Star Formation and Evolution in Spiral Galaxies

Abstract

Evolutionary models for regions of M31 and M33 and the solar neighborhood are based on a stellar birthrate suggested by the dynamics of spiral structure: we assume that stars are formed very efficiently until the gas content reaches equilibrium at its present value, which takes about 10 years; thereafter, the birthrate just equals the rate at which gas enters the system from stellar mass-loss or infall of intergalactic matter. Each model represents an average around a cylindrical- shell-shaped region, which is homogeneous and closed except for possible infall. The disk and spiral-arm populations only are considered. Each star is followed in the H-R diagram from the main sequence to death as an invisible remnant. Integrated magnitudes, colors, mass-to-light ratio (M/L), gas content, helium and metal abundance (Z), are computed in steps of 10 years. For regions of M31 and M33 we use only the Salpeter initial mass function (IMF). With this restriction, successful models can be obtained provided the infall rate and lower mass limit are suitably chosen. The gradient of lower limit required to give the observed gradient of MIL results in a decrease outwards of Z, in qualitative agreement with observation. The solar-neighborhood IMF is adjusted to give rise to the observed present-day luminosity function at age 11 x 10 years. The rapid initial birthrate results in such depletion of deuterium that, if there is no infalling intergalactic gas, the solar abundance cannot be explained; also, the solar Z is far too great unless at least half of the local hydrogen is hidden in molecular form. But if infall occurs at 2 percent of the local mass per 10 years, we can obtain a model which solves these problems and is consistent with the local M/L, distribution of Z among Ci dwarfs, abundances of long-lived r-process elements, supernova rate, and birthrate of white dwarfs. Subject headings: abundances, stellar - galaxies - galaxies, motions in - star formation - stellar statistics