Chemical evolution of galaxies - II. Variation of the heavy element yield with Z

Recent determinations of the heavy element yield, p, are reviewed. The data indicate that p increases with metallicity or that the accretion rate is larger than the star formation rate in many galaxies. It is proposed that p = 0.002 + 0.6 Z; with this relation it is possible to explain the heavy element abundances derived from irregular galaxies, blue compact galaxies, the Galaxy and M83. Within the uncertainties of the observations all the objects but one fall in the Z versus log $$M_\text{gas}/M_\text{tot}$$ region limited by the simple model and an infall model with M_gas = const. The exception is A 1228 + 15 where apparently the infall rate at present is considerably higher than the star formation rate. It is suggested that the increase of the yield with Z is due to a decrease in the relative amount of low mass stars ($$M\lt1M_\odot$$) with Z. The proposed yield together with negative gradients in abundances produce positive gradients in M/L in agreement with observations. Moreoever, the proposed yield implies that $$20\lt(M/L_\text B)_\text{halo}\lt 40$$.