Hypernova Nucleosynthesis and Galactic Chemical Evolution

We study nucleosynthesis in ’hypernovae’, i.e., supernovae with very large explosion energies ( ∼ > 10 ergs) for both spherical and aspherical explosions. The hypernova yields compared to those of ordinary corecollapse supernovae show the following characteristics: 1) Complete Siburning takes place in more extended region, so that the mass ratio between the complete and incomplete Si burning regions is generally larger in hypernovae than normal supernovae. As a result, higher energy explosions tend to produce larger [(Zn, Co)/Fe], small [(Mn, Cr)/Fe], and larger [Fe/O], which could explain the trend observed in very metalpoor stars. 2) Si-burning takes place in lower density regions, so that the effects of α-rich freezeout is enhanced. Thus Ca, Ti, and Zn are produced more abundantly than in normal supernovae. The large [(Ti, Zn)/Fe] ratios observed in very metal poor stars strongly suggest a significant contribution of hypernovae. 3) Oxygen burning also takes place in more extended regions for the larger explosion energy. Then a larger amount of Si, S, Ar, and Ca (”Si”) are synthesized, which makes the ”Si”/O ratio larger. The abundance pattern of the starburst galaxy M82 may be attributed to hypernova explosions. Asphericity in the explosions strengthens the nucleosynthesis properties of hypernovae except for ”Si”/O. We thus suggest that hypernovae make important contribution to the early Galactic (and cosmic) chemical evolution.