CHEMICAL EVOLUTION OF PRIMARY ELEMENTS IN THE GALACTIC DISC - AN ANALYTICAL MODEL

We model the evolution of abundances of oxygen, iron, alpha and r-proc ess elements in the Galactic disc using the instantaneous and delayed production approximations which enable the problem to be handled analy tically. In addition to using the new greatly improved data published by Edvardsson et al. and the oxygen abundance distribution function of G dwarfs in the solar neighbourhood as improved by Sommer-Larsen, we take into account new survey data on metal-deficient stars which show that the thick-disc population extends to arbitrarily low metallicitie s, meaning that a significant proportion of stars previously classifie d as belonging to the halo, on grounds of low metallicity, actually be long to the disc and should be modelled as such, without bringing in a ny prior phase associated with the halo. Comparison with theoretical y ields from Type II and Type Ia supernovae shows good agreement, as has previously been found by Tsujimoto et al. using a different approach.