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EVOLUTION OF GALACTIC DISCS:
TWO MODES OF STAR FORMATION

ABSTRACT


We develop the model of the evolution of surface gas density,
stellar density, star formation rates and chemical composition of
galactic disc taking into account the gravitational instability of
rotating gaseous layer in the scales of several kpc.

The central proposition was maden is the existence of two modes in
the star formation. First mode associated with the large - scale
grav.instability and takes place only when a local gas surface
density is more than a critical value, which depends on the
dynamical parameters of a gaseous disc. Second one don't associated
with a large - scale grav.instability and determined by the surface
density of the gas and stars.

In the model we took into account the gas return from evolved stars,
a radial movement of the gas in a spiral arms and accretion of the
gas into the galactic disc.

We found that our model is in a good agreement with observations for
our Galaxy and for four others galaxies only if to assume relatively
high low-mass limit for IMF ($1 \div 2 \, M_{\odot}$) at the mode I
and upper-mass limit ($\approx 40\, M_{\odot}$) for
mode II. The model distributions are in agreement with observations
as for the cases with the accretion of the gas into the disc, as when
no accretion.