Electron exchange and polarization effects on electron captures and neutron emissions by nuclei in white dwarfs and neutron stars

TL;DR

This paper investigates how electron exchange and polarization influence electron capture and neutron emission processes in dense stellar environments, providing analytical expressions and assessing their impact on stellar core and crust compositions.

Contribution

It extends the instability conditions for electron captures and neutron emissions to include electron exchange and polarization effects, with derived analytical formulas and impact assessments.

Findings

Corrections to electron-capture thresholds are very small.

Neutron-drip density and pressure are only slightly shifted.

Electron polarization can alter crust composition depending on nuclear mass models.

Abstract

In dense stellar environments, nuclei may become unstable against electron captures and/or neutron emissions. These processes are of particular importance for determining the internal constitution of white-dwarf cores and neutron-star crusts. In this paper, the role of electron exchange and polarization effects is studied. In particular, the instability condition for the onset of electron captures and neutron emissions is extended so as to account for electron exchange and polarization. Moreover, general analytical expressions for the corresponding density and pressure are derived. The corrections to the electron-capture threshold in white-dwarf cores are found to be very small. Likewise, the neutron-drip density and pressure in the crusts of accreting and nonaccreting neutron stars are only slightly shifted. Depending on the nuclear mass model employed, electron polarization may change the composition of the crust of nonaccreting neutron stars. On the other hand, the current uncertainties in the masses of neutron-rich Kr and Sr isotopes are found to be more important than electron exchange and polarization effects.