Stellar Evolution and Nuclear Reaction Rate Uncertainties

Abstract

Among the uncertainties of nuclear reaction rate, this chapter focuses on how the¹²C(α, γ)¹⁶O reaction rate affects the evolution of massive stars with the initial masses of M(ZAMS) = 13–40 M_⊙. It is shown that the final fates of these stars, i.e., the formation of a neutron star or a black hole, are sensitive to the strength of convective C-shell burning and thus the mass fraction X(C) and distribution of carbon in the convective shell. If the ¹²C(α, γ)¹⁶O reaction rate is small as σ_C12α = −1, enhancement of X(C) by mixing of carbon from the overlying layer strengthens C-shell burning. For M(ZAMS) = 28 M_⊙, e.g., carbon shell burning becomes strong enough to prevent the contraction of outer layers and make the compactness parameter at M_r = 2.5 M_⊙ small. It is shown that the small compactness leads to a steep gradient of density at the Fe- and Si-rich core, which helps to transform collapse into explosion and thus to form a neutron star. This is in contrast to the case of σ_C12α = 0 where X(C) is too small to make the compactness small, which leads to a black hole formation. Thus the final fates of massive stars are quite sensitive to the ¹²C(α, γ)¹⁶O reaction rate that affects the carbon shell burning behavior. The accurate prediction of the fate of massive stars needs more accurate 3D simulations of convective mixing and carbon shell burning. Constraints from nucleosynthesis yields and the black hole mass distribution are discussed. In Appendix, it is also presented how the uncertainty of the ¹²C+¹²C reaction rate affects the presupernova nucleosynthesis yields.