Electron Captures on $^{14}{\rm N}$ as a Trigger for Helium Shell Detonations

TL;DR

This study investigates how electron captures on nitrogen-14 influence helium shell detonations on white dwarfs, revealing that these reactions lead to ignitions at the shell base, which are more likely to cause detonations than previous models suggested.

Contribution

The paper introduces a detailed binary evolution model incorporating the NCO reaction chain, showing it significantly affects ignition conditions and shell mass predictions for helium detonations.

Findings

NCO reactions cause ignitions at the shell base, unlike 3α ignitions.

Predicted helium shell mass for ignition is nearly doubled.

Shells of 0.153 M_sun can ignite dynamically, enabling detonations.

Abstract

White dwarfs (WDs) that accrete helium at rates $\sim 10^{-8} \, M_\odot \, \rm yr^{-1}$, such as those in close binaries with sdB stars, can accumulate large ($\gtrsim 0.1 \, M_\odot$) helium envelopes which are likely to detonate. We perform binary stellar evolution calculations of sdB+WD binary systems with MESA, incorporating the important reaction chain $^{14}{\rm N}(e^-, \nu) {^{14}{\rm C}} ( \alpha, \gamma) {^{18}{\rm O}}$ (NCO), including a recent measurement for the ${^{14}{\rm C}} ( \alpha, \gamma) {^{18}{\rm O}}$ rate. In large accreted helium shells, the NCO reaction chain leads to ignitions at the dense base of the freshly accreted envelope, in contrast to $3\alpha$ ignitions which occur away from the base of the shell. In addition, at these accretion rates, the shells accumulate on a timescale comparable to their thermal time, leading to an enhanced sensitivity of the outcome on the accretion rate history. Hence, time dependent accretion rates from binary stellar evolution are necessary to determine the helium layer mass at ignition. We model the observed sdB+WD system ${\rm CD}\,-30^\circ 11223$ and find that the inclusion of these effects predicts ignition of a $0.153 \, M_\odot$ helium shell, nearly a factor of two larger than previous predictions. A shell with this mass will ignite dynamically, a necessary condition for a helium shell detonation.