Carbon Synthesis in Steady-State Hydrogen and Helium Burning On Accreting Neutron Stars

TL;DR

This paper investigates how steady-state hydrogen and helium burning on accreting neutron stars can produce sufficient carbon to trigger superbursts, highlighting the role of nuclear reaction rates and accretion conditions.

Contribution

It demonstrates that stable hydrogen and helium burning can generate enough carbon for superbursts across various accretion scenarios, with a focus on nuclear reaction sensitivities.

Findings

Large carbon production occurs in steady-state burning across diverse accretion rates.

Stable hydrogen and helium burning can be a viable source of superburst-triggering carbon.

The $^{14}$O($ ext{α}$,p)$^{17}$F reaction rate significantly affects carbon yield.

Abstract

Superbursts from accreting neutron stars probe nuclear reactions at extreme densities ($\rho \approx 10^{9}~g\,cm^{-3}$) and temperatures ($T>10^9~K$). These bursts ($\sim$1000 times more energetic than type I X-ray bursts) are most likely triggered by unstable ignition of carbon in a sea of heavy nuclei made during the rp-process of regular type I X-ray bursts (where the accumulated hydrogen and helium are burned). An open question is the origin of sufficient amounts of carbon, which is largely destroyed during the rp-process in X-ray bursts. We explore carbon production in steady-state burning via the rp-process, which might occur together with unstable burning in systems showing superbursts. We find that for a wide range of accretion rates and accreted helium mass fractions large amounts of carbon are produced, even for systems that accrete solar composition. This makes stable hydrogen and helium burning a viable source of carbon to trigger superbursts. We also investigate the sensitivity of the results to nuclear reactions. We find that the $^{14}$O($\alpha$,p)$^{17}$F reaction rate introduces by far the largest uncertainties in the $^{12}$C yield.