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

Abstract

Superbursts from accreting neutron stars probe nuclear reactions at extreme densities ($\rho \approx 10^{9}gc{m}^{-3}$) 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.

arXiv comment8 pages, submitted to ApJ

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