-year maps, which is a strong indication of the consistency of this method. Of all the sources, 97% are identified with either the WMAP three-year source catalog or plausible extrapolations of lower frequency data, which indicates that our method is also reliable. The two unidentified sources have been recently confirmed to be false detections, using the WMAP five-year data. We derive the source count distribution at the WMAP V band by combining our verified detections with sources from the WMAP three-year catalog. If we assume that the effect of source clustering is negligible, the contribution to the power spectrum from faint sources below 0.75 Jy is estimated to be (2.4 ± 0.8) × 10⁻³ μK² sr for the V band, which implies a source correction amplitude of A = 0.012 ± 0.004 μK² sr.

We present general properties of ionized hydrogen (H II) bubbles and their growth based on a state-of-the-art, large-scale (100 Mpc h⁻¹) cosmological radiative transfer simulation. The simulation resolves all halos with atomic cooling at the relevant redshifts and simultaneously performs radiative transfer and dynamical evolution of structure formation. Our major conclusions include the following: (1) For significant H II bubbles, the number distribution is peaked at a volume of ~0.6 Mpc³h⁻³ at all redshifts. But at z ⩽ 10, one large, connected network of bubbles dominates the entire H II volume. (2) H II bubbles are highly nonspherical. (3) The H II regions are highly biased with respect to the underlying matter distribution, with the bias decreasing with time. (4) The non-Gaussianity of the H II region is small when the universe becomes 50% ionized. The non-Gaussianity reaches its maximum near the end of the reionization epoch z ∼ 6. But at all redshifts of interest there is a significant non-Gaussianity in the H II field. (5) Population III galaxies may play a significant role in the reionization process. Small bubbles are initially largely produced by Population III stars. At z ⩾ 10 even the largest H II bubbles have a balanced ionizing photon contribution from Population II and Population III stars, while at z ⩽ 8 Population II stars start to dominate the overall ionizing photon production for large bubbles, although Population III stars continue to make a nonnegligible contribution. (6) The relationship between halo number density and bubble size is complicated, but a strong correlation is found between halo number density and bubble size for large bubbles.

We consider the radiative feedback processes that operate during the formation of the first stars. (1) Photodissociation of H₂ in the local dark matter minihalo occurs early in the growth of the protostar but does not affect subsequent accretion. (2) Lyα radiation pressure acting at the boundary of the H II region that the protostar creates in the accreting envelope reverses infall in the polar directions when the star reaches ~20-30 M_☉ but cannot prevent infall from other directions. (3) Expansion of the H II region beyond the gravitational escape radius for ionized gas occurs at masses ~50-100 M_☉. However, accretion from the equatorial regions can continue since the neutral accretion disk shields a substantial fraction of the accretion envelope from direct ionizing flux. (4) At higher stellar masses, ~140 M_☉ in the fiducial case, photoevaporation-driven mass loss from the disk, together with declining accretion rates, halts the increase in the protostellar mass. We identify this process as the mechanism that determines the mass of Population III.1 stars (i.e., stars with primordial composition that have not been affected by prior star formation). The initial mass function of these stars is set by the distribution of entropy and angular momentum. The Appendix gives approximate solutions to a number of problems relevant to the formation of the first stars: the effect of Rayleigh scattering on line profiles in media of very large optical depth, the intensity of Lyα radiation in very opaque media, radiative acceleration in terms of the gradient of a modified radiation pressure, the flux of radiation in a shell with an arbitrary distribution of opacity, and the vertical structure of an accretion disk supported by gas pressure with constant opacity.

The non-Gaussian contribution to the intrinsic halo spin alignments is analytically modeled and numerically detected. Assuming that the growth of non-Gaussianity in the density fluctuations caused the tidal field to have a nonlinear-order effect on the orientations of the halo angular momentum, we model the intrinsic halo spin alignments as a linear scaling of the density correlations on large scales, which is different from the previous quadratic scaling model based on the linear tidal torque theory. Then, we analyze the halo catalogs from the recent high-resolution Millenium Run simulation at four different redshifts (z = 0, 0.5, 1, and 2) and measure quantitatively the degree of the nonlinear effect on the halo spin alignments and its changes with redshifts. A clear signal of spin correlations is found on scales as large as 10 h⁻¹ Mpc at z = 0, which marks a detection of the nonlinear tidal effect on the intrinsic halo alignments. We also investigate how the nonlinear effect depends on the intrinsic properties of the halos. It is found that the degree of the nonlinear tidal effect increases as the halo mass scale decreases, the halo specific angular momentum increases, and the halo peculiar velocity decreases. We discuss the implications of our results for weak gravitational lensing.

We estimate the rate of near-field microlensing events expected from all-sky surveys and investigate the properties of the events. Under the assumption that lenses are composed of stars, our estimation of the event rate ranges from Γ_tot ∼ 0.1 yr ⁻¹ for a survey with a magnitude limit of V_lim = 12 to Γ_tot ∼ 23 yr ⁻¹ for a survey with V_lim = 18. We find that the average distances to the source star and lens vary considerably depending on the magnitude limit, while the dependencies of the event timescale and lens-source transverse speed are weak. We also find that the average lens-source proper motion of events expected from a survey with V_lim = 18 would be ⟨ μ ⟩ ≳ 40 mas yr ⁻¹, and the value further increases as the magnitude limit becomes lower, implying that the source and lens of a significant fraction of near-field events can be resolved from high-resolution follow-up observations conducted several years after the lensing magnification. From the investigation of the variation of the event characteristics depending on the position of the sky, we find that the average distances to source stars and lenses become shorter, the lens-source transverse speed increases, and the timescale becomes shorter as the Galactic latitude of the field increases. Because of the concentration of events near the Galactic plane, we find that ≳50% of events would be detected in the field with b ⩽ 30°.

We present a new approach to gravitational lens mass map reconstruction. Our mass map solutions perfectly reproduce the positions, fluxes, and shears of all multiple images, and each mass map accurately recovers the underlying mass distribution to a resolution limited by the number of multiple images detected. We demonstrate our technique given a mock galaxy cluster similar to Abell 1689, which gravitationally lenses 19 mock background galaxies to produce 93 multiple images. We also explore cases in which as few as four multiple images are observed. Mass map solutions are never unique, and our method makes it possible to explore an extremely flexible range of physical (and unphysical) solutions, all of which perfectly reproduce the data given. Each reconfiguration of the source galaxies produces a new mass map solution. An optimization routine is provided to find those source positions (and redshifts, within uncertainties) that produce the "most physical" mass map solution, according to a new figure of merit developed here. Our method imposes no assumptions about the slope of the radial profile or mass following light. However, unlike "nonparametric" grid-based methods, the number of free parameters that we solve for is only as many as the number of observable constraints (or slightly greater if fluxes are constrained). For each set of source positions and redshifts, mass map solutions are obtained "instantly" via direct matrix inversion by smoothly interpolating the deflection field using a recently developed mathematical technique. Our LensPerfect software is straightforward and easy to use, and is publicly available on our Web site.

We measure the evolution of the intergalactic Lyα effective optical depth, τ_eff, over the redshift range 2 ⩽ z⩽ 4.2 from a sample of 86 high-resolution, high-S/N quasar spectra obtained with the ESI and HIRES spectrographs on Keck and with the MIKE spectrograph on Magellan. This represents an improvement over previous analyses of the Lyα forest from high-resolution spectra in this redshift interval of a factor of 2 in the size of the data set alone. We pay particular attention to robust error estimation and extensively test for systematic effects. We find that our estimates of the quasar continuum levels in the Lyα forest obtained by spline fitting are systematically biased low, with the magnitude of the bias increasing with redshift, but that this bias can be accounted for using mock spectra. The mean fractional error ⟨ Δ C/C_true⟩ is <1% at z = 2, 4% at z = 3, and 12% at z = 4. Previous measurements of τ_eff at z≳ 3 based on directly fitting the quasar continua in the Lyα forest, which have generally neglected this effect, are therefore likely biased low. We provide estimates of the level of absorption arising from metals in the Lyα forest based on both direct and statistical metal removal results in the literature, finding that this contribution is ≈6%-9% at z = 3 and decreases monotonically with redshift. The high precision of our measurement, attaining 3% in redshift bins of width Δ z = 0.2 around z = 3, indicates significant departures from the best-fit power-law redshift evolution [τ_eff = 0.0018(1 + z)^3.92, when metals are left in], particularly near z = 3.2. The observed downward departure is statistically consistent with a similar feature detected in a precision statistical measurement using SDSS spectra by Bernardi and coworkers using an independent approach.

We have conducted a long-slit search for low surface brightness Lyα emitters at redshift 2.67 < z < 3.75. A 92 hr long exposure with the ESO VLT FORS2 instrument down to a 1 σ surface brightness detection limit of 8 × 10⁻²⁰ erg cm⁻² s⁻¹ arcsec⁻² per arcsec² aperture yielded a sample of 27 single line emitters with fluxes of a few × 10⁻¹⁸ erg s⁻¹ cm⁻². We present arguments that most objects are indeed Lyα. The large comoving number density, 3 × 10⁻²h³₇₀ Mpc⁻³, the large covering factor, dN/dz ∼ 0.2–1, and the often extended Lyα emission suggest that the emitters can be identified with the elusive host population of damped Lyα systems (DLAS) and high column density Lyman limit systems (LLS). A small inferred star formation rate, perhaps supplemented by cooling radiation, appears to energetically dominate the Lyα emission, and is consistent with the low metallicity, low dust content, and theoretically inferred low masses of DLAS, and with the relative lack of success of earlier searches for their optical counterparts. Some of the line profiles show evidence for radiative transfer in galactic outflows. Stacking surface brightness profiles, we find emission out to at least 4^''. The centrally concentrated emission of most objects appears to light up the outskirts of the emitters (where LLS arise) down to a column density where the conversion from UV to Lyα photon becomes inefficient. DLAS, high column density LLS, and the emitter population discovered in this survey appear to be different observational manifestations of the same low-mass, protogalactic building blocks of present-day L_* galaxies.

We report evidence for a bimodality in damped Lyα systems (DLAs). Using [C II] 158 μm cooling rates, ℓ_c, we find a distribution with peaks at ℓ_c = 10^−27.4 and 10^−26.6 ergs s⁻¹ H⁻¹ separated by a trough at ℓ^crit_c ≈ 10^−27.0 ergs s⁻¹ H⁻¹. We divide the sample into "low cool" DLAs with ℓ_c ⩽ ℓ^crit_c and "high cool" DLAs with ℓ_c > ℓ^crit_c and find the K-S probabilities that velocity width, metallicity, dust-to-gas ratio, and Si II equivalent width in the two subsamples are drawn from the same parent population are small. These quantities are significantly larger in the high cool population, while the H I column densities are indistinguishable in the two populations. We find the DLA gas is heated by local radiation fields and background radiation, rather than background radiation alone. The rare appearance of faint, extended objects in the Hubble Ultra Deep Field rules out in situ star formation as the dominant star-formation mode for the high cool population, but is compatible with in situ star formation as the dominant mode for the low cool population. Star formation in the high cool DLAs likely arises in Lyman break galaxies. Using Si II equivalent width as a mass indicator, we construct bivariate distributions of metallicity, ℓ_c, and areal SFR versus the mass indicators. Tentative evidence is found for correlations and parallel sequences, which suggest similarities with the bimodality found in nearby galaxies. We suggest that the transition-mass model provides a plausible scenario for the bimodality we have found. As a result, the bimodality in current galaxies may have originated in DLAs.

A detailed analysis of the evolution of the properties of core-jet systems within the VLBA Imaging and Polarimetry Survey (VIPS) is presented. We find a power-law relationship between jet intensity and width that suggests that for the typical jet, little if any energy is lost as it moves away from its core. Using VLA images at 1.5 GHz, we have found evidence that parsec-scale jets tend to be aligned with the direction of emission on kiloparsec scales. We also found that this alignment improves as the jets move farther from their cores on projected scales as small as ~50-100 pc. This suggests that realignment of jets on these projected scales is relatively common. We typically find a modest amount of bending (a change in jet position angle of ~5°) on these scales, suggesting that this realignment may typically occur relatively gradually.

We present the results of a broadband simultaneous campaign on the nearby low-luminosity active galactic nucleus M81*. From 2005 February through August, we observed M81* five times using the Chandra X-Ray Observatory with the HETGS, complemented by ground-based observations with the Giant Meterwave Radio Telescope, the Very Large Array and Very Large Baseline Array, the Plateau de Bure Interferometer at IRAM, the Submillimeter Array, and Lick Observatory. We discuss how the resulting spectra vary over short and longer timescales compared to previous results, especially in the X-rays where this is the first ever longer term campaign at spatial resolution high enough to nearly isolate the nucleus (17 pc). We compare the spectrum to our Galactic center weakly active nucleus Sgr A*, which has undergone similar campaigns, as well as to weakly accreting X-ray binaries in the context of outflow-dominated models. In agreement with recent results suggesting that the physics of weakly accreting black holes scales predictably with mass, we find that the exact same model that successfully describes hard-state X-ray binaries applies to M81*, with very similar physical parameters.

