GALACTIC CHEMICAL EVOLUTION WITH NEARLY CONSTANT STAR-FORMATION RATES

In this work we have investigated a set of closed models of Galactic chemical evolution with nearly constant star-formation rates. The star-formation rates (psi) we have considered depend both on the surface density of gas and the metallicity, and are of the form psi is-proportional-to SIGMA(g)(alpha)Z(beta). In Rana & Basu (1992) we showed how such a form can probably be used to model the solar neighborhood and reproduce the age-metallicity relation (AMR) which had been derived from the observational age and metallicity distribution of stars. A more detailed investigation is reported here. We have first modeled the solar neighborhood and then extended the results to the rest of the Galaxy. One novel feature of this investigation is that the initial mass function (IMF) used for each model with a set of (alpha, beta) is calculated from the observed present-day mass function of main-sequence stars in the solar neighborhood and using the star-formation rate corresponding to that model. The different parameters of the model, such as the present rate of star formation at the solar neighborhood (psi1).. the returned gas fraction, R, and the total surface mass density, SIGMA(tot) are calculated from the initial mass function and hence are not free parameters. We find that the parameter range alpha = 1.15 +/- 0.05, beta = 1.25 +/- 0.05 fits various observational data of the solar neighborhood better than the parameter range used in our previous work. The resulting model is such that the yield of iron, y, obtained from the model is consistent with that calculated from the IMF. The fit to the Galactic data is reasonable. We find that the fit to the data on Galactic radial variations improves if it is further assumed that the efficiency of star formation varies across the face of the Galaxy by about 10%. We also find that the resultant star-formation rate per unit area of the disk is consistent with one which is proportional to some power of the surface mass density of molecular hydrogen.