Implications of O and Mg abundances in metal-poor halo stars for stellar iron yields

Abstract

Inhomogeneous chemical evolution models of galaxies which try to reproduce the scatter seen in element-to-iron ratios of metal-poor halo stars are heavily dependent on theoretical nucleosynthesis yields of core-collapse supernovae. Hence inhomogeneous models present themselves as a test for stellar nucleosyn- thesis calculations. Applying an inhomogeneous chemical evolution model to our Galaxy reveals a number of shortcomings of existing theoretical nucleosynthesis yields. One problem is the predicted scatter in [O/Fe] and [Mg/Fe] which is too large compared to the one observed in metal-poor halo stars. This can be either due to the O or Mg yields or due to the Fe yields (or both). However, O and Mg are alpha-elements that are produced mainly during hydrostatic burning and thus are not affected by the theoretical uncertainties afflicting the collapse and explosion of a massive star. Stellar iron yields, on the other hand, depend heavily on the choice of the mass-cut between ejecta and proto neutron star and are therefore very uncertain. We present Fe yield distributions as function of progenitor mass that are consistent with the abundance distribution of metal- poor halo stars and are in agreement with observed Ni yields of SNe II with known progenitor masses. The iron yields of lower-mass SNe II (in the range 10-20 Msol) are well constrained by those observations. Present observations, however, do not allow to determine a unique solution for higher-mass SNe. Nevertheless, the main dependence of the stellar iron yield as function of progenitor mass may be derived and can be used as constraint for future supernova/hypernova models. The results are of importance for the earliest stages of galaxy formation when the ISM is dominated by chemical inhomogeneities and the instantaneous mixing approximation is not valid.