We measure the evolution of the correlation between black hole mass and host spheroid velocity dispersion (M_BH-σ_*) over the last 6 billion years, by studying three carefully selected samples of active galaxies at z = 0.57, z = 0.36 and z < 0.1. For all three samples, virial black hole masses are consistently estimated using the line dispersion of Hβ and the continuum luminosity at 5100 Å or Hα line luminosity, based on our cross calibration of the broad-line region size-luminosity relation. For the z = 0.57 sample, new stellar velocity dispersions are measured from high signal-to-noise ratio spectra obtained at the Keck Telescope, while for the two lower redshift samples they are compiled from previous works. Extending our previous result at z = 0.36, we find an offset from the local relation, suggesting that for fixed M_BH, distant spheroids have on average smaller velocity dispersions than local ones. The measured offset at z = 0.57 is Δ log σ_* = 0.12 ± 0.05 ± 0.06 (or Δ log M_BH = 0.50 ± 0.22 ± 0.25), i.e., Δ log M_BH = (3.1 ± 1.5) log (1 + z) + 0.05 ± 0.21. This is inconsistent with a tight and nonevolving universal M_BH-σ_* relation at the 95% CL.

Using Chandra X-ray observations in the All-Wavelength Extended Groth Strip International Survey (AEGIS) we identify 241 X-ray-selected active galactic nuclei (AGNs; L_2–10 > 10⁴² ergs s⁻¹) and study the properties of their host galaxies in the range 0.4 < z < 1.4. By making use of infrared photometry from the Palomar Observatory and BRI imaging from the Canada-France-Hawaii Telescope, we estimate AGN host galaxy stellar masses and show that both stellar mass and photometric redshift estimates (where necessary) are robust to the possible contamination from AGNs in our X-ray-selected sample. Accounting for the photometric and X-ray sensitivity limits of the survey, we construct the stellar mass function of X-ray-selected AGN host galaxies and find that their abundance decreases by a factor of ~2 since z ∼ 1 but remains roughly flat as a function of stellar mass. We compare the abundance of AGN hosts to the rate of star formation quenching observed in the total galaxy population. If the timescale for X-ray-detectable AGN activity is roughly 0.5-1 Gyr, as suggested by black hole demographics and recent simulations, then we deduce that the inferred AGN "trigger" rate matches the star formation quenching rate, suggesting a link between these phenomena. However, given the large range of nuclear accretion rates we infer for the most massive and red hosts, X-ray-selected AGNs may not be directly responsible for quenching star formation.

All but three (M87, BL Lac, and 3C 279) extragalactic sources detected so far at very high energy γ-rays belong to the class of high-frequency-peaked BL Lac objects. This suggested to us a systematic scan of candidate sources with the MAGIC telescope, based on the Donato et al. compilation of X-ray blazars. The observations took place from 2004 December to 2006 March and cover northern sky sources visible under small zenith distances zd < 30° at culmination, constraining the declination to –2° to +58°. The sensitivity of the search was planned for detecting X-ray-bright [F(1 keV) > 2 μ Jy ] sources emitting at least the same energy flux at 200 GeV as at 1 keV. To avoid strong γ-ray attenuation close to the energy threshold, source redshift was constrained to z < 0.3. Of the 14 sources observed, 1ES 1218+304 (for the first time at VHE) and 1ES 2344+514 (strong detection in a low flux state) were detected in addition to the known bright TeV blazars Mrk 421 and Mrk 501. A marginal excess of 3.5 σ from the position of 1ES 1011+496 was observed and then confirmed as a VHE γ-ray source by a second MAGIC observation triggered by a high optical state. For the remaining sources, we present 99% c.l. upper limits on the integral flux ≳200 GeV. We characterize the HBL sample (including all HBLs detected at VHE so far) by looking for correlations between their multifrequency spectral indices determined from simultaneous optical, archival X-ray, and radio luminosities, finding that VHE-emitting HBLs do not seem to constitute a unique subclass. The HBLs' absorption-corrected γ-ray luminosities at 200 GeV are generally not higher than their X-ray luminosities at 1 keV.

We present high-resolution spectroscopic VLT observations of the outflow seen in QSO 2359–1241. These data contain absorption troughs from five resonance Fe II lines with a resolution of ~7 km s⁻¹ and a signal-to-noise ratio per resolution element of order 100. We use this unprecedented high-quality data set to investigate the physical distribution of the material in front of the source and by that to determine the column densities of the absorbed troughs. We find that the apparent optical depth model gives a very poor fit to the data and greatly underestimates the column density measurements. Power-law distributions and partial covering models give much better fits, with some advantage to power-law models, while both models yield similar column density estimates. The better fit of the power-law model solves a long-standing problem plaguing the partial covering model when applied to large distance scale outflow: how to obtain a velocity-dependent covering factor for an outflow situated at distances thousands of time greater than the size of the AGN emission source. This problem does not affect power-law models. Therefore, based on the better fit and plausibility of the physical model, we conclude that in QSO 2359–1241, the outflow covers the full extent of the emission source but in a nonhomogeneous way.

We present an analysis of our Chandra Low Energy Transmission Grating Spectrometer (LETGS) observation of the quasar MR 2251–178. The warm absorber of MR 2251–178 is well described by a hydrogen column density N_H ≈ 2 × 10²¹ cm⁻² and an ionization parameter log (ξ ) ≈ 0.6. We find in the spectrum weak evidence for narrow absorption lines from carbon and nitrogen which indicate that the ionized material is in outflow. We note changes (in time) of the absorption structure in the band 0.6-1 keV (around the unresolved transition arrays [UTAs] plus the O VII and O VIII K edges) at different periods of the observation. We measure a 0.1-2 keV flux of 2.58 × 10⁻¹¹ ergs cm⁻² s⁻¹. This flux implies that the nuclear source of MR 2251–178 is in a relatively low state. No significant variability is seen in the light curve. We do not find evidence for extra cold material in the line of sight, and set an upper limit of N_H ≈ 1.2 × 10²⁰ cm⁻². The X-ray spectrum does not appear to show evidence for dusty material, although an upper limit in the neutral carbon and oxygen column densities can only be set to cm⁻² and cm⁻², respectively.

We report the discovery of strong soft X-ray emission lines and a hard continuum above 2 keV in the narrow-line Seyfert 1 galaxy Mrk 335 during an extremely low X-ray flux state. Mrk 335 was observed for 22 ks by XMM-Newton in 2007 July as a Target of Opportunity to examine it in its X-ray low flux state, which was discovered with Swift. Long-term light curves suggest that this is the lowest flux state this AGN has ever been seen in. However, Mrk 335 is still sufficiently bright that its X-ray properties can be studied in detail. The X-ray continuum spectrum is very complex and requires several components to model. Statistically, partial covering and blurred reflection models work well. We confirm the presence of a strong narrow Fe line at 6.4 keV. High-resolution spectroscopy with the XMM-Newton RGS reveals strong, soft X-ray emission lines not detected in previous, higher signal-to-noise ratio, XMM-Newton observations, such as highly ionized Fe lines, O VII, and Ne IX and Mg XI lines. The optical/UV fluxes are similar to those previously measured with Swift. Optical spectroscopy taken in 2007 September does not show any changes to optical spectra obtained 8 years earlier.

We present a numerical study of the evolution of galaxy clustering when galaxies flow passively from high redshift, respecting the continuity equation throughout. While passive flow is a special case of galaxy evolution, it allows a well-defined study of galaxy ancestry and serves as an interesting limit to be compared to nonpassive cases. We use dissipationless N-body simulations, assign galaxies to massive halos at z = 1 and 2 using various halo occupation distribution (HOD) models, and trace these galaxy particles to lower redshift while conserving their number. We find that passive flow results in an asymptotic convergence at low redshift in the HOD and in galaxy clustering on scales above ~3 h⁻¹ Mpc for a wide range of initial HODs. As galaxies become less biased with respect to mass asymptotically with time, the HOD parameters evolve such that M₁/M_min decreases while α converges toward unity, where ⟨ N_g(M) ⟩ = exp (− M_min/M) [ 1 + (M/M₁)^α] . The satellite populations converge toward the Poisson distribution at low redshift. The convergence is robust for different number densities and is enhanced when galaxies evolve from higher redshift. We compare our results with the observed luminous red galaxy (LRG) sample from SDSS that has the same number density. We claim that if LRGs have experienced a strict passive flow, their ⟨ N_g(M) ⟩ should be close to a power law with an index of unity in halo mass. Discrepancies could be due to dry galaxy merging or new members arising between the initial and the final redshifts. The spatial distribution of passively flowing galaxies within halos appears on average more concentrated than the halo mass profile at low redshift. The evolution of bias for passively flowing galaxies is consistent with linear bias evolution on quasi-linear as well as large scales.

We present results from a systematic investigation of the X-ray properties of a sample of moderate-redshift (0.3 < z < 0.6) galaxy groups. These groups were selected not by traditional X-ray or optical search methods, but rather by an association, either physical or along the line of sight, with a strong gravitational lens. We calculate the properties of seven galaxy groups in the fields of six lens systems. Diffuse X-ray emission from the intragroup medium is detected in four of the groups. All of the detected groups have X-ray luminosities greater than 10⁴²h⁻² ergs s⁻¹ and lie on the L_X-σ_v relations defined by local groups and clusters. The upper limits for the nondetections are also consistent with the local L_X-σ_v relationships. Although the sample size is small and deeper optical and X-ray data are needed, these results suggest that lens-selected groups are similar to X-ray-selected samples and thus are more massive than the typical poor-group environments of local galaxies.

Quillen et al. presented an imaging survey with the Spitzer Space Telescope of 62 brightest cluster galaxies with optical line emission located in the cores of X-ray-luminous clusters. They found that at least half of these sources have signs of excess IR emission. Here we discuss the nature of the IR emission and its implications for cool core clusters. The strength of the mid-IR excess emission correlates with the luminosity of the optical emission lines. Excluding the four systems dominated by an AGN, the excess mid-IR emission in the remaining brightest cluster galaxies is likely related to star formation. The mass of molecular gas (estimated from CO observations) is correlated with the IR luminosity as found for normal star-forming galaxies. The gas depletion timescale is about 1 Gyr. The physical extent of the IR excess is consistent with that of the optical emission-line nebulae. This supports the hypothesis that star formation occurs in molecular gas associated with the emission-line nebulae and with evidence that the emission-line nebulae are mainly powered by ongoing star formation. We find a correlation between mass deposition rates () estimated from the X-ray emission and the star formation rates estimated from the IR luminosity. The star formation rates are 1/10 to 1/100 of the mass deposition rates, suggesting that the reheating of the intracluster medium is generally very effective in reducing the amount of mass cooling from the hot phase but not eliminating it completely.

A new algorithm is developed, based on the friends-of-friends (FOF) algorithm, to identify galaxy groups in a galaxy catalog in which the redshift errors have large dispersions (e.g., a photometric redshift galaxy catalog in which a portion of the galaxies also have much more precise spectroscopic redshifts). The DEEP2 mock catalogs, with our additional simulated photometric redshift errors, are used to test the performance of our algorithm. The association of the reconstructed galaxy groups with the dark halos in the mock catalogs gives an idea about the completeness and purity of the derived group catalog. Our results show that in a 0.6 ⩽ z⩽ 1.6 galaxy catalog with an R-band limiting magnitude of 24.1 and an average 1 σ photometric redshift error of ~0.03, the overall purity of our new algorithm for richness 4-7 (line-of-sight velocity dispersion ~300 km s⁻¹) groups is higher than 70% (i.e., 70% of the groups reconstructed by our algorithm are related to real galaxy groups). The performance of the new algorithm is compared with the performance of the FOF algorithm, and it is suggested that this new algorithm is better than FOF for a database, given the same redshift uncertainties.

We have performed simple simulations which suggest that the phase-space structure of halos identified in cosmological calculations is invariant under the dynamics induced by sinking substructure satellites. This contention is confirmed using a two-component Fokker-Planck formulation, representing the dynamics of a smooth background and a system of massive clumps. It is shown that, to very high accuracy, when the clumps sink in under the action of dynamical friction, the background expands so as to leave the total distribution unchanged. This holds for the inner and intermediate regions of isotropic systems in dynamical equilibrium, and is valid for any mass spectrum of substructure, because the governing equation is linear in their mass-weighted phase-space distribution. If the clumps are considered solid, the process whereby background particles are driven out of low-energy states takes the form of an exponential instability, with a characteristic timescale on the order of the dynamical friction time, on which develops a low-energy cutoff in the phase-space distribution of these lighter particles and a constant-density core in their spatial distribution. This could correspond to a situation in which the clumps are made of dense baryonic material. We also considered the case when stripping is strong enough for a low-energy cutoff to develop in the clump distribution (as in the situation with dissipationless substructure). The results of this paper suggest that halo profiles similar to those found in dissipationless cosmological simulations are approximately invariant under the interaction induced by the presence of substructure, a necessary condition for the observed "universality." In addition, the total profile, including baryons, should also be invariant, provided that the latter are initially in the form of dense clumps, whose distribution initially follows that of dark matter.

Cold dark matter cosmogony predicts triaxial dark matter halos, whereas observations find quite round halos. This is most likely due to the condensation of baryons leading to rounder halos. We examine the halo phase space distribution basis for such shape changes. Triaxial halos are supported by box orbits, which pass arbitrarily close to the density center. The decrease in triaxiality caused by baryons is thought to be due to the scattering of these orbits. We test this hypothesis with simulations of disks grown inside triaxial halos. After the disks are grown we check whether the phase space structure has changed by evaporating the disks and comparing the initial and final states. While the halos are substantially rounder when the disk is at full mass, their final shape after the disk is evaporated is not much different from the initial. Likewise, the halo becomes (more) radially anisotropic when the disk is grown, but the final anisotropy is consistent with the initial. Only if the baryons are unreasonably compact or massive does the halo change irreversibly. We show that the character of individual orbits is not generally changed by the growing mass. Thus, the central condensation of baryons does not destroy enough box orbits to cause the shape change. Rather, box orbits merely become rounder along with the global potential. However, if angular momentum is transferred to the halo, either via satellites or via bars, a large irreversible change in the halo distribution occurs. The ability of satellites to alter the phase space distribution of the halo is of particular concern to galaxy formation simulations since halo triaxiality can profoundly influence the evolution of disks.

