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 iron-peak element abundance ra tios ([Cr/Fe], [Mn/Fe], [Co/Fe], and [Ni/Fe]) in halo stars as a function o f metallicity for the range -4 less than or equal to [Fe/H] less than or eq ual to -2.5. [Cr/Fe] and [Mn/Fe] decrease with decreasing [Fe/H], whereas [ Co/Fe] increases. We show that such behavior can be explained by a variatio n of mass cuts in Type IT supernovae as a function of progenitor mass, whic h provides a changing mix of nucleosynthesis from an alpha-rich freeze-out of Si burning and incomplete Si burning. This explanation is consistent wit h the amount of ejected Ni-56 determined from modeling the early light curv es of individual supernovas. We also suggest that the ratio [H/Fe] of halo stars is mainly determined by the mass of interstellar hydrogen mixed with the ejecta of a single supernova, which is larger for larger explosion ener gy and the larger Stromgren radius of the progenitor.