Mark Pecaut

The concept of kinematic assemblages representing dissolved clusters has remained contentious since Eggen's formulation of moving groups [Eggen58]. The availability of precise astrometric data from Hipparcos has led to evidence that the large scale kinematic structure of the solar neighborhood is dominated by dynamical streams composed of stars of various ages and metallicities[Famaey05], rather than the relic cluster/supercluster picture of Eggen. In order to address this dissolved cluster versus dynamic structure paradigm, we have flagged solar-type (F,G, and K) members of two kinematic groups. In order to unravel their origin, we propose to obtain high resolution stellar spectroscopy of the alleged group members. By applying the novel approach of chemically tagging the members across multiple elemental abundances, we will ascertain whether the kinematic structures plausibly represent a dissolved cluster or are dynamically formed entities. The results of this work will build a foundation for further use of kine-chemical tagging in exploring both the dynamical and chemical evolution of the Galactic disk.

We present an analysis of the ages and star-formation history of the F-type stars in the Upper Scorpius (US), Upper Centaurus-Lupus (UCL) and Lower Centaurus-Crux (LCC) subgroups of Scorpius-Centaurus. We find that 1) our empirical isochrones are consistent with the previously published age-rank of the Sco-Cen subgroups, 2) LCC and UCL appear to reach the main sequence turn-on at spectral types ~F4 and ~F2, respectively. An analysis of the A-type stars shows US reaching the main sequence at about spectral type ~A3. 3) The median ages for the pre-main sequence members of UCL and LCC are 16 Myr and 17 Myr, respectively, in agreement with previous studies, however we find that 4) Upper Sco is much older than previously thought. The luminosities of the F-type stars in US are typically a factor of ~2.5 less luminous than predicted for a 5 Myr old population for four sets of evolutionary tracks. We reexamine the evolutionary state and isochronal ages for the B-, A-, and G-type Upper Sco members, and the evolved M supergiant Antares, and estimate a revised mean age for Upper Sco of 11+/-1+/-2 Myr (statistical, systematic). Using radial velocities and Hipparcos parallaxes we calculate a lower limit on the kinematic expansion age for Upper Sco of >10.5 Myr (99% confidence). However, the data are statistically consistent with no expansion. We reevaluate the inferred masses for the known substellar companions in Upper Sco using the revised age. Specifically, we estimate the mass of 1RXS J1609-2105b to be 14^{+2}_{-3} Mjup, suggesting that it is a brown dwarf rather than a planet. Finally, we find the fraction of F-type stars exhibiting Ha emission and/or a K-band excess consistent with accretion to be 0/17 (<19%; 95% C.L.) in US at ~11 Myr, while UCL has 1/41 (2^{+5}_{-1}%; 68% C.L.) accretors and LCC has 1/50 (2^{+4}_{-1}%; 68% C.L.) accretors at ~16 Myr and ~17 Myr, respectively. [Abridged]

The chemical evolution of the galaxy can be examined on a small scale locally by searching for evidence of enrichment by core collapse (type II) supernova explosions among stars belonging to the same star- forming complex. We propose to obtain high resolution spectroscopy of a sample of slowly rotating members of nearby, young stellar groups associated with the nearest star-forming complex: Scorpius-Centaurus. These spectra will be used to perform a precise spectroscopic abundance analysis to test for enrichment of the ~5 Myr-old Upper Scorpius region and neighboring regions by supernova explosions in the neighboring ~ 15 Myr-old Upper Centaurus Lupus and Lower Centaurus Crux subgroups. Enrichment by core-collapse supernovae can be traced by enhancements in oxygen and other alpha- element abundances compared to Fe-peak elements. These abundances can also be used for constraining the chemical homogeneity of members of Upper-Sco. This study will allow us to explore the processes of Galactic chemical evolution and SN enrichment on a small scale (< 0.1 kpc, <10-20 SN) in a complex with a relatively well constrained star formation history and high mass stellar membership.

ABSTRACT We present the results of a survey of ~70 stars in the nearby Scorpius-Centaurus OB association based mainly on high-resolution spectra from the Magellan Clay 6.5-m telescope. A subsample of these stars are lithium rich, have common velocities, and are clustered near the southeastern part of the Upper Centaurus-Lupus sub-region of Scorpius-Centaurus. Given their separation away from known members, clustering, and youth, we believe that many of these stars constitute a new sub-region of Scorpius-Centaurus, which we propose to call &amp;quot;Lower Scorpius&amp;quot;. For this new group, we estimated stellar parameters including temperature, surface gravity, metallicity, radial velocity, and projected rotational velocity.