We use a combination of deep, high angular resolution imaging data from the CDFS (HST/ACS GOODS survey) and ground-based near-IR K_s images to derive the evolution of the galaxy major merger rate in the redshift range 0.2 ⩽ z⩽ 1.2. We select galaxies solely on the basis of their J-band rest-frame absolute magnitude, which is a good tracer of the stellar mass. We find steep evolution with redshift, with the merger rate ∝(1 + z)^{3.43 ± 0.49} for optically selected pairs and ∝(1 + z)^{2.18 ± 0.18} for pairs selected in the near-IR. Our result is unlikely to be affected by luminosity evolution that is relatively modest when using rest-frame J-band selection. The apparently more rapid evolution that we find in the visible is likely caused by biases relating to incompleteness and spatial resolution affecting the ground-based near-IR photometry, underestimating pair counts at higher redshifts in the near-IR. The major merger rate was ~5.6 times higher at z ∼ 1.2 than at the current epoch. Overall, 41% × (0.5 Gyr/τ) of all galaxies with M_J ⩽ − 19.5 have undergone a major merger in the last ~8 Gyr, where τ is the merger timescale. Interestingly, we find no effect on the derived major merger rate due to the presence of the large-scale structure at z = 0.735 in the CDFS.

We present K-band imaging of two ~30^' × 30^' fields covered by the Multiwavelength Survey by Yale-Chile (MUSYC) Wide NIR Survey. The SDSS 1030+05 and Cast 1255 fields were imaged with the Infrared Side Port Imager (ISPI) on the 4 m Blanco telescope at the Cerro Tololo Inter-American Observatory (CTIO) to a 5 σ point-source limiting depth of K ∼ 20 (Vega). Combining these data with the MUSYC optical UBVRIz imaging, we created multiband K-selected source catalogs for both fields. These catalogs, together with the MUSYC K-band catalog of the Extended Chandra Deep Field South (ECDF-S) field, were used to select K < 20 BzK galaxies over an area of 0.71 deg². This is the largest area ever surveyed for BzK galaxies. We present number counts, redshift distributions, and stellar masses for our sample of 3261 BzK galaxies (2502 star-forming [sBzK] and 759 passively evolving [pBzK]), as well as reddening and star formation rate estimates for the star-forming BzK systems. We also present two-point angular correlation functions and spatial correlation lengths for both sBzK and pBzK galaxies and show that previous estimates of the correlation function of these galaxies were affected by cosmic variance due to the small areas surveyed. We have measured correlation lengths r₀ of 8.89 ± 2.03 and 10.82 ± 1.72 Mpc for sBzK and pBzK galaxies, respectively. This is the first reported measurement of the spatial correlation function of passive BzK galaxies. In the ΛCDM scenario of galaxy formation, these correlation lengths at z ∼ 2 translate into minimum masses of ~4 × 10¹² and ~9 × 10¹²M_☉ for the dark matter halos hosting sBzK and pBzK galaxies, respectively. The clustering properties of the galaxies in our sample are consistent with their being the descendants of bright Lyman break galaxies at z ∼ 3, and the progenitors of present-day >1L^* galaxies.

We present a proof-of-concept study that dust extinction curves can be extracted from the infrared (IR), optical, ultraviolet (UV), and X-ray afterglow observations of GRBs without assuming known extinction laws. We focus on GRB 050525A (z = 0.606), for which we also present IR observations from the Spitzer Space Telescope at t = t_IR ≈ 2.3 days postburst. We construct the spectral energy distribution of the afterglow at t = t_IR and use it to derive the dust extinction curve of the host galaxy in seven optical/UV bands. By comparing our derived extinction curve to known templates, we see that the Galactic or Milky Way extinction laws are disfavored versus those of the Small and Large Magellanic Clouds (SMC and LMC), but that we cannot rule out the presence of a LMC-like 2175 Å bump in our extinction curve. The dust-to-gas ratio present within the host galaxy of GRB 050525A is similar to that found in the LMC, while about 10%-40% more dust is required if the SMC template is assumed. Our method is useful to observatories that are capable of simultaneously observing GRB afterglows in multiple wave bands from the IR to the X-ray.

Using the local three-dimensional galaxy density in a comoving sphere with a radius of the distance to the fifth-nearest galaxy, we construct two samples at the two density extremes from the luminous red galaxy (LRG) sample of the Sixth Data Release of the Sloan Digital Sky Survey (SDSS DR6), which contains 53,453 LRGs with redshift z = 0.2-0.36. We compare the basic properties of LRGs in the lowest density regimes with those of LRGs in the highest density regimes. It is found that the sample at low density has a higher proportion of faint galaxies (M_g⩾ − 21.92) and a lower proportion of luminous galaxies (M_g⩽ − 22.2) than the one at high density, but the colors of the LRGs nearly are independent of the local density. We also measure the projected local density ∑₅, which is computed from the distance to the fifth-nearest neighbor within a redshift slice ±1000 km s⁻¹ of each galaxy. For the color distributions of the two samples, we get the same results as when using the three-dimensional local density, but the tendency for LRG luminosity to change with the local density is opposite to what is found using the three-dimensional local density. This shows that projection effects or redshift space distortions may seriously influence any final conclusions about the dependence of LRG luminosity on environment.

The VLA-COSMOS Large Project has imaged the 2 deg² COSMOS field with a resolution of 1.5^'' and a sensitivity of about 11 μJy (1 σ), yielding a catalog of ~3600 radio sources. In this paper we present a further analysis of the VLA-COSMOS Large Project catalog of radio sources aimed to (1) quantify and correct for the effect of bandwidth smearing in the catalog, (2) determine the incompleteness produced by the noise bias and the resolution bias in the new catalog, and (3) derive the radio source counts at 1.4 GHz. The effect of bandwidth smearing on the radio sources in the catalog was quantified comparing the peak and total flux densities in the final mosaic and in each of the individual pointings where the source was closest to the center of the field. We find that the peak flux densities in the original VLA-COSMOS Large Project catalog have to be divided by a factor of about 0.8 or 0.9, depending on the distance from the mosaic center. The completeness of the radio catalog has been tested using samples of simulated radio sources with different angular size distributions. These simulated sources have been added to the radio image and recovered using the same techniques used to produce the radio catalog. The fraction of missed sources as a function of the total flux density is a direct measure of the incompleteness. Finally, we derived the radio source counts down to 60 μJy with unprecedented good statistics. Comparison to the findings of other surveys shows good agreement in the flux density range 0.06-1 mJy confirming the upturn at ~0.5 mJy and a possible decline of the source counts below ~0.1 mJy.

In this series of papers, we present the results of detailed N-body simulations of the interaction of a sample of four massive globular clusters in the inner region of a triaxial galaxy for two different sets of initial conditions that correspond to different initial density concentrations. A full merging of the clusters takes place, leading to a slowly evolving cluster that is quite similar to observed nuclear clusters. Actually, both the density and the velocity dispersion profiles match qualitatively, and also quantitatively after scaling to a larger number of merger globulars, with the observed features of many nucleated galaxies. In the case of dense initial clusters, the merger remnant shows a density profile more concentrated than that of the progenitors, with a central density higher than the sum of the progenitors' central densities and an effective radius compatible with observed nuclear values. These findings support the idea that a massive nuclear cluster may have formed in the early phases of the mother galaxy's evolution and led to the formation of a nucleus, which in many galaxies has a luminosity profile similar to that of an extended King model. A correlation with galactic nuclear activity is suggested.

Using one-dimensional hydrodynamic simulations including interstellar heating, cooling, and thermal conduction, we investigate nonlinear evolution of gas flow across galactic spiral arms. We model the gas as a non-self-gravitating, unmagnetized fluid and follow its interaction with a stellar spiral potential in a local frame comoving with the stellar pattern. Initially uniform gas with density n₀ in the range 0.5 cm ⁻³ ⩽ n₀ ⩽ 10 cm ⁻³ rapidly separates into warm and cold phases as a result of thermal instability (TI) and also forms a quasi-steady shock that prompts phase transitions. After saturation, the flow follows a recurring cycle: warm and cold phases in the interarm region are shocked and immediately cool to become a denser cold medium in the arm; postshock expansion reduces the mean density to the unstable regime in the transition zone and TI subsequently mediates evolution back into warm and cold interarm phases. For our standard model with n₀ = 2 cm ⁻³, the gas resides in the dense arm, thermally unstable transition zone, and interarm region for 14%, 22%, and 64% of the arm-to-arm crossing time, respectively. These regions occupy 1%, 16%, and 83% of the arm-to-arm distance, respectively. Gas at intermediate temperatures (i.e., neither warm stable nor cold states) represents ~25%-30% of the total mass, similar to the fractions estimated from H I observations (larger interarm distances could reduce this mass fraction, whereas other physical processes associated with star formation could increase it). Despite transient features and multiphase structure, the time-averaged shock profiles can be matched to that of a diffusive isothermal medium with temperature 1000 K (which is < T_warm) and a "particle" mean free path of l₀ = 100 pc . Finally, we quantify numerical conductivity associated with translational motion of phase-separated gas on the grid and show that convergence of numerical results requires the numerical conductivity to be comparable to or smaller than the physical conductivity.

We report on the X-ray evolution over the last ≈9 Gyr of cosmic history (i.e., since z = 1.4) of late-type galaxy populations in the Chandra Deep Field-North and Extended Chandra Deep Field-South (CDF-N and E-CDF-S, respectively; jointly CDFs) survey fields. Our late-type galaxy sample consists of 2568 galaxies, which were identified using rest-frame optical colors and HST morphologies. We utilized X-ray stacking analyses to investigate the X-ray emission from these galaxies, emphasizing the contributions from normal galaxies that are not dominated by active galactic nuclei (AGNs). Over this redshift range, we find significant increases (factors of ≈5-10) in the X-ray-to-optical mean luminosity ratio (L_X/L_B) and the X-ray-to-stellar mass mean ratio (L_X/M_⋆) for galaxy populations selected by L_B and M_⋆, respectively. When analyzing galaxy samples selected via SFR, we find that the mean X-ray-to-SFR ratio (L_X/SFR) is consistent with being constant over the entire redshift range for galaxies with SFR = 1-100 M_☉ yr⁻¹, thus demonstrating that X-ray emission can be used as a robust indicator of star formation activity out to z ≈ 1.4. We find that the star formation activity (as traced by X-ray luminosity) per unit stellar mass in a given redshift bin increases with decreasing stellar mass over the redshift range z = 0.2-1, which is consistent with previous studies of how star formation activity depends on stellar mass. Finally, we extend our X-ray analyses to Lyman break galaxies at z ∼ 3 and estimate that L_X/L_B at z ∼ 3 is similar to its value at z = 1.4.

We investigate the effect of metallicity calibrations, AGN classification, and aperture covering fraction on the local mass-metallicity (M-Z) relation using 27,730 star-forming galaxies from the SDSS Data Release 4. We analyze the SDSS M-Z relation with 10 metallicity calibrations, including theoretical and empirical methods. We show that the choice of metallicity calibration has a significant effect on the shape and y-intercept [12 + log (O/H) ] of the M-Z relation. The absolute metallicity scale (y-intercept) varies up to Δ [ log (O/H) ] = 0.7 dex, depending on the calibration used, and the change in shape is substantial. These results indicate that it is critical to use the same metallicity calibration when comparing different luminosity-metallicity or M-Z relations. We present new metallicity conversions that allow metallicities that have been derived using different strong-line calibrations to be converted to the same base calibration. These conversions facilitate comparisons between different samples, particularly comparisons between galaxies at different redshifts for which different suites of emission lines are available. Our new conversions successfully remove the large 0.7 dex discrepancies between the metallicity calibrations, and we reach agreement in the M-Z relation to within 0.03 dex on average. We investigate the effect of AGN classification and aperture covering fraction on the M-Z relation. We find that different AGN classification methods have negligible effect on the SDSS M-Z relation. We compare the SDSS M-Z relation with nuclear and global relations from the NFGS. The turnover of the M-Z relation at M_* ∼ 10¹⁰M_☉ depends on the aperture covering fraction. We find that a lower redshift limit of z < 0.04 is insufficient for avoiding aperture effects in fiber spectra of the highest stellar mass (M_* > 10¹⁰M_☉) galaxies.

Archival Spitzer observations of 41 starburst galaxies that span a wide range in metallicity reveal for the first time a correlation between the [Fe II]/[Ne II] 26.0 μm/12.8 μm ratio and the electron gas density as traced by the 18.7 μm/33.4 μm [S III] ratio, with the [Fe II]/[Ne II] ratio decreasing with increasing gas density. The correlations of the [Fe II]/[Ne II] ratio, the PAH peak-to-continuum strength, and metallicity found in an earlier paper were confirmed for a larger sample. We also find a strong correlation between the gas density and the PAH peak-to-continuum strength. Using shock and photoionization models, we see that the driver of the observed [Fe II ]/[Ne II] ratios is metallicity. The majority of [Fe II] emission in low-metallicity galaxies may be shock-derived, while at high metallicity, the [Fe II] emission may be instead dominated by contributions from H II and in particular from dense PDR regions. However, the observed [Fe II]/[Ne II] ratios may instead be following a metallicity-abundance relationship, with iron being less depleted onto grains in low-metallicity galaxies, a result that would have profound implications for the use of iron emission lines as unambiguous tracers of shocks.

