Low- and Intermediate-Mass Stars

Abstract

Energy in stars is provided by nuclear reactions, which, in many cases, produce radioactive nuclei. When stable nuclei are irradiated by a flux of protons or neutrons, capture reactions push stable matter out of stability into the regime of unstable species. The ongoing production of radioactive nuclei in the deep interior of the Sun via proton-capture reactions is recorded by neutrinos emitted during radioactive decay. These neutrinos escape the inner region of the Sun and can be detected on Earth. Radioactive nuclei that have relatively long half lives may also be detected in stars via spectroscopic observations and in stardust recovered from primitive meteorites via laboratory analysis. The vast majority of these stardust grains originated from Asymptotic Giant Branch (AGB) stars. This is the final phase in the evolution of stars initially less massive than ≃10 M⊙, during which nuclear energy is produced by alternate hydrogen and helium burning in shells above the core. The long-lived radioactive nucleus26Al is produced in AGB stars by proton captures at relatively high temperatures, above 60 MK. Efficient production of 26Al occurs in massive AGB stars (> 4:5 M⊙), where the base of the convective envelope reaches such temperatures. Several other long-lived radioactive nuclei, including 60Fe, 87Rb, and 99Tc, are produced in AGB stars when matter is exposed to a significant neutron flux leading to the synthesis of elements heavier than iron. Here, neutron captures occur on a timescale that is typically slower than β-decay timescales, resulting in a process known as slow neutron captures (the s-process). However, when radioactive nuclei with half lives greater than a few days are produced, depending on the temperature and the neutron density, they may either decay or capture a neutron, thus branching up the path of neutron captures and defining the final s-process abundance distribution. The effect of these branching points is observable in the composition of AGB stars and stardust. This nucleosynthesis in AGB stars could produce some long-living radioactive nuclei in relative abundances that resemble those observed in the early solar system.