Presupernova Yields and Their Dependence on the ¹²C(α,γ)¹⁶O Reaction

Abstract

Massive stars (M ≳ 20 M⊙) produce abundance patterns during the high temperature shell burning phases of hydrostatic evolution before the core collapse which are little changed by explosive processing of the ejected envelope in a type II supernova explosion. Thus a prediction for presupernova abundances can be taken as an approximation to the final products of explosive nucleosynthesis. A calculation using a detailed nucleosynthesis network (254 nuclear species), which adds shell burning products at typical burning conditions (temperature, density) weighted in mass according to stellar evolution calculations, reproduces the qualitative behavior of previous yield curves from postprocessing calculations, including the (too) large enhancement of carbon burning products and (too) small enhancement of oxygen burning products. When the ¹²C(α,γ)^16O rate is increased, as indicated by recent experiments, this leads to a lower 12_C and higher 16_O abundance after core helium burning. Consequently the subsequent burning phases give less carbon burning products and more oxygen (and silicon) burning products. These changes are shown to lead to a uniform enhancement of nucleosynthesis products (2 < Z ≲ 32 at least) from an “average” type II supernova.