Most of the X-ray-emitting gas in early-type galaxies probably originates from red giant mass loss, and here we model the interaction between this stellar mass loss and the hot ambient medium. Using two-dimensional hydrodynamic simulations, we adopt a temperature for the ambient medium of 3 × 10⁶ K along with a range of ambient densities and stellar velocities. When the stellar velocity is supersonic relative to the ambient medium, a bow shock occurs, along with a shock driven into the stellar ejecta, which heats only a fraction of the gas. Behind the bow shock, a cool wake develops, but the fast flow of the hot medium causes Kelvin-Helmholtz instabilities to grow and these fingers are shocked and heated (without radiative cooling). Along with the mixing of this wake material with the hot medium, most of the stellar ejecta is heated to approximately the temperature of the hot ambient medium within 2 pc of the star. With the addition of radiative cooling, some wake material remains cool (<10⁵ K), accounting for up to 25% of the stellar mass loss. Less cooled gas survives when the ambient density is lower or when the stellar velocity is higher than in our reference case. These results suggest that some cooled gas should be present in the inner part of early-type galaxies that have a hot ambient medium. These calculations may explain the observed distributed optical emission line gas as well as the presence of dust in early-type galaxies.

We present extensive new photometry in (g', i') of the large globular cluster (GC) system around NGC 3311, the central cD galaxy in the Hydra cluster. Our GMOS data cover a 5.5' field of view and reach a limiting magnitude i' ≃ 26.0, about 0.5 mag fainter than the turnover point of the GC luminosity function. We find that NGC 3311 has a huge population of ≃16,000 GCs, closely similar to the prototypical "high specific frequency" Virgo giant M87. The color-magnitude distribution shows that the metal-poor "blue" GC sequence and the more metal-rich "red" sequence are both present, with nearly equal numbers of clusters. Bimodal fits to the color distributions confirm that the blue sequence shows the same trend of progressively increasing metallicity with GC mass that has previously been found in many other large galaxies; the correlation we find corresponds to a scaling of GC metallicity with mass of Z ∼ M^0.6. By contrast, the red sequence shows no change of mean metallicity with mass but does show an upward extension to much higher than normal luminosity into the UCD-like range, strengthening the potential connections between massive GCs and UCDs. The GC luminosity function, which we measure down to the turnover point at M_I≃ − 8.4, also has a normal form like those in other giant ellipticals. Within the Hydra field, another giant elliptical NGC 3309 is sitting just 100'' from the cD NGC 3311. We use our data to solve simultaneously for the spatial structure and total GC populations of both galaxies. Their specific frequencies are S_N(N 3311) = 12.5 ± 1.5 and S_N(N 3309) = 0.6 ± 0.4. NGC 3311 is completely dominant and entirely comparable with other cD-type systems such as M87 in Virgo.

The autocorrelation function provides an objective test for the existence of special scales in the hierarchical clustering of young stars. We apply this measure to single-star photometry for the brightest main-sequence stars in the Small Magellanic Cloud (SMC), the Large Magellanic Cloud (LMC), M33, and M31, using data from the Magellanic Clouds Photometric Survey and the Massey Local Group Survey. Our primary result is the identification of a transition to a higher correlation dimension (weaker clustering) at 1 kpc in the LMC and M31, and at 300 pc in M33. We suggest that this transition marks the large-scale regime where disk geometry and dynamics set the scale for structure. On smaller scales, the correlation functions for each galaxy are scale-free over at least 2 orders of magnitude, with a projected correlation dimension varying from 1.0 for M31 to 1.8 for the SMC. This variation is probably caused by a combination of differences in stellar ages and masses, physical environment, and extinction.

We present results from K-band slit scan observations of a ~20'' × 20'' region of the Galactic center (GC) in two separate epochs more than 5 years apart. The high-resolution (R = λ/Δ λ ⩾ 14,000) observations allow the most accurate radial velocity and acceleration measurements of the stars in the central parsec of the Galaxy. Detected stars can be divided into three groups based on the CO absorption band heads at ~2.2935 μm and the He II lines at ~2.0581 and ~2.112, 2.113 μm: cool, narrow-line hot, and broad-line hot. The radial velocities of the cool, late-type stars have approximately a symmetrical distribution with its center at ~–7.8 ± 10.3 km s⁻¹ and a standard deviation ~113.7 ± 10.3 km s⁻¹. Although our statistics are dominated by the brightest stars, we estimate a central black hole mass of (3.9 ± 1.1) × 10⁶M_☉, consistent with current estimates from complete orbits of individual stars. Our surface density profile and the velocity dispersion of the late-type stars support the existence of a low-density region at the Galactic center suggested by earlier observations. Many hot, early-type stars show radial velocity changes higher than maximum values allowed by pure circular orbital motions around a central massive object, suggesting that the motions of these stars greatly deviate from circular orbital motions around the Galactic center. The correlation between the radial velocities of the early-type He I stars and their declination offsets from Sagittarius A* suggests that a systematic rotation is present for the early-type population. No figure rotation around the Galactic center for the late-type stars is supported by the new observations.

We investigate the deflections of UHE protons by Galactic magnetic field (GMF) using four conventional GMF models in order to discuss the positional correlation between the arrival distribution of UHECRs and their sources. UHE protons coming from the direction around the Galactic center are highly deflected above 8° by the dipole magnetic field during their propagation in Galactic space. However, in bisymmetric spiral field models, there are directions in which the deflection angle is below 1°. One of these directions is toward Centaurus A, the nearest radio-loud active galactic nucleus that is a possible UHECR source candidate. On the other hand, UHE protons arriving from the direction of the Galactic anticenter are generally less deflected, especially in bisymmetric spiral field models. Thus, the Northern Hemisphere, not including the Galactic center, is suitable for studies of correlation with sources. The dependence on model parameters is also investigated. The deflection angles of UHE protons are dependent on the pitch angle of the spiral field. We also investigate distortion of the supergalactic plane by the GMF. Since the distortion in the direction around the Galactic center strongly depends on the GMF model, we can obtain information on the GMF around the Galactic center if Pierre Auger Observatory finds significant positional correlation around the supergalactic plane.

We present Very Large Array (VLA) radio interferometry observations of the 1720 MHz OH masers in the Galactic center (GC). Most 1720 MHz OH masers arise in regions where the supernova remnant Sgr A East is interacting with the interstellar medium. The majority of the newly found 1720 MHz OH masers are located to the northeast, independently indicating and confirming an area of shock interaction with the +50 km s⁻¹ molecular cloud (M–0.02–0.07) on the far side of Sgr A East. The previously known bright masers in the southeast are suggested to be the result of the interaction between two supernova remnants, instead of between Sgr A East and the surrounding molecular clouds, as generally found elsewhere in the Galaxy. Together with masers north of the circumnuclear disk (CND) they outline an interaction on the near side of Sgr A East. In contrast to the interaction between the +50 km s⁻¹ cloud and Sgr A East, OH absorption data do not support a direct interaction between the CND material and Sgr A East. We also present three new high-negative velocity masers, supporting a previous single detection. The location and velocities of the high-negative and high-positive velocity masers are consistent with being near the tangent points of, and physically located in, the CND. We argue that the high-velocity masers in the CND are pumped by dissipation between density clumps in the CND instead of a shock generated by the supernova remnant. That is, the CND masers are not coupled to the supernova remnant and are sustained independently.

We present the results of a 25 yr program to monitor the radio flux evolution of the planetary nebula NGC 7027. We find significant evolution of the spectral flux densities. The flux density at 1465 MHz, where the nebula is optically thick, is increasing at a rate of 0.251% ± 0.015% yr ⁻¹, caused by the expansion of the ionized nebula. At frequencies where the emission is optically thin, the spectral flux density is changing at a rate of –0.145% ± 0.005% yr ⁻¹, caused by a decrease in the number of ionizing photons coming from the central star. A distance of 980 ± 100 pc is derived. By fitting interpolated models of post-AGB evolution to the observed changes, we find that over the 25 yr monitoring period, the stellar temperature has increased by 3900 ± 900 K and the stellar bolometric luminosity has decreased by 1.75% ± 0.5% . We derive a distance-independent stellar mass of 0.655 ± 0.010 M_☉ adopting the Blöcker stellar evolution models, or about 0.04 M_☉ higher when using models of Vassiliadis & Wood. A Cloudy photoionization model is used to fit all epochs at all frequencies simultaneously. The differences between the radio flux density predictions and the observed values show some time-independent residuals of typically 1%. A possible explanation is inaccuracies in the radio flux scale of Baars and coworkers. We propose an adjustment to the flux density scale of the primary radio flux calibrator 3C 286, based on the Cloudy model of NGC 7027. We also calculate precise flux densities for NGC 7027 for all standard continuum bands used at the VLA, as well as for some new 30 GHz experiments.

We used the FUSE (Far Ultraviolet Spectroscopic Explorer ) satellite to observe O VI emission along two sight lines toward the edge of the interaction zone (IZ) between the Loop I superbubble and the Local Bubble. One sight line was chosen because material in the interaction zone blocks distant X-ray emission, and should thus do the same for nonlocal O VI emission. We measured an O VI intensity of I_shad = 2750 ± 550 photons cm⁻² s⁻¹ sr⁻¹ along this "shadowed" sight line, and I_unshad = 10,800 ± 1200 photons cm⁻² s⁻¹ sr⁻¹ along the other sight line. Given these results, very little (≲ 800 photons cm⁻² s⁻¹ sr⁻¹) of the emission arises from the near side of the interaction zone, which likely has an H I column density of about 4 × 10²⁰ cm⁻² along the "shadowed" sight line. The O VI emission arising within Loop I (~10⁴ photons cm⁻² s⁻¹ sr⁻¹) is probably associated with gas of n_e ∼ 0.1 cm⁻³ and an emitting path length of ~2.5 pc, suggesting it arises at interfaces rather than from gas filling Loop I. In contrast, the C III emission is similar along both sight lines, indicating that much of the emission likely arises on the near side of the interaction zone.

The low abundance of molecular oxygen in cold cores of interstellar clouds poses a continuing problem to modelers of the chemistry of these regions. In chemical models O₂ is formed principally by the reaction between O and OH, which has been studied experimentally down to 39 K. It remains possible that the rate coefficient of this reaction at 10 K is considerably less than its measured value at 39 K, which might inhibit the production of O₂ and possibly bring theory and observation closer together over a wider range of times. Two theoretical determinations of the rate coefficient for the O + OH reaction at temperatures down to 10 K have been undertaken recently; both results show that the rate coefficient is indeed lower at 10 K than at 39 K, although they differ in the magnitude of the decrease. Here we show, using gas-phase models, how the calculated interstellar O₂ abundance in cold cores is affected by a decrease in the rate coefficient. We also consider its effect on other species. Our major finding is that for standard O-rich abundances, the calculated abundance of O₂ in cold cores is sufficiently low to explain observations only at early times regardless of the value of k₁ in the range investigated here. For C-rich abundances, on the other hand, late-time solutions can also be possible.

Here we address the question of whether the ionized shells associated with giant H II regions can be progenitors of the larger H I shell-like objects found in the Milky Way and other spiral and dwarf irregular galaxies. We use for our analysis a sample of 12 H II shells presented recently by Relaño et al. We calculate the evolutionary tracks that these shells would have if their expansion is driven by multiple supernovae explosions from the parental stellar clusters. We find, contrary to Relaño et al., that the evolutionary tracks of their sample H II shells are inconsistent with the observed parameters of the largest and most massive neutral hydrogen supershells. We conclude that H II shells found inside giant H II regions may represent the progenitors of small or intermediate H I shells; however, they cannot evolve into the largest H I objects unless, aside from the multiple supernovae explosions, an additional energy source contributes to their expansion.

Galactic cosmic-ray composition reflects the effects of nuclear interactions during propagation through the interstellar medium. In order to use measurements to determine the source of cosmic rays, one needs a model to deconvolute the propagation effect. This paper presents a new numerical method to solve cosmic-ray diffusive transport equations with a complete network of nuclear interactions. The new method uses the time backward Markov stochastic processes to solve the transport equation by tracing the particles' trajectories starting from the solar system back to their sources in the Galaxy. To make the calculation efficient for large arrays of equations for many cosmic-ray species, a matrix representing the composition of all cosmic-ray heavy nuclei and location is used. The results for abundance ratios of key elements such as B/C and sub-Fe/Fe compared with observations from HEAO-3, ACE, and Ulysses and for isotopic ratios such as ³⁶Cl/³⁷Cl and ⁵⁴Mn/⁵⁵Mn are shown to be consistent with other numerical approaches. The ability to track particles through the Galaxy, which is inherent in this technique, offers new opportunities for investigations. For example, one can look at the spatial and propagation time distributions and element composition of source particles that arrive in the solar system.

We present an analysis of wind-blown, parsec-sized, mid-infrared bubbles and associated star formation using the GLIMPSE IRAC, MIPSGAL MIPS, and MAGPIS VLA surveys. Three bubbles from the Churchwell et al. catalog were selected. The relative distribution of the ionized gas (based on 20 cm emission), PAH emission (based on 8 μm, 5.8 μm, and lack of 4.5 μm emission), and hot dust (24 μm emission) is compared. At the center of each bubble there is a region containing ionized gas and hot dust surrounded by PAHs. We identify the likely source(s) of the stellar wind and ionizing flux producing each bubble based on SED fitting to numerical hot stellar photosphere models. Candidate YSOs are also identified using SED fitting, including several sites of possible triggered star formation.

