EVOLUTION OF SPIRAL GALAXIES .2. THE STAR-FORMATION HISTORY IN THE SOLAR NEIGHBORHOOD

This paper extends the discussion about the evolution of stellar populations in the solar vicinity, begun with the paper of Ferrini et al. (1992) where our new model of the evolution of spirals was presented in detail. We compare model predictions obtained under different assumptions about the halo collapse timescale, star formation efficiency, and initial mass function, with a wide set of observational constraints which need simultaneous fulfillment; halo-to-disk mass ratio, disk gas fraction, present star formation rate, Type I and II supernova rates, enrichment time for a number of chemical elements, age-metallicity relation, G dwarf distribution, and present-day mass function. From this comparison we draw the following conclusions: 1. The star formation rate in the solar neighborhood had to rise slowly, on a timescale of about 2 Gyr, and its maximum was not greater than about 10 times the present value nor less than about 3 times, and the ratio of the past average star formation rate to the present rate in our preferred model solution is psi/psi(now) similar or equal to 5; this value depends on the initial mass function and on the assumed Galactic age. 2. The above-mentioned star formation history is obtained only if the disk formed from the halo with a long timescale (greater than or equal to 5 Gyr). Such a request conflicts with halo observational properties, which seem to be fulfilled better by a quicker collapse (less than or equal to 2 Gyr). We suggest that this contradiction could be overcome allowing for a thick disk phase during the collapse. 3. Focusing the analysis on the differences in production timescale and enrichment history of different chemical elements, we can place some limits on the initial mass function. The model therefore constitutes a test for such functions, and we favor the one proposed by Ferrini, Palla, & Pence (1990).