Hypernova Nucleosynthesis and Galactic Chemical Evolution

Abstract

We study nucleosynthesis in ‘hypernovae’, i.e., supernovae with very large explosion energies (≳ 10⁵² ergs) for both spherical and aspherical explosions. The hypernova yields compared to those of ordinary core-collapse supernovae show the following characteristics: 1) Complete Si-burning 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 metal-poor stars. 2) Si-burning takes place in lower density regions, so that the effects of a-rich freezeout is enhanced. Thus ⁴⁴Ca, 48T1, and ⁶⁴Zn 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 M82b may be attributed to hypernova explosions. Asphericity in the explosions strengthens the nucleosynthesis properties of hypernovae as summarized above. We thus suggest that hypernovae make important contribution to the early Galactic (and cosmic) chemical evolution.