Magnetic fields are usually considered dynamically important in star formation when the dimensionless mass-to-flux ratio is close to, or less than, unity (λ ≲ 1). We show that, in disk formation, the requirement is far less stringent. This conclusion is drawn from a set of 2D (axisymmetric) simulations of the collapse of rotating, singular isothermal cores magnetized to different degrees. We find that a weak field corresponding to λ ∼ 100 can begin to disrupt the rotationally supported disk through magnetic braking, by creating regions of rapid, supersonic collapse in the disk. These regions are separated by one or more centrifugal barriers, where the rapid infall is temporarily halted. The number of centrifugal barriers increases with the mass-to-flux ratio λ. When λ ≳ 100, they merge together to form a more or less contiguous, rotationally supported disk. Even though the magnetic field in such a case is extremely weak on the scale of dense cores, it is amplified by collapse and differential rotation, to the extent that its pressure dominates the thermal pressure in both the disk and its surrounding region. For relatively strongly magnetized cores with λ ≲ 10, the disk formation is suppressed completely, as found previously. A new feature is that the mass accretion is highly episodic, due to reconnection of the magnetic field lines accumulated near the center. For rotationally supported disks to appear during the protostellar mass accretion phase of star formation in dense cores with realistic field strengths, the powerful magnetic brake must be weakened, perhaps through nonideal MHD effects. Another possibility is to remove, through protostellar winds, the material that acts to brake the disk rotation. We discuss the possibility of observing a generic product of the magnetic braking, an extended circumstellar region that is supported by a combination of toroidal magnetic field and rotation—a "magnetogyrosphere"—interferometrically.

The periodic eclipses of the pre-main-sequence binary KH 15D have been explained by a circumbinary dust ring inclined to the orbital plane, which causes occultations of the stars as they pass behind the ring edge. We compute the extinction and forward scattering of light by the edge of the dust ring in order to explain (1) the gradual slope directly preceding total eclipse, (2) the gradual decline at the end of ingress, and (3) the slight rise in flux at mideclipse. The size of the forward-scattering halo indicates that the dust grains have a radius of a ∼ 6(D/3 AU) μm, where D is the distance of the edge of the ring from the system barycenter. This dust size estimate agrees well with estimates of the dust grain size from polarimetry, adding to the evidence that the ring lies at several AU. Finally, the ratio of the fluxes during an eclipse to those not during an eclipse independently indicates that the ring lies at a few AU.

Sakai et al. have observed long-chain unsaturated hydrocarbons and cyanopolyynes in the low-mass star-forming region L1527, and have attributed this result to a gas-phase ion-molecule chemistry, termed warm carbon-chain chemistry, which occurs during and after the evaporation of methane from warming grains. The source L1527 is an envelope surrounding a Class 0/I protostar with regions that possess a slightly elevated temperature of ≈30 K. The molecules detected by Sakai et al. are typically associated only with dark molecular clouds, and not with the more evolved hot corino phase. In order to determine whether L1527 is chemically distinct from a dark cloud, we compute models including various degrees of heating. The results indicate that the composition of L1527 is somewhat more likely to be due to warm carbon-chain chemistry than to be a remnant of a colder phase. If so, the molecular products provide a signature of a previously uncharacterized early phase of low-mass star formation, which can be characterized as a "lukewarm" corino. We also include predictions for other molecular species that might be observed toward candidate lukewarm corino sources. Although our calculations show that unsaturated hydrocarbons and cyanopolyynes can be produced in the gas phase as the grains warm up to 30 K, they also show that such species do not disappear rapidly from the gas as the temperature reaches 200 K, implying that such species might be detected in hot corinos and hot cores.

We present Submillimeter Array observations of several deuterated species in the disk around the classical T Tauri star TW Hydrae at arcsecond scales, including detections of the DCN J = 3–2 and DCO⁺J = 3–2 lines and upper limits to the HDO 3_1,2–2_2,1, ortho-H₂D⁺ 1_1,0–1_1,1, and para-D₂H⁺ 1_1,0–1_0,1 transitions. We also present observations of the HCN J = 3–2, HCO⁺J = 3–2, and H¹³CO⁺J = 4–3 lines for comparison with their deuterated isotopologues. We constrain the radial and vertical distributions of various species in the disk by fitting the data using a model where the molecular emission from an irradiated accretion disk is sampled with a two-dimensional Monte Carlo radiative transfer code. We find that the distribution of DCO⁺ differs markedly from that of HCO⁺. The D/H ratios inferred change by at least 1 order of magnitude (0.01-0.1) for radii <30 to ≥70 AU, and there is a rapid falloff of the abundance of DCO⁺ at radii larger than 90 AU. Using a simple analytical chemical model, we constrain the degree of ionization, x(e −) = n(e −)/n(H₂) , to be ~10⁻⁷ in the disk layer(s) where these molecules are present. Provided the distribution of DCN follows that of HCN, the ratio of DCN to HCN is determined to be (1.7 ± 0.5) × 10⁻²; however, this ratio is very sensitive to the poorly constrained vertical distribution of HCN. The resolved radial distribution of DCO⁺ indicates that in situ deuterium fractionation remains active within the TW Hydrae disk and must be considered in the molecular evolution of circumstellar accretion disks.

Many gamma-ray bursts (GRBs) have been observed with the Burst Alert Telescope and X-Ray Telescope of the Swift satellite. The successive "pulses" of these GRBs end with a fast decline and a fast spectral softening, until they are overtaken by another pulse or the last pulse's decline is overtaken by a less rapidly varying "afterglow." The fast-decline phase has been attributed, in the currently explored standard fireball model of GRBs, to "high-latitude" synchrotron emission from a collision of two conical shells. This high-latitude emission does not explain the observed spectral softening. In contrast, the temporal behavior and the spectral evolution during the fast-decline phase agree with the predictions of the cannonball model for GRBs.

We analyzed the available LIGO data coincident with GRB 070201, a short-duration, hard-spectrum γ-ray burst (GRB) whose electromagnetically determined sky position is coincident with the spiral arms of the Andromeda galaxy (M31). Possible progenitors of such short, hard GRBs include mergers of neutron stars or a neutron star and a black hole, or soft γ-ray repeater (SGR) flares. These events can be accompanied by gravitational-wave emission. No plausible gravitational-wave candidates were found within a 180 s long window around the time of GRB 070201. This result implies that a compact binary progenitor of GRB 070201, with masses in the range 1 M_☉ < m₁ < 3 M_☉ and 1 M_☉ < m₂ < 40 M_☉, located in M31 is excluded at >99% confidence. If the GRB 070201 progenitor was not in M31, then we can exclude a binary neutron star merger progenitor with distance D < 3.5 Mpc, assuming random inclination, at 90% confidence. The result also implies that an unmodeled gravitational-wave burst from GRB 070201 most probably emitted less than 4.4 × 10⁻⁴M_☉c² (7.9 × 10⁵⁰ ergs) in any 100 ms long period within the signal region if the source was in M31 and radiated isotropically at the same frequency as LIGO's peak sensitivity (f ≈ 150 Hz). This upper limit does not exclude current models of SGRs at the M31 distance.

Gravitational waves (GWs) from the inspiral of a neutron star (NS) or stellar-mass black hole (BH) into an intermediate-mass black hole (IMBH) with mass M ∼ 50–350 M_☉ may be detectable by the planned advanced generation of ground-based GW interferometers. Such intermediate mass ratio inspirals (IMRIs) are most likely to be found in globular clusters. We analyze four possible IMRI formation mechanisms: (1) hardening of an NS-IMBH or BH-IMBH binary via three-body interactions, (2) hardening via Kozai resonance in a hierarchical triple system, (3) direct capture, and (4) inspiral of a CO from a tidally captured main-sequence star; we also discuss tidal effects when the inspiraling object is an NS. For each mechanism we predict the typical eccentricities of the resulting IMRIs. We find that IMRIs will have largely circularized by the time they enter the sensitivity band of ground-based detectors. Hardening of a binary via three-body interactions, which is likely to be the dominant mechanism for IMRI formation, yields eccentricities under 10⁻⁴ when the GW frequency reaches 10 Hz. Even among IMRIs formed via direct captures, which can have the highest eccentricities, around 90% will circularize to eccentricities under 0.1 before the GW frequency reaches 10 Hz. We estimate the rate of IMRI coalescences in globular clusters and the sensitivity of a network of three Advanced LIGO detectors to the resulting GWs. We show that this detector network may see up to tens of IMRIs per year, although rates of one to a few per year may be more plausible. We also estimate the loss in signal-to-noise ratio that will result from using circular IMRI templates for data analysis and find that, for the eccentricities we expect, this loss is negligible.

We report the results of a series of three-dimensional (3D) simulations of the deflagration phase of the gravitationally confined detonation mechanism for Type Ia supernovae. In this mechanism, ignition occurs at one or several off-center points, resulting in a burning bubble of hot ash that rises rapidly, breaks through the surface of the star, and collides at a point opposite the breakout on the stellar surface. We find that detonation conditions are robustly reached in our 3D simulations for a range of initial conditions and resolutions. Detonation conditions are achieved as the result of an inwardly directed jet that is produced by the compression of unburnt surface material when the surface flow collides with itself. A high-velocity outwardly directed jet is also produced. The initial conditions explored in this paper lead to conditions at detonation that can be expected to produce large amounts of ⁵⁶Ni and small amounts of intermediate-mass elements. These particular simulations are therefore relevant only to high-luminosity Type Ia supernovae. Recent observations of Type Ia supernovae imply a compositional structure that is qualitatively consistent with that expected from these simulations.

We present time-resolved photometry of the optical counterpart to the black hole candidate Swift J1753.5–0127 which has remained in the low/hard X-ray state and bright at optical/IR wavelengths since its discovery in 2005. At the time of our observations Swift J1753.5–0127 does not show a decay trend but remains stable at R = 16.45 with a night-to-night variability of ~0.05 mag. The R-band light curves, taken from 2007 June 3 to August 31, are not sinusoidal, but exhibit a complex morphology with remarkable changes in shape and amplitude. The best period determination is 3.2443 ± 0.0010 hr. This photometric period is likely a superhump period, slightly larger than the orbital period. Therefore, Swift J1753.5–0127 is the black hole candidate with the shortest orbital period observed to date. Our estimation of the distance is comparable to values previously published and likely places Swift J1753.5–0127 in the Galactic halo.

GRB 070201 was a bright, short-duration, hard-spectrum gamma-ray burst detected by the Interplanetary Network. Its error quadrilateral, which has an area of 0.124 deg², intersects some prominent spiral arms of the nearby M31 (Andromeda) galaxy. Given the properties of this GRB, along with the fact that LIGO data argue against a compact binary merger origin in M31, it is an excellent candidate to have been an extragalactic soft gamma-ray repeater (SGR) giant flare, with an energy of 1.4 × 10⁴⁵ ergs. However, we cannot rule out the possibility that it was a short-duration GRB in the background. Analysis of ROTSE-IIIb visible-light observations of M31, taken 10.6 hr after the burst and covering 42% of the GRB error region, does not reveal any optical transient down to a limiting magnitude of 17.1. We inspected archival and proprietary XMM-Newton X-ray observations of the intersection of the GRB error region and M31, obtained about 4 weeks prior to the outburst, in order to look for periodic variable X-ray sources. No SGR or anomalous X-ray pulsar (AXP) candidates (periods in the range 1-20 s) were detected. We discuss the possibility of detecting extragalactic SGRs/AXPs by identifying their periodic X-ray light curves. Our simulations suggest that the probability of detecting the periodic X-ray signal of one of the known Galactic SGRs/AXPs, if placed in M31, is about 10% using a 50 ks XMM-Newton exposure, increasing to 50% for a 2 Ms observation.

This paper presents a sensitive and comprehensive IRAC 3-8 μm photometric survey of white dwarfs for companions in the planetary-mass regime with temperatures cooler than the known T dwarfs. The search focuses on descendents of intermediate-mass stars with M≳ 3 M_☉ whose inner, few hundred AU regions cannot be probed effectively for massive planets and brown dwarfs by any alternative existing method. Furthermore, examination for mid-infrared excess explores an extensive range of orbital semimajor axes, including the intermediate 5-50 AU range, poorly covered and incompletely accessible by other techniques at main-sequence or evolved stars. Three samples of white dwarfs are chosen which together represent relatively young as well as older populations of stars: nine open cluster white dwarfs, 22 high-mass field white dwarfs, and 17 metal-rich field white dwarfs. In particular, these targets include: seven Hyades and four field white dwarfs of similar age, one Pleiades and 19 field white dwarfs of similar age, and van Maanen 2 and 16 similarly metal-rich white dwarfs with ages between 1 and 7 Gyr. No substellar companion candidates were identified at any star. By demanding a 15% minimum photometric excess at 4.5 μm to indicate a companion detection, upper limits in the planetary-mass regime are established at 34 of the sample white dwarfs, 20 of which have limits below 10 M_J according to substellar cooling models. Specifically, limits below the minimum mass for deuterium burning are established at all Pleiades and Hyades white dwarfs, as well as similarly young field white dwarfs, half a dozen of which receive limits at or below 5 M_J. Two IRAC epochs at vMa 2 rule out T≳ 200 K proper-motion companions within 1200 AU.

