Abstract

Evolutionary models of the starburst in M82 provide new, independent support for the suggestion that the initial mass function (IMF) is heavily weighted toward massive stars. All relevant details of stellar evolution are included using recent evolutionary tracks, including pre-main-sequence evolution and mass loss as a wind from the starbursting region. The free parameters of the starburst models are the IMF, the initial mass of cold star-forming gas, and the star formation rate. From these parameters we compute the time variation of the mass in stars and remaining cold gas, the star formation rate, the supernova rate, the bolometric luminosity, the ionizing photon luminosity, and the iron abundance in the wind and remaining gas as enhanced by Type II supernovae.

In comparing with observations of M82, the computed supernova rate, mass of molecular gas, and the total dynamical mass are most critical in restricting the IMF. The observed supernova rate in M82 can be understood with normal Salpeter-type IMFs that extend down to stars of mass m_l ∼ 0.1 and a star formation rate ∼ 10³ times that in the solar neighborhood. However, success with a Salpeter IMF requires fine-tuning the input parameters, and the total mass of stars residing in the burst region before the burst began must be negligibly small.

By comparison, IMFs restricted to stars of mass ≳3 M_sun, as suggested by Rieke et al. (1980), can easily match the observed M82 supernova rate and the dynamical mass. The allowed range of initial parameters for agreement with observations increases as the low-mass cutoff on the IMF increases from m_l ≈ 0.1 to m_l ≈ 3. Stars of mass ≲3 M_sun, many of which would not have reached the main sequence during the starburst lifetime, cannot be present even as incipient protostars. The likely absence of low-mass stars can be understood if slowly forming low-mass stars do not reach or complete their Hayashi or radiative tracks before being disrupted by supernova blast waves of more rapidly forming massive stars. In addition, the cosmic-ray intensity is ≳100 times greater in M82 than in the plane of our Galaxy; this (and the strong ambient X-ray field in M82) produces a higher level of ionization in star-forming clouds, suppressing ambipolar diffusion within molecular clouds. For successful Rieke-type models, the star formation rate is ≳100 times that in the local solar neighborhood, the initial mass of star-forming gas is 1 × 10⁸ M_sun-14 × 10⁸ M₀, and the starburst age is ∼(3-6) × 10⁷ yr. The total duration of the burst is significantly reduced by mass loss in the starburst (galactic) wind.