Impact of nuclear reactions on the fate of intermediate-mass stars

Abstract

Stars in the mass range of 8 to 12 M (solar masses) represent the transition region between those that end their lives producing white dwarfs and those that undergo a core-collapse supernova and produce neutron stars. The final phases of stellar evolution of these intermediate-mass stars are tightly connected to the behavior of nuclear reactions at high densities. Despite their importance, the stellar evolution of intermediate-mass stars has received little attention in the past. The pioneering work in this mass range was published by Nomoto in the 80’s [1, 2]. Numerical and computational difficulties had hindered progress in the past, however two recent stellar evolution studies [3, 4] revived the interest in intermediate-mass stars. We explore nuclear processes that may be relevant for the modeling of these stars, starting from the neon burning stage. We show, that due to electron captures on 20Ne, 20O becomes abundant in the stellar core. This opens new reaction channels that have so far not been considered. These reactions modify the standard neon-burning that now proceeds by the reactions 20Ne(γ,α) 16O, followed by 20O(α,γ) 24Ne. Once the stellar core reaches a sufficiently high temperature, also the fusion reactions of neutron-rich oxygen isotopes, 16O + 20O→ 36S∗ and 20O + 20O→ 40S∗, may become important.