Star formation in molecular clouds of intermediate mass

Abstract

Self-consistent multicomponent models of evolution of the interstellar medium have been computed by extending the scheme of Habeet al. (1981) and adding some processes of star formation in molecular clouds, induced by supersonic collisions. A monochromatic spectrum of the molecular clouds has been adopted with a cloud mass of 104M⊙. The consequences of these simplifying assumptions have been discussed and moreover the influence of several parameters (efficiency of star formation, photoionization rate, cloud radius, and mass) and of the initial conditions has been analyzed. Emphasis has been put on the following points: (1) there is a strong conditioning of the physical state of the intercloud gas on the star formation rate; (2) depending on the total initial mass of the molecular clouds per unit volume δ, two different regimes of star formation are possible: one, when δ is larger than a critical value δcr, dominated by collisions between clouds, with a total star formation rate practically constant and a long lifetime for the system, the other, characterized by δ<δcr, in which the dominant process is due to the expansion ofHii regions: the resulting star formation rate causes the system exhaustion in a relatively short lifetime. Some suggestions are derived concerning the evolution of galaxies.