2006ApJ...642..443C

We have investigated the physical conditions under which accreting neutron stars in low-mass X-ray binaries can both produce and preserve sufficient quantities of carbon fuel to trigger superbursts. Our theoretical models span the plausible ranges of neutron star thermal conductivities, core neutrino emission mechanisms, and areal radii, as well as the CNO abundances in the accreted material. We find that neutron stars that accrete hydrogen-rich material with CNO mass fractions Z_CNO≲Z_CNO,☉will not exhibit superbursts under any circumstances. Neutron stars that accrete material with CNO mass fractions ≳4Z_CNO,☉will exhibit superbursts at accretion rates in the observed range. On this basis, we suggest that the mass donors of superburst systems must have enhanced CNO abundances. The accreted CNO acts only as a catalyst for hydrogen burning via the hot CNO cycle, and therefore it is the sum of the three elements' mass fractions, not the individual mass fractions themselves, that is important. Systems that exhibit superbursts are observed to differ from those that do not exhibit superbursts in the nature of their helium-triggered type I X-ray bursts: the bursts have shorter durations and much greater α-values. Increasing the CNO abundance of the accreted 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 abundances of the accreting material are distinctly nonsolar. Although abundance analyses of the systems that exhibit superbursts currently do not exist, Bowen fluorescence blend profiles of 4U 1636-536 and Ser X-1 suggest that the mass donor stars may indeed have nonsolar CNO metallicities. More detailed abundance analyses of the accreting matter in systems that exhibit superbursts are needed to verify our assertion that the matter is rich in CNO elements.