Electron capture in stars

• Published in: Reports on progress in physics Vol. 84; no. 6; pp. 66301 - 66335
• Main Authors: Langanke, K, Martínez-Pinedo, G, Zegers, R G T
• Format: Journal Article
• Language: English
• Published: England IOP Publishing 01.06.2021
• Subjects: charge-exchange reactions; electron capture; Gamow–Teller strength; supernova; Supernova; Electron capture; Charge-exchange reactions; Gamow-Teller strength
• ISSN: 0034-4885; 1361-6633
• DOI: 10.1088/1361-6633/abf207

Electron captures on nuclei play an essential role for the dynamics of several astrophysical objects, including core-collapse and thermonuclear supernovae, the crust of accreting neutron stars in binary systems and the final core evolution of intermediate mass stars. In these astrophysical objects, the capture occurs at finite temperatures and at densities at which the electrons form a degenerate relativistic electron gas. The capture rates can be derived in perturbation theory where allowed nuclear transitions (Gamow-Teller transitions) dominate, except at the higher temperatures achieved in core-collapse supernovae where forbidden transitions also contribute significantly to the rates. There has been decisive progress in recent years in measuring Gamow-Teller (GT) strength distributions using novel experimental techniques based on charge-exchange reactions. These measurements provide not only data for the GT distributions of ground states for many relevant nuclei, but also serve as valuable constraints for nuclear models which are needed to derive the capture rates for the many nuclei for which no data yet exist. In particular models are needed to evaluate the stellar capture rates at finite temperatures, where the capture can also occur on thermally excited nuclear states. There has also been significant progress in recent years in the modelling of stellar capture rates. This has been made possible by advances in nuclear many-body models as well as in computer soft- and hardware. Specifically, to derive reliable capture rates for core-collapse supernovae, a dedicated strategy has been developed based on a hierarchy of nuclear models specifically adapted to the abundant nuclei and astrophysically conditions present at the various collapse conditions. In particular, for the challenging conditions where the electron chemical potential and the nuclear $Q$ values are of the same order, large-scale diagonalization shell model calculations have been proven as an appropriate tool to derive stellar capture rates, often validated by experimental data. Such situations are relevant in the early stage of the core collapse of massive stars, for the nucleosynthesis of thermonuclear supernovae as well for the final evolution of the cores of intermediate-mass stars, involving nuclei in the mass range $A \sim 20$--65. This manuscript reviews the experimental and theoretical progress achieved recently in deriving stellar electron capture rates. It also discusses the impact these improved rates have on the various astrophysical objects.