The Thermal Stability of Helium Burning on Accreting Neutron Stars

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 ${\dot{m}}_{\mathrm{crit}}$ as a function of the base flux, and derive analytic fitting formulae for ${\dot{m}}_{\mathrm{crit}}$ 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 $d{y}_{\mathrm{burn}}/d\dot{m}=0$ 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 $\sim 3$ 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.

11 pages, 8 figures, accepted to MNRAS

References in corpus (9)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• Models of Type I X-ray Bursts from GS 1826-24: A Probe of rp-Process Hydrogen Burning
• Spread of Matter over a Neutron-Star Surface During Disk Accretion: Deceleration of Rapid Rotation
• First superburst from a classical low-mass X-ray binary transient
• Superburst Models for Neutron Stars with Hydrogen and Helium-Rich Atmospheres
• Millihertz Oscillation Frequency Drift Predicts the Occurrence of Type I X-ray Bursts
• Turbulent Mixing in the Surface Layers of Accreting Neutron Stars
• Reaction Rate and Composition Dependence of the Stability of Thermonuclear Burning on Accreting Neutron Stars

Cited by in corpus (2)

• On the dependence of X-ray burst rate on accretion and spin rate
• The efficiency of nuclear burning during thermonuclear (Type I) bursts as a function of accretion rate