Evolution of galactic disks: Two modes of star formation

A model for the evolution of the gas surface density, star formation rate, and chemical composition of galactic disks is constructed, taking into account large-scale gravitational instabilities of the rotating gaseous layer on scales of the order of several kiloparsecs. A key factor is the existence of two star formation modes. The first is associated with large-scale gravitational instability, and is realized only when the local gas surface density exceeds a critical value, which is determined by the dynamical parameters of the gaseous disk. The second mode is not associated with gravitational instabilities of the disk on large scales, and is determined by the surface density of the stars and gas. The model allows for the return of gas from evolved stars, radial mixing of gas in spiral arms, and gas accretion onto the galactic disk. The model agrees well with observations of our Galaxy and a number of other galaxies if a relatively high lower limit to the initial mass function (1-2M.) is adopted for the first mode, and an upper limit approximate to 40M. is adopted for the second mode. The model calculations are in agreement with observations both for the case of gas accretion onto the disk and for the case of no accretion.