Chemical evolution of spiral galaxies: models with star formation proportional to molecular hydrogen

The possibility of assuming the star formation-rate ψ in spiral discs to be proportional to their H2 density is analysed. Since a good correlation between the molecular gas fraction and the logarithm of the oxygen abundance is derived from a sample of several nearby spirals, we examine in particular the possibility that ψ∝ΣH2/Σgas∝log(O/H). Numerical models have been computed to evaluate the implications on the chemical evolution of spiral galaxies of two alternative assumptions on this kind of star-formation rate. In one case ψ is always proportional only to H2 through the empirical relation found with the oxygen abundance, and in the other it is proportional to both the H2 and the total gas density. Model results are presented for the Galaxy and other spirals. (author)