We present Multiband Imaging Photometer for Spitzer (MIPS) observations at 24 and 70 μm for 30 stars, and at 160 μm for a subset of 12 stars, in the nearby (~30 pc), young (~12 Myr) β Pictoris moving group (BPMG). In several cases, the new MIPS measurements resolve source confusion and background contamination issues in the IRAS data for this sample. We find that 7 members have 24 μm excesses, implying a debris disk fraction of 23%, and that at least 11 have 70 μm excesses (disk fraction of ≥37%). Five disks are detected at 160 μm (out of a biased sample of 12 stars observed), with a range of 160/70 flux ratios. The disk fraction at 24 and 70 μm, and the size of the excesses measured at each wavelength, are both consistent with an "inside-out" infrared excess decrease with time, wherein the shorter wavelength excesses disappear before longer wavelength excesses, and consistent with the overall decrease of infrared excess frequency with stellar age, as seen in Spitzer studies of other young stellar groups. Assuming that the infrared excesses are entirely due to circumstellar disks, we characterize the disk properties using simple models and fractional infrared luminosities. Optically thick disks, seen in the younger TW Hya and η Cha associations, are entirely absent in the BPMG. Additional flux density measurements at 24 and 70 μm are reported for nine Tucana-Horologium association member stars. Since this is <20% of the association membership, limited analysis on the complete disk fraction of this association is possible.

We present radial velocities and Fe and Al abundances for 180 red giant branch (RGB) stars in the Galactic globular cluster Omega Centauri (ω Cen). The majority of our data lie in the range 11.0 < V < 13.5, which covers the RGB from about 1 mag above the horizontal branch to the RGB tip. The selection procedures are biased toward preferentially observing the more metal-poor and luminous stars of ω Cen. Abundances were determined using equivalent width measurements and spectrum synthesis analyses of moderate resolution spectra (R ≈ 13,000) obtained with the Blanco 4 m telescope and Hydra multifiber spectrograph. Our results are in agreement with previous studies as we find at least four different metallicity populations with [ Fe/H ] = − 1.75, –1.45, –1.05, and –0.75, with a full range of –2.20≲ [ Fe/H ] ≲ − 0.70. [Al/Fe] ratios exhibit large star-to-star scatter for all populations, with the more than 1.0 dex range of [Al/Fe] decreasing for stars more metal-rich than [ Fe/H ] ∼ − 1.4. The minimum [Al/Fe] abundance observed for all metallicity populations is [ Al/Fe ] ∼ + 0.15. The maximum abundance of log (Al) is reached for stars with [ Fe/H ] ∼ − 1.4 and does not increase further with stellar metallicity. We interpret these results as evidence for Type II SNe providing the minimum [Al/Fe] ratio and a mass spectrum of intermediate-mass asymptotic giant branch stars causing the majority of the [Al/Fe] scatter. These results seem to fit in the adopted scheme that star formation occurred in ω Cen over >1 Gyr.

We present the results of an abundance analysis for a sample of stars with –4 < [ Fe/H ] < − 2. The data were obtained with the HIRES spectrograph at Keck Observatory. The set includes 28 stars, with effective temperature ranging from 4800 to 6600 K. For 13 stars with [ Fe/H ] < − 2.6, including nine with [ Fe/H ] < − 3.0 and one with [ Fe/H ] = − 4.0, these are the first reported detailed abundances. For the most metal-poor star in our sample, CS 30336–049, we measure an abundance pattern that is very similar to stars in the range [ Fe/H ] ∼ − 3.5, including a normal C + N abundance. We also find that it has very low but measurable Sr and Ba, indicating some neutron-capture activity even at this low of a metallicity. We explore this issue further by examining other very neutron capture-deficient stars and find that, at the lowest levels, [Ba/Sr] exhibits the ratio of the main r-process. We also report on a new r-process-enhanced star, CS 31078–018. This star has [ Fe/H ] = − 2.85, [ Eu/Fe ] = 1.23, and [ Ba/Eu ] = − 0.51. CS 31078–018 exhibits an "actinide boost," i.e., much higher [Th/Eu] than expected and at a similar level to CS 31082–001. Our spectra allow us to further constrain the abundance scatter at low metallicities, which we then use to fit to the zero-metallicity Type II supernova yields of Heger & Woosley (2008). We find that supernovae with progenitor masses between 10 and 20 M_☉ provide the best matches to our abundances.

We present Spitzer IRS data on 19 asymptotic giant branch (AGB) stars in the Large Magellanic Cloud, complementing existing published data sets of carbon stars in both Magellanic Clouds and the Milky Way, to investigate the effects of metallicity on dust and molecular spectral features arising from the circumstellar envelope. We find that the C₂H₂P- and R-branches at 7.5 μm are affected by dust dilution at higher mass-loss rates—albeit to a lesser extent for sources in the Magellanic Clouds, compared to the Milky Way—while the narrow 13.7 μm C₂H₂Q-branch only shows the effect of dust dilution at low mass-loss rates. A strong metallicity dependence is not observed for the Q-branch. Independent of metallicity, we also provide an explanation for the observed shifts in the central wavelength of the SiC emission feature, as we show that these are largely caused by molecular band absorption on either side of the dust emission feature, dominating over shifts in the central wavelength caused by self-absorption. We have devised a method to study the dust condensation history in carbon-rich AGB stars in different metallicity environments, by measuring the strength of the 11.3 μm SiC and 30 μm MgS features with respect to the continuum, as a function of mass-loss rate. With this method, it is possible to distinguish in what order SiC and graphite condense, which is believed to be sensitive to the metallicity, prior to the eventual deposit of the MgS layer.

We report the first Very Long Baseline Array (VLBA) observations of v = 1, J = 1–0 SiO masers at 43 GHz in the circumstellar envelope of the M-type semiregular supergiant variable star AH Sco at two epochs separated by 12 days in 2004 March. These high-resolution VLBA images reveal that the distribution of the SiO masers is roughly on a persistent elliptical ring, with the lengths of the major and minor axes being about 18.5 and 15.8 mas, respectively, along a position angle of 150°. The redshifted masers are found to be slightly closer to the central star than are the blueshifted masers. The line-of-sight velocity structure of the SiO masers shows that with respect to the systemic velocity of –6.8 km s⁻¹, the higher velocity features are closer to the star, which can be well explained by the simple outflow or infall, without rotation kinematics, of SiO masers around AH Sco. A study of the proper motions of 59 matched features between our two epochs clearly indicates that the SiO maser shell around AH Sco was undergoing an overall contraction toward the star at a velocity of ≈13 km s⁻¹ at a distance of 2.26 kpc to AH Sco. Our three-dimensional maser kinematics model further suggests that such an inward motion is very likely to be due to the gravitation of the central star. The distance to AH Sco of 2.26 ± 0.19 kpc that we obtained from our three-dimensional kinematics model fitting is consistent with its kinematic distance of 2.0 kpc.

In our analysis of Spitzer IRS archival data on the stellar and substellar members of the TW Hydrae association (TWA), we have discovered two new brown dwarf disks: a flat, optically thick disk around SSSPM J1102–3431 (SSSPM 1102) and a transition disk around 2MASS J1139511–315921 (2M1139). The disk structure for SSSPM 1102 is found to be very similar to the known brown dwarf disk 2MASSW J1207334–393254 (2M1207), with excess emission observed at wavelengths as short as 5 μm. No excess emission shortward of ~20 μm is seen from 2M1139, but it flares up at longer wavelengths and is the first transition disk detected among the substellar members of the TWA. We also report on Spitzer 70 μm observations and the presence of a 10 μm silicate absorption feature for 2M1207. The absorption can be attributed to a nearly edge-on disk, at 75° inclination. The 10 μm spectrum for 2M1207 shows crystalline forsterite features, with a peak in absorption near 11.3 μm. No silicate absorption or emission is observed toward SSSPM 1102. While only six of 25 stellar members show excess emission at these mid-infrared wavelengths, all of the TWA brown dwarfs that have been observed so far with Spitzer show signs of disks around them, resulting in a disk fraction of at least 60%. This is a considerable fraction at the relatively old age of ~10 Myr. A comparison with younger clusters indicates that by the age of the TWA (~10 Myr), the disk fraction for brown dwarfs has not decreased, whereas it drops by a factor of ~2 for the higher mass stars. This suggests longer disk decay timescales for brown dwarfs as compared with higher mass stars.

The frequency of microlensing planet detections, particularly in difficult-to-model high-magnification events, is increasing. Their analysis can require tens of thousands of processor hours or more, primarily because of the high density and high precision of measurements whose modeling requires time-consuming finite-source calculations. I show that a large fraction of these measurements, those that lie at least one source diameter from a caustic or the extension from a cusp, can be modeled using a very simple hexadecapole approximation, which is one to several orders of magnitude faster than full-fledged finite-source calculations. Moreover, by restricting the regions that actually require finite-source calculations to a few isolated "caustic features," the hexadecapole approximation will, for the first time, permit the powerful "magnification map" approach to be applied to events for which the planet's orbital motion is important.

We study the dynamics of planetesimals embedded in a circumbinary protoplanetary disk. A hybrid numerical approach is developed where the evolution of the gaseous component of the disk is computed with the hydrodynamical code FARGO while the planetesimal trajectories are computed with an N-body code. The local gas density and velocity derived from the hydrodynamical portion are used to calculate the drag force and the gravitational attraction of the disk on the planetesimals. We explore the effects of spiral density wave patterns and of the disk eccentricity, both excited by the binary tidal perturbations, on the dynamical evolution of planetesimal orbits. A new definition of osculating orbital elements is given to properly account for the gravitational attraction of the disk. The outcomes of the numerical simulations show that the pericenter alignment of the planetesimal orbits is a robust result. It occurs for different values of the binary eccentricity and surface density profiles of the disk. However, the pericenters are less collimated compared to early predictions based on codes adopting a stationary and axisymmetric approximation for the disk. In addition, the eccentricity values are higher and depend on the semimajor axis of the bodies. Both these effects favor higher relative velocities between colliding planetesimals, making accretion less likely than previously thought. Small 100 m size bodies (planetesimal precursors) have a very high inward drift rate that might lead to a high-density belt in the proximity of the inner border of the disk. Fast accretion into larger bodies might occur in this region.

According to the standard liquid-water definition, the Earth is only partially habitable. We reconsider planetary habitability in the framework of energy balance models, the simplest seasonal models in physical climatology, to assess the spatial and temporal habitability of Earth-like planets. We quantify the degree of climatic habitability of our models with several metrics of fractional habitability. Previous evaluations of habitable zones may have omitted important climatic conditions by focusing on close solar system analogies. For example, we find that model pseudo-Earths with different rotation rates or different land-ocean fractions have fractional habitabilities that differ significantly from that of the Earth itself. Furthermore, the stability of a planet's climate against albedo-feedback snowball events strongly impacts its habitability. Therefore, issues of climate dynamics may be central in assessing the habitability of discovered terrestrial exoplanets, especially if astronomical forcing conditions are different from the moderate solar system cases.

We study the formation conditions of icy planetesimals in protoplanetary disks in order to determine the composition of ices in small and cold extrasolar planets. Assuming that ices are formed from hydrates, clathrates, and pure condensates, we calculate their mass fractions with respect to the total quantity of ices included in planetesimals, for a grid of disk models. We find that the composition of ices weakly depends on the adopted disk thermodynamic conditions, and is rather influenced by the initial composition of the gas phase. The use of a plausible range of molecular abundance ratios and the variation of the relative elemental carbon over oxygen ratio in the gas phase of protoplanetary disks allow us to apply our model to a wide range of planetary systems. Our results can thus be used to constrain the icy/volatile phase composition of cold planets evidenced by microlensing surveys, hypothetical ocean planets, and carbon planets, which could be detected by COROT or Kepler.

Extrasolar planets close to their host stars have likely undergone significant tidal evolution since the time of their formation. Tides probably dominated their orbital evolution once the dust and gas cleared away, and as the orbits evolved there was substantial tidal heating within the planets. The tidal heating history of each planet may have contributed significantly to the thermal budget governing the planet's physical properties, including its radius, which in many cases may be measured by observing transit events. Typically, tidal heating increases as a planet moves inward toward its star and then decreases as its orbit circularizes. Here we compute the plausible heating histories for several planets with measured radii, using the same tidal parameters for the star and planet that have been shown to reconcile the eccentricity distribution of close-in planets with other extrasolar planets. Several planets are discussed, including, for example, HD 209458b, which may have undergone substantial tidal heating during the past billion years, perhaps enough to explain its large measured radius. Our models also show that GJ 876d may have experienced tremendous heating and is probably not a solid, rocky planet. Theoretical models should include the role of tidal heating, which is large but time-varying.

We present a numerical study of several two-planet systems based on the motions of Jupiter and Saturn, in which the two giant planets move in low eccentric orbits close to a mean motion resonance. It is more likely to find two planets with similar characteristics in a system than a clone of the Jupiter-Saturn pair of our solar system. Therefore, we vary the distance between the two planets and their mass ratio by changing Saturn's semimajor axis from 8 to 11 AU and increasing its mass by factors of 2-40. The different two-planets were analyzed for the interacting perturbations due to the mean motion resonances of the giant planets. We select several mass ratios for the gas giants, for which we study their influence on test bodies (with negligible mass) moving in the habitable zone (HZ) of a Sun-like star. The orbits are calculated for 2 × 10⁷ yr. In all cases the HZ is dominated by a significant curved band, indicating higher eccentricity, which corresponds to a secular resonance with Jupiter. Interesting results of this study are finding (1) an increase of Venus's eccentricity for the real Jupiter and Saturn masses and the actual semimajor axis of Saturn; (2) an increase of the eccentricity of a test planet at Earth's position when Saturn's mass was increased by a factor of 3 or more; and (3) if the two giant planets are in 2:1 resonance, we observe a strong influence on the outer region of the HZ.

