The Thermal Stability of Helium Burning on Accreting Neutron Stars

TL;DR

This study investigates how base heating flux influences the stability of helium burning on accreting neutron stars, using simulations and models to reconcile theoretical predictions with observations.

Contribution

It provides new calculations of the critical accretion rate for helium burning stability incorporating base flux effects, and evaluates the limitations of steady-state models.

Findings

Base heating flux significantly affects the critical accretion rate.

Steady-state models do not reliably predict the stability boundary.

Linear stability analysis yields higher critical accretion rates than time-dependent simulations.

Abstract

Thermonuclear burning on the surface of accreting neutron stars is observed to stabilize at accretion rates almost an order of magnitude lower than theoretical models predict. One way to resolve this discrepancy is by including a base heating flux that can stabilize the layer. We focus our attention on pure helium accretion, for which we calculate the effect of a base heating flux on the critical accretion rate at which thermonuclear burning stabilizes. We use the MESA stellar evolution code to calculate $\dot m_{\rm crit}$ as a function of the base flux, and derive analytic fitting formulae for $\dot m_{\rm crit}$ and the burning temperature at that critical accretion rate, based on a one-zone model. We also investigate whether the critical accretion rate can be determined by examining steady-state models only, without time-dependent simulations. We examine the argument that the stability boundary coincides with the turning point $dy_{\rm burn}/d\dot m=0$ in the steady-state models, and find that it does not hold outside of the one-zone, zero base flux case. A linear stability analysis of a large suite of steady-state models is also carried out, which yields critical accretion rates a factor of $\sim3$ larger than the MESA result, but with a similar dependence on base flux. Lastly, we discuss the implications of our results for the ultracompact X-ray binary 4U~1820-30.