Galactic evolution with bimodal star formation. 1

Models for the history of the star formation rate are presented which essentially follow three assumptions: (i) high-mass star formation is mainly initiated by collisions of molecular clouds while low-mass stars are formed in a spontaneous mode; (ii) the bimodal IMF is responsible for the structure in the present-day mass function of nearby stars; (iii) the global molecular gas density can be described in terms of the total gas density and the metal abundance. Some properties of these models are confronted with observations in the Solar neighborhood (total density, gas fraction, OB star formation rate, star formation efficiency, present-day mass function, age-metallicity relation, white dwarf luminosity function). Special attention is given to the question of the nature of dark matter in the galactic disk within the frame of these models. 123 refs. (author)