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

Inhomogeneous chemical evolution models of galaxies that 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 (SNe II). Thus, inhomogeneous chemical evolution models present themselves as a test for stellar nucleosynthesis calculations. Applying such a model to our Galaxy reveals a number of shortcomings of existing nucleosynthesis yields. One problem is the predicted scatter in [O/Fe] and [Mg/Fe] which is too large compared to that observed in metal-poor halo stars. This can be either due to the oxygen or magnesium yields or due to the iron yields (or both). However, oxygen and magnesium are alpha-elements that are produced mainly during hydrostatic burning and thus are not affected by the theoretical uncertainties in 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 iron yield distributions as a function of progenitor mass that are consistent with the abundance distribution of metal-poor halo stars and are in agreement with observed Ni-56 yields of core-collapse supernovae with known progenitor masses. The iron yields of lower-mass SNe II (in the range 10-20 M-.) are well constrained by these observations. Present observations, however, do not allow us to determine a unique solution for higher-mass SNe. Nevertheless, the main dependence of the stellar iron yields as function of progenitor mass can be derived and may be used as a constraint for future core-collapse supernova/hypernova models. A prediction of hypernova models is the existence of ultra alpha-element enhanced stars at metallicities [Fe/H] less than or equal to -2.5, which can be tested by future observations. The results are of importance for the earliest stages of galaxy formation when the ISM is dominated by local chemical inhomogeneities and the instantaneous mixing approximation is not valid.