Turbulence is ubiquitous in solar system planetary atmospheres. In hot Jupiter atmospheres, the combination of moderately slow rotation and thick pressure scale height may result in dynamical weather structures with unusually large, planetary-size scales. Using equivalent-barotropic, turbulent circulation models, we illustrate how such structures can generate a variety of features in the thermal phase curves of hot Jupiters, including phase shifts and deviations from periodicity. Such features may have been spotted in the recent infrared phase curve of HD 189733b. Despite inherent difficulties with the interpretation of disk-integrated quantities, phase curves promise to offer unique constraints on the nature of the circulation regime present on hot Jupiters.

We introduce a simplified method to calculate the cross sections and rates of ionization and recombination of accelerated ions with arbitrary nuclear charge Z and atomic mass number A. Calculations of equilibrium and nonequilibrium charge states of the element Tellurium (Te, Z = 52) are presented for the first time. The validity of the proposed method is demonstrated by showing that predictions for Si and Fe are in agreement at energies characteristic for energetic (≥0.15 MeV nucleon⁻¹) ultraheavy ions with the results of a more sophisticated model. We find that while the charge states for Te come out higher than those for Fe under similar conditions, the Q/A values for Te fall consistently below those for Fe over the entire energy range and under all comparable conditions, thus extending the trend in Q/A that is observed when going to higher mass elements. Implications of our results for the observed enrichments of ultraheavy ions in solar energetic particle events are discussed.

Coronal loop emission profiles are often of remarkably constant width along their entire lengths, contradicting expectations based on model coronal magnetic field strengths decreasing with height. Meanwhile P. Bellan has produced a theoretical model in which an initially empty, twisted force-free loop, on being filled with plasma via upflow at each footpoint, in the absence of significant gravitational effects, forms a narrow, filamentary loop of constant cross section. In this paper, we focus on equilibrium states that include stratification by uniform gravity while retaining the effects of magnetic field twist. Comparing these with related force-free equilibria, it is found that injection of low-β plasma under coronal conditions is not likely to change the shape of a loop significantly. These linear equilibria apply to the interiors and boundaries of loops only, with external influences modeled by boundary total pressures. The effects of total pressure balance with surroundings and of gravitational stratification are to inhibit the pinching of a loop to a constant cross section. Only if the plasma β were high enough for the plasma to reconfigure the external field and the hydrostatic scale height much greater than the loop size could the final state have nearly constant cross section. We do not expect this to occur in the corona.

Recent studies have demonstrated without doubt that the magnetic field in the photosphere and corona is an intermittent structure, opening new views of the underlying physics. In particular, such problems as the existence in the corona of localized areas with extremely strong resistivity (required to explain magnetic reconnection at all scales) and the interchange between small and large scales (required in the study of photospheric-coronal coupling), to name a few, can be easily captured by the concept of intermittency. This study focuses on simultaneous time variations of intermittency properties derived in the photosphere, chromosphere, and corona. We analyze data for NOAA Active Region 10930 acquired between 2006 December 8, 12:00 UT, and December 13, 18:45 UT. Photospheric intermittency is inferred from Hinode magnetic field measurements, while intermittency in the transition region and corona is derived from Nobeyama 9 GHz radio polarization measurements and high-cadence Hinode XRT (thin-Be) data, as well as GOES 1-8 Å flux. The photospheric dynamics and its possible relationship with the intermittency variations are also analyzed by calculating the kinetic vorticity. In this case study, we find the following chain of events: The intermittency of the photospheric magnetic field peaked after the specific kinetic vorticity of plasma flows in the active region reached its maximum (4 hr time delay). In turn, a gradual increase of coronal intermittency occurred after the peak of the photospheric intermittency. The time delay between the peak of photospheric intermittency and the occurrence of the first strong (X3.4) flare was approximately 1.3 days. Our analysis seems to suggest that the enhancement of intermittency/complexity first occurs in the photosphere and is later transported toward the corona.

Continuous observations of sunspot penumbrae with the Solar Optical Telescope aboard Hinode clearly show that the outer boundary of the penumbra fluctuates around its averaged position. The penumbral outer boundary moves inward when granules appear in the outer penumbra. We discover that such granules appear one after another while moving magnetic features (MMFs) are separating from the penumbral "spines" (penumbral features that have fields that are stronger and more vertical than those of their surroundings). These granules that appear in the outer penumbra often merge with bright features inside the penumbra that move with the spines as they elongate toward the moat region. This suggests that convective motions around the penumbral outer boundary are related to the disintegration of magnetic flux in the sunspot. We also find that dark penumbral filaments frequently elongate into the moat region in the vicinity of MMFs that detach from penumbral spines. Such elongating dark penumbral filaments correspond to nearly horizontal fields extending from the penumbra. Pairs of MMFs with positive and negative polarities are sometimes observed along the elongating dark penumbral filaments. This strongly supports the notion that such elongating dark penumbral filaments have magnetic fields with a "sea serpent"-like structure. Evershed flows, which are associated with the penumbral horizontal fields, may be related to the detachment of the MMFs from the penumbral spines, as well as to the formation of the MMFs along the dark penumbral filaments that elongate into the moat region.

We analyze a dense cluster of solar radio spikes registered at 4.5-6 GHz by the Purple Mountain Observatory spectrometer (Nanjing, China), operating in the 4.5-7.5 GHz range with 5 ms temporal resolution. To handle the data from the spectrometer, we developed a new technique that uses a nonlinear multi-Gaussian spectral fit based on χ² criteria to extract individual spikes from the originally recorded spectra. Applying this method to the experimental raw data, we eventually identified about 3000 spikes for this event, which allows us to make a detailed statistical analysis. Various statistical characteristics of the spikes have been evaluated, including the intensity distributions, the spectral bandwidth distributions, and the distribution of the spike mean frequencies. The most striking finding of this analysis is the distributions of the spike bandwidth, which are remarkably asymmetric. To reveal the underlaying microphysics, we explore the local-trap model with the renormalized theory of spectral profiles of the electron cyclotron maser (ECM) emission peak in a source with random magnetic irregularities. The distribution of the solar spike relative bandwidths calculated within the local-trap model represents an excellent fit to the experimental data. Accordingly, the developed technique may offer a new tool with which to study very low levels of magnetic turbulence in the spike sources, when the ECM mechanism of the spike cluster is confirmed.

Full-disk photometric solar images at a wavelength of 672.3 nm have been obtained daily since 1986 using the CFDT1 (Cartesian Full Disk Telescope No. 1). An analysis of these images from 1986 through the end of 2004 December has shown a peak-to-peak variation in the geocentric north-south solar radius of 0.136 ± 0.01, approximately in phase with the solar cycle. The multiple correlation coefficient squared is R² = 0.0404 (R = 0.2). While this correlation coefficient is small, due to the large number of data points (N = 4042), the level of significance is less than 0.02. The radius had a maximum value near the times of maximum activity for solar cycles 22 and 23.

In this paper we report for the first time the absolute abundance of P (A_P = 6.1 ± 0.5), Al (A_Al = 7.2 ± 0.2), K (A_K = 5.7 ± 0.4), and Na (A_Na = 6.5 ± 0.3) in the slow solar wind for three wind speed ranges (380, 390, 400 km s⁻¹, with a tolerance of ±2 km s⁻¹) using 4 years of CELIAS (Charge, Element, and Isotope Analysis System) data. In addition, we give a new evaluation of the ratios X/Mg and X/Ca. Finally, using these new abundance measurements we give the element enrichment as a function of the FIP (first ionization potential) and the first ionization time (FIT). For the latter we evaluated for the first time the FIT of phosphorus (3.37 s), and we reevaluated the FIT of sulfur (2.7 s).

We calculate the γ-ray albedo flux from cosmic-ray (CR) interactions with the solid rock and ice in Main Belt asteroids (MBAs), Jovian and Neptunian Trojan asteroids, and Kuiper Belt objects (KBOs) using the Moon as a template. We show that the γ-ray albedo for the Main Belt, Trojans, and Kuiper Belt strongly depends on the small-body size distribution of each system. Based on an analysis of the Energetic Gamma-Ray Experiment Telescope (EGRET) data we infer that the diffuse emission from the MBAs, Trojans, and KBOs has an integrated flux of less than ~6 × 10⁻⁶ cm⁻² s⁻¹ (100-500 MeV), which corresponds to ~12 times the lunar albedo, and may be detectable by the forthcoming Gamma-Ray Large Area Space Telescope (GLAST). If detected by GLAST, it can provide unique direct information about the number of small bodies in each system that is difficult to assess by any other method. In addition, the KBO albedo flux can be used to probe the spectrum of CR nuclei at close-to-interstellar conditions. The orbits of MBAs, Trojans, and KBOs are distributed near the ecliptic, which passes through the Galactic center and high Galactic latitudes. Therefore, the asteroid γ-ray albedo has to be taken into account when analyzing weak γ-ray sources close to the ecliptic, especially near the Galactic center, and signals at high Galactic latitudes, such as the extragalactic γ-ray emission. The asteroid albedo spectrum also exhibits a 511 keV line due to secondary positrons annihilating in the rock. This may be an important and previously unrecognized celestial foreground for the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) observations of the Galactic 511 keV line emission including the direction of the Galactic center.

The dissociative recombination (DR) of D₃S⁺ has been studied at the heavy ion storage ring CRYRING. The absolute DR cross sections have been measured over the interaction energy range between ~0 and 0.1 eV, and the energy-dependent cross section obtained is best fitted by the expression σ (E) = (1.3 ± 0.3) × 10⁻¹⁶E^{−1.36 ± 0.04} cm². From the cross section the thermal rate coefficient has been deduced to be (2.8 ± 0.7) × 10⁻⁷(T/300)^{−0.86 ± 0.03} cm³ s⁻¹. The branching fractions in the reaction have been measured at ~0 eV collision energy. The break-up into D₂S+D occurs with a probability of only 0.17 ± 0.08, whereas the three-body channel DS+2D dominates with a probability of 0.58 ± 0.11. The remaining channels DS+D₂ and S+D₂+D contribute with 0.15 ± 0.07 and 0.10 ± 0.06, respectively. The impact of these results on the sulfur chemistry and deuterium fractionation in dark molecular clouds was investigated through chemical kinetic models. With these data, the calculated fractional abundance of H₂S is lower than observed in TMC-1, which supports the assertion that H₂S is formed on grain surfaces and not through gas-phase reactions.

Here we investigate some aspects of stochastic acceleration of ultrarelativistic electrons by magnetic turbulence. In particular, we discuss the steady state energy spectra of particles undergoing momentum diffusion due to resonant interactions with turbulent MHD modes, taking rigorously into account direct energy losses connected with different radiative cooling processes. For the magnetic turbulence we assume a given power spectrum of the type W(k) ∝ k^−q. In contrast to the previous approaches, however, we assume a finite range of turbulent wavevectors k, consider a variety of turbulence spectral indices 1 ⩽ q⩽ 2, and concentrate on the case of a very inefficient particle escape from the acceleration site. We find that for different cooling and injection conditions, stochastic acceleration processes tend to establish a modified ultrarelativistic Maxwellian distribution of radiating particles, with the high-energy exponential cutoff shaped by the interplay between cooling and acceleration rates. For example, if the timescale for the dominant radiative process scales with the electron momentum as ∝p^r, the resulting electron energy distribution is of the form n_e(p) ∝ p²exp [ − (1/a)(p/p_eq)^a] , where a = 2 − q − r and p_eq is the equilibrium momentum defined by the balance between the stochastic acceleration and energy loss timescales. We also discuss in more detail the synchrotron and inverse-Compton emission spectra produced by such an electron energy distribution, taking into account Klein-Nishina effects. We point out that the curvature of the high-frequency segments of these spectra, even though they are produced by the same population of electrons, may be substantially different between the synchrotron and inverse-Compton components.

We report the spectroscopic confirmation of a submillimeter galaxy (SMG) at z = 4.547 with an estimated L_IR = (0.5-2.0) × 10¹³L_☉. The spectra, mid-IR, and X-ray properties indicate the bolometric luminosity is dominated by star formation at a rate of >1000 M_☉ yr⁻¹. Multiple, spatially separated components are visible in the Lyα line with an observed velocity difference of up to 380 km s⁻¹ and the object morphology indicates a merger. The best-fit spectral energy distribution and spectral line indicators suggest the object is 2-8 Myr old and contains >10¹⁰M_☉ of stellar mass. This object is a likely progenitor for the massive early-type systems seen at z ∼ 2.

We report the discovery of a bright (m_R = 22.2) Lyman break galaxy at z = 3.03 that appears to be a massive system in a late stage of merging. Deep imaging reveals multiple peaks in the flux profile with angular separations of ~0.8^'' (~20 h⁻¹ kpc, comoving). In addition, high signal-to-noise ratio rest-frame UV spectroscopy shows evidence for at least three individual components based on stellar photospheric and ISM absorption lines. We find a 1D velocity dispersion of σ ∼ 450 km s⁻¹ for the three strongest components. Both the dynamics and high luminosity as well as our analysis of a ΛCDM numerical simulation suggest that this is a system with halo mass M ∼ 10¹³M_☉. We find in the simulation that all halos of this mass at z = 3 contain massive subhalos that agree with the observed component properties. These halos typically evolve into M ∼ 10¹⁴–10^14.5M_☉ halos in groups and clusters by z = 0. This discovery provides a rare opportunity to study the properties and components of a z ∼ 3 system that is likely to be the progenitor of a brightest cluster galaxy.

