Nucleosynthesis in Type Ii Supernovae and the Abundances in Metal-poor Stars

We explore the effects on nucleosynthesis in Type II supernovae of various parameters (mass cut, neutron excess, explosion energy, progenitor mass) in order to explain the observed trends of the ironpeak element abundance ratios ([Cr/Fe], [Mn/Fe], [Co/Fe] and [Ni/Fe]) in halo stars as a function of metallicity for the range −4 ≤ [Fe/H] ≤ −2.5. [Cr/Fe] and [Mn/Fe] decrease with decreasing [Fe/H], while [Co/Fe] behaves the opposite way and increases. We show that such behavior can be explained by a variation of mass cuts in Type II supernovae as a function of progenitor mass, which provides a changing mix of nucleosynthesis from an alpha-rich freeze-out of Si-burning and incomplete Si-burning. This explanation is consistent with the amount of ejected Ni determined from modeling the early light curves of individual supernovae. We also propose that the metallicity of halo stars depends on the extent of the progenitors’ Strömgren spheres in very early galaxies. Subject headings: supernovae: general — Galaxy: halo — stars: abundances