On the Production and Survival of Carbon Fuel for Superbursts on Accreting Neutron Stars: Implications for Mass Donor Evolution

Abstract

We have investigated the physical conditions under which accretingneutron stars in low-mass X-ray binaries can both produce and preservesufficient quantities of carbon fuel to trigger superbursts. Ourtheoretical models span the plausible ranges of neutron star thermalconductivities, core neutrino emission mechanisms, and areal radii, aswell as the CNO abundances in the accreted material. We find thatneutron stars that accrete hydrogen-rich material with CNO massfractions Z_{CNO}{\lt}~Z_{CNO,solar} will not exhibitsuperbursts under any circumstances. Neutron stars that accrete materialwith CNO mass fractions {\gt}~4Z_{CNO,solar} will exhibitsuperbursts at accretion rates in the observed range. On this basis, wesuggest that the mass donors of superburst systems must have enhancedCNO abundances. The accreted CNO acts only as a catalyst for hydrogenburning via the hot CNO cycle, and therefore it is the sum of the threeelements' mass fractions, not the individual mass fractions themselves,that is important. Systems that exhibit superbursts are observed todiffer from those that do not exhibit superbursts in the nature of theirhelium-triggered type I X-ray bursts: the bursts have shorter durationsand much greater {α}-values. Increasing the CNO abundance of theaccreted material in our models reproduces both of these observations,thus once again suggesting enhanced CNO abundances in the mass donors.Many compact binary systems have been observed in which the abundancesof the accreting material are distinctly nonsolar. Although abundanceanalyses of the systems that exhibit superbursts currently do not exist,Bowen fluorescence blend profiles of 4U 1636-536 and Ser X-1 suggestthat the mass donor stars may indeed have nonsolar CNO metallicities.More detailed abundance analyses of the accreting matter in systems thatexhibit superbursts are needed to verify our assertion that the matteris rich in CNO elements.