We present cosmological magnetohydrodynamic simulations of the formation of a galaxy cluster with magnetic energy feedback from an active galactic nuclei (AGNs). We demonstrate that X-ray cavities can be produced by the magnetically dominated jet-lobe system that is supported by a central axial current. The cavities are magnetically dominated, and their morphology is determined jointly by the magnetic fields and the background cluster pressure profile. The expansion and motion of the cavities are driven initially by the Lorentz force of the magnetic fields, and the cavities only become buoyant at late stages (>500 Myr). We find that up to 80%-90% of the injected magnetic energy goes into doing work against the hot cluster medium, heating it, and lifting it in the cluster potential.

The role of thermal conduction in regulating the thermal behavior of cooling flows in galaxy clusters is reexamined. Recent investigations have shown that the anisotropic Coulomb heat flux caused by a magnetic field in a dilute plasma drives a dynamical instability. A long-standing problem of cooling flow theory has been to understand how thermal conduction can offset radiative core losses without completely preventing them. In this Letter, we propose that magnetohydrodynamic turbulence driven by the heat flux instability regulates field-line insulation and drives a reverse convective thermal flux, both of which may mediate the stabilization of the cooling cores of hot clusters. This model suggests that turbulent mixing should accompany strong thermal gradients in cooling flows. This prediction seems to be supported by the spatial distribution of metals in the central galaxies of clusters, which shows a much stronger correlation with the ambient hot gas temperature gradient than with the parent stellar population.

We present a sequence of 12 monthly polarimetric 15, 22, and 43 GHz VLBA observations of the radio galaxy 3C 120 revealing a systematic presence of gradients in Faraday rotation and degree of polarization across and along the jet. The degree of polarization increases with distance from the core and toward the jet edges and has an asymmetric profile in which the northern side of the jet is more highly polarized. The Faraday rotation measure is also stratified across the jet width, with larger values for the southern side. We find a localized region of high Faraday rotation measure superposed on this structure between approximately 3 and 4 mas from the core, with a peak of ~6000 rad m⁻². Interaction of the jet with the external medium or a cloud would explain the confined region of enhanced Faraday rotation, as well as the stratification in degree of polarization and the flaring of superluminal knots when crossing this region. The data are also consistent with a helical field in a two-fluid jet model, consisting of an inner, emitting jet and a sheath containing nonrelativistic electrons. However, this helical magnetic field model cannot by itself explain the localized region of enhanced Faraday rotation. The polarization electric vectors, predominantly perpendicular to the jet axis once corrected for Faraday rotation, require a dominant component parallel to the jet axis (in the frame of the emitting plasma) for the magnetic field in the emitting region.

We present observations made with the 10 m Heinrich Hertz Submillimeter Telescope of HCN(3-2) emission from a sample of 30 nearby galaxies ranging in infrared luminosity from 10¹⁰ to 10^12.5L_☉ and HCN(3-2) luminosity from 10⁶ to 10⁹ K km s⁻¹ pc². We examine the correlation between the infrared luminosity and HCN(3-2) luminosity and find that the best-fit linear regression has a slope (in log-log space) of 0.74 ± 0.12. Including recently published data from Graciá-Carpio et al. tightens the constraints on the best-fit slope to 0.79 ± 0.09. This slope below unity suggests that the HCN(3-2) molecular line luminosity is not linearly tracing the amount of dense gas. Our results are consistent with predictions from recent theoretical models that find slopes below unity when the line luminosity depends on the average gas density with a power-law index greater than a Kennicutt-Schmidt index of 1.5.

We investigate the relationship between the star formation rate (SFR) and dense molecular gas mass in the nuclei of galaxies. To do this, we utilize the observed 850 μm luminosity as a proxy for the infrared luminosity (L_IR) and SFR, and we correlate this with the observed CO(J = 3–2) luminosity. We find tentative evidence that the L_IR-CO(J = 3–2) index is similar to the Kennicutt-Schmidt (KS) index (N ≈ 1.5) in the central ~1.7 kpc of galaxies, and it flattens to a roughly linear index when including emission from the entire galaxy. This result may imply that the volumetric Schmidt relation is the underlying driver behind the observed SFR-dense gas correlations, and it provides tentative confirmation for recent numerical models. While the data exclude the possibility of a constant L_IR-CO(J = 3–2) index for both galaxy nuclei and global measurements at the ~80% confidence level, the considerable error bars cannot preclude alternative interpretations.

We report the discovery of an extensive system of scattered-light echo arclets associated with the recent supernovae in the local neighborhood of the Milky Way: Tycho (SN 1572) and Cassiopeia A. Existing work suggests that the Tycho SN was a thermonuclear explosion while the Cas A supernova was a core-collapse explosion. Precise classifications according to modern nomenclature require spectra of the outburst light. In the case of ancient SNe, this can only be done with spectroscopy of their light echo, where the discovery of the light echoes from the outburst light is the first step. Adjacent light echo positions suggest that Cas A and Tycho may share common scattering dust structures. If so, it is possible to measure precise distances between historical Galactic supernovae. Ongoing surveys that alert on the development of bright scattered-light echo features have the potential to reveal detailed spectroscopic information for many recent Galactic supernovae, both directly visible and obscured by dust in the Galactic plane.

A two-dimensional (2D) shock-rest-frame model for particle simulations is developed. Then full kinetic dynamics of a perpendicular collisionless shock is examined by means of a 2D full particle simulation. We found that in the 2D simulation there are fewer nonthermal electrons due to surfing acceleration, which was seen in the previous 1D simulations of a high Mach number perpendicular shock in a low-beta and weakly magnetized plasma. This is because the particle motion along the ambient magnetic field disturbs the formation of coherent electrostatic solitary structures, which is necessary for electron surfing acceleration.

The discovery of X-ray afterglows accompanying two short bursts from SGR 1900+14 is presented. The afterglow luminosities at the end of each observation are lower by 30%-50% than their initial luminosities, and decay with power-law indices p ∼ 0.2–0.4. Their initial bolometric luminosities are L ∼ 10³⁴-10³⁵ erg s⁻¹. We discuss analogies and differences between the X-ray afterglows of SGR short bursts and short gamma-ray bursts.

We show that the Weibel-mediated collisionless shocks are driven at nonrelativistic propagation speed (0.1c < V < 0.45c) in unmagnetized electron-ion plasmas by performing two-dimensional particle-in-cell simulations. It is shown that the profiles of the number density and the mean velocity in the vicinity of the shock transition region, which are normalized by the respective upstream values, are almost independent of the upstream bulk velocity, i.e., the shock velocity. In particular, the width of the shock transition region is ~100 ion inertial lengths, independent of the shock velocity. For these shocks the energy density of the magnetic field generated by the Weibel-type instability within the shock transition region reaches typically 1%-2% of the upstream bulk kinetic energy density. This mechanism probably explains the robust formation of collisionless shocks, for example, driven by young supernova remnants, with no assumption of an external magnetic field in the universe.

We report the discovery of a circumbinary disk around the Herbig Ae/Be system V892 Tau. Our detailed mid-infrared images were made using segment-tilting interferometry on the Keck I telescope and reveal an asymmetric disk inclined at ~60° with an inner hole diameter of 250 mas (35 AU), approximately 5 times larger than the apparent separation of the binary components. In addition, we report a new measurement along the binary orbit using near-infrared Keck aperture masking, allowing a crude estimate of orbital parameters and the system mass for the first time. The size of the inner hole appears to be consistent with the minimum size prediction from tidal truncation theory, bearing a resemblance to the recently unmasked binary CoKu Tau/4. Our results have motivated a reanalysis of the system spectral energy distribution, concluding the luminosity of this system has been severely underestimated. With further study and monitoring, V892 Tau should prove a powerful testing ground for both predictions of dynamical models for disk-star interactions in young systems with gas-rich disks and for calibrations of pre-main-sequence tracks for intermediate-mass stars.

This Letter reports on a search for infrared emissions of isotopologues of CO₂ in the atmosphere of Titan using spectral data recorded by the Cassini Composite Infrared Spectrometer (CIRS). We have made a successful 6.5 σ detection of ¹³CO₂ at a fraction CO₂/¹³CO₂ = 84 ± 17, consistent with measurements of ¹²C/¹³C in other species, and also the terrestrial value (89). We also find a probable 3.5 σ detection of C¹⁶O¹⁸O at a fraction CO₂/C¹⁶O¹⁸O = 173 ± 55, slightly lower than the terrestrial value (253) and consistent with the twofold enhancement in ¹⁸O reported previously in CO, or with an intermediate value as suggested by chemistry. These isotopic ratios provide important constraints on models of the formation, evolution, and current processes in Titan's atmosphere.

Microstructural studies by transmission electron microscope (TEM) techniques of a microtomed cometary enstatite (Mg/Si 0.858; Fe/Si 0.027; Ca/Si 0.01; Al/Si 0.009; Cr/Si 0.01) from Wild 2 sampled during NASA's Stardust mission were conducted. The enstatite is characterized by high stacking disorder parallel (100) which includes alternating clinoenstatite (CLEN) and orthoenstatite (OREN) lamellae and (100) twins. In addition a widespread occurrence of 4.5 Å wide half-planes parallel (100) are detected, which leads to 13.5 and 22.5 Å polytypes of the structure. These microstructural features are indicative of the direct transformation from a protoenstatite (PEN) precursor, which requires temperatures of more than 1275 K and rapid cooling (>10 K hr⁻¹). Our finding represents a further high-temperature component originally present in the cold icy region where comet Wild 2 was formed.

This Letter reports the first detection of the three ¹³C isotopologues of HC₃N on Titan, from Cassini Composite Infrared Spectrometer (CIRS) infrared spectra. The data are limb spectra taken at latitudes N54°-N69° in 2006 and 2007 when HC₃N was enhanced in the north. Using a new line list for the ν₅ bands of all isotopologues, we have modeled the isolated emission of H¹³CCCN at 658.7 cm⁻¹ and both HC¹³CCN and HCC¹³CN at 663.0 cm⁻¹, which are blended with the Q-branch of HC₃N at 663.3 cm⁻¹ at the resolution of CIRS (0.5 cm⁻¹) and detectable as an increase in the intensity of the low-frequency wing. Using the resolved pair H¹³CCCN/HC₃N we find ¹²C/¹³C = 79 ± 17, in line with other measurements on Titan from Cassini and Huygens.

We report a large-scale coronal wave (so-called EIT wave) observed with high cadence by EUVI on board STEREO in association with the GOES B9.5 flare and double CME event on 2007 May 19. The EUVI instruments provide us with the unprecedented opportunity to study the dynamics of flare/CME associated coronal waves. The coronal wave under study reveals deceleration, indicative of a freely propagating MHD wave. Complementary analysis of the associated flare and erupting filament/CME hint at wave initiation by the CME expanding flanks, which drive the wave only over a limited distance. The associated flare is very weak and occurs too late to account for the wave initiation.

The Ar XVII X-ray line group principally due to transitions 1s²–1s2l (l = s, p) near 4 Å was observed in numerous flares by the RESIK bent crystal spectrometer aboard CORONAS-F between 2001 and 2003. The three line features include Ar XVII w (resonance line), a blend of x and y (intercombination lines), and z (forbidden line), all of which are blended with Ar XVI dielectronic satellites. The ratio G, equal to [ I(x) + I(y) + I(z) ]/I(w) , varies with electron temperature T_e mostly because of unresolved dielectronic satellites. With temperatures estimated from GOES X-ray emission, the observed G ratios agree fairly well with those calculated from CHIANTI and other data. With a two-component emission measure, better agreement is achieved. Some S XV and S XVI lines blend with the Ar lines, the effect of which occurs at temperatures ≳8 MK, allowing the S/Ar abundance ratio to be determined. This is found to agree with coronal values. A nonthermal contribution is indicated for some spectra in the repeating-pulse flare of 2003 February 6.

We performed a systematic study of the Doppler shifts and electron densities measured in an EUV bright point (hereafter BP) observed in more than 10 EUV lines with formation temperatures from log (T/K) = 4.5 to 6.3. Those parts of a BP seen in transition region and coronal lines are defined as its cool and hot components, respectively. We find that the transition from cool to hot occurs at a temperature around log (T/K) = 5.7. The two components of the BP reveal a totally different orientation and Doppler-shift pattern, which might result from a twist of the associated magnetic loop system. The analysis of magnetic field evolution and topology seems to favor a two-stage heating process, in which magnetic cancellation and separator reconnection are powering, respectively, the cool and hot components of the BP. We also found that the electron densities of both components of the BP are higher than those of the surrounding quiet Sun, and comparable to or smaller than active region densities.

Stably stratified fluids, such as stellar and planetary atmospheres, can support and propagate gravity waves. On Earth these waves, which can transport energy and momentum over large distances and can trigger convection, contribute to the formation of our weather and global climate. Gravity waves also play a pivotal role in planetary sciences and modern stellar physics. They have also been proposed as an agent for the heating of stellar atmospheres and coronae, the exact mechanism behind which is one of the outstanding puzzles in solar and stellar physics. Using a combination of high-quality observations and 3D numerical simulations we have the first unambiguous detection of propagating gravity waves in the Sun's (and hence a stellar) atmosphere. Moreover, we are able to determine the height dependence of their energy flux and find that at the base of the Sun's chromosphere it is around 5 kW m⁻². This amount of energy is comparable to the radiative losses of the entire chromosphere and points to internal gravity waves as a key mediator of energy into the solar atmosphere.