Star formation in an unstable interstellar medium

Abstract

Critical column densities for gravitational instabilities and for cooling to diffuse cloud temperatures are discussed. The fundamental scale for star formation in the outer regions of galaxies, in spiral arms, and in resonance rings seems to be always related to the local unstable length. In spiral arms and rings, this length is proportional to the arm or ring thickness at the threshold of instability. The instability model gives a star formation rate proportional to the 1.5 power of the density, and a gas consumption time inversely proportional to the 0.5 power of the density. Because the critical gas density for collapse scales directly with the local density in all forms, the star formation rate is much higher in the inner regions of galaxies, and the gas consumption time much smaller, once the threshold is exceeded. This implies that the inner regions of galaxies usually form stars in bursts, while the main disks can be more-or-less steady for a large fraction of the Hubble time. The burst-like nature of star formation in dwarf galaxies is probably not related to gas consumption, however, but to gas clearing from the low potential well near the star formation region. Thus there are two reasons for starbursts. © Universidad Nacional Autónoma de México.