We obtained Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS) 24 μm and 70 μm observations of 182 nearby, Hipparcos F- and G-type common proper motion single and binary systems in the nearest OB association, Scorpius-Centaurus. We also obtained Magellan/MIKE R â 50,000 visual spectra at 3500-10500 A for 181 candidate ScoCen stars in single and binary systems. Combining

ABSTRACT We present the results of a survey of ~70 stars in the nearby Scorpius-Centaurus OB association based mainly on high-resolution spectra from the Magellan Clay 6.5-m telescope. A subsample of these stars are lithium rich, have common velocities, and are clustered near the southeastern part of the Upper Centaurus-Lupus sub-region of Scorpius-Centaurus. Given their separation away from known members, clustering, and youth, we believe that many of these stars constitute a new sub-region of Scorpius-Centaurus, which we propose to call &amp;quot;Lower Scorpius&amp;quot;. For this new group, we estimated stellar parameters including temperature, surface gravity, metallicity, radial velocity, and projected rotational velocity.

ABSTRACT The chemical evolution of the galaxy can be examined on a small scale locally by searching for evidence of enrichment by core collapse (type II) supernova explosions among stars belonging to the same star- forming complex. We propose to obtain high resolution spectroscopy of a sample of slowly rotating members of nearby, young stellar groups associated with the nearest star-forming complex: Scorpius-Centaurus. These spectra will be used to perform a precise spectroscopic abundance analysis to test for enrichment of the ~5 Myr-old Upper Scorpius region and neighboring regions by supernova explosions in the neighboring ~ 15 Myr-old Upper Centaurus Lupus and Lower Centaurus Crux subgroups. Enrichment by core-collapse supernovae can be traced by enhancements in oxygen and other alpha- element abundances compared to Fe-peak elements. These abundances can also be used for constraining the chemical homogeneity of members of Upper-Sco. This study will allow us to explore the processes of Galactic chemical evolution and SN enrichment on a small scale (&amp;lt; 0.1 kpc,

ABSTRACT The concept of kinematic assemblages representing dissolved clusters has remained contentious since Eggen&amp;#39;s formulation of moving groups [Eggen58]. The availability of precise astrometric data from Hipparcos has led to evidence that the large scale kinematic structure of the solar neighborhood is dominated by dynamical streams composed of stars of various ages and metallicities[Famaey05], rather than the relic cluster/supercluster picture of Eggen. In order to address this dissolved cluster versus dynamic structure paradigm, we have flagged solar-type (F,G, and K) members of two kinematic groups. In order to unravel their origin, we propose to obtain high resolution stellar spectroscopy of the alleged group members. By applying the novel approach of chemically tagging the members across multiple elemental abundances, we will ascertain whether the kinematic structures plausibly represent a dissolved cluster or are dynamically formed entities. The results of this work will build a foundation for further use of kine-chemical tagging in exploring both the dynamical and chemical evolution of the Galactic disk.

ABSTRACT We have conducted a WISE/Spitzer MIPS search for dusty debris around around ~400 B- through G-type members of Scorpius-Centaurus, the nearest OB association with an age ~10 - 20 Myr. We measure 24 micron excesses, indicative of the presence of debris dust around 92 young stars. Comparing our observations with those of other nearby (&amp;lt;200 pc), young (&amp;lt;200 Myr) stars, we find that 24 micron excess peaks at ages ~15 Myr for stars with masses 1.5 - 2.5 Msun stars, consistent with oligarchic growth models, but that excess decreases monotonically around 1.0 Msun stars. We additionally measure 12 micron excess, indicative of the presence of terrestrial temperature dust, around half of our excess sources, consistent with the presence of giant planets on eccentric orbits. We estimate dust grain properties (e.g. temperature and fractional infrared luminosity) from follow-up IRS spectra, assuming a two temperature black body model.

ABSTRACT As part of a long-term observing program to elucidate the membership, star-formation history, and substructure of the nearest OB association (Sco-Cen; d = 100-150 pc; ages 5-20 Myr), we present results from a low-resolution optical spectroscopy survey of 200 F-type members of Sco-Cen selected via their convergent proper motions. Using low-resolution spectra over red ( 5700A-6800A) and blue ( 3700A-5300A) wavelengths obtained using the SMARTS 1.5-m telescope, we determined accurate MK spectral classifications and searched for evidence of accretion among these intermediate-mass stars ( 1.3-2 Msun). Combining the spectral types, revised Hipparcos trigonometric parallaxes, and Hipparcos and 2MASS photometry, we obtain luminosities for these stars, and estimate their masses and ages through interpolation of published evolutionary tracks. Combining these results with previously published results for the higher- and lower-mass stars, we discuss the substructure, age distribution, and kinematics of the Sco-Cen OB association, as well as the accretion disk fraction amongst young F-type stars in the 5-20 Myr range.