Fusion Rate and Superburst Ignition

Observationally inferred superburst ignition depths are shallower than models predict. We address this discrepancy by reexamining the superburst trigger mechanism. We first explore the hypothesis of Kuulkers et al. that exothermic electron captures trigger superbursts. We find that all electron capture reactions are thermally stable in accreting neutron star oceans and thus are not a viable trigger mechanism. Fusion reactions other than 12C + 12C are infeasible as well since the possible reactants either deplete at much shallower depths or have prohibitively large Coulomb barriers. Thus we confirm the proposal of Cumming & Bildsten and Strohmayer & Brown that 12C + 12C triggers superbursts. We then examine the 12C + 12C fusion rate. The reaction crosssection is experimentally unknown at astrophysically relevant energies, but resonances exist in the 12C + 12C system throughout the entire measured energy range. Thus it is likely, and in fact has been predicted, that a resonance exists near the Gamow peak energy Epk ≈ 1.5 MeV. For such a hypothetical 1.5 MeV resonance, we derive both a fiducial value and upper limit to the resonance strength (ωγ)R and find that such a resonance could decrease the theoretically predicted superburst ignition depth by up to a factor of 4; in this case, observationally inferred superburst ignition depths would accord with model predictions for a range of plausible neutron star parameters. Said differently, such a resonance would decrease the temperature required for unstable 12C ignition at a column depth 1012 gcm−2 from 6×108 K to 5×108 K. A resonance at 1.5 MeV would not strongly affect the ignition density of Type Ia supernovae, but it would lower the temperature at which 12C ignites in massive post–main-sequence stars. Determining the existence of a strong resonance in the Gamow window requires measurements of the 12C + 12C cross-section down to a center-of-mass energy near 1.5 MeV, which is within reach of the proposed DUSEL facility. Subject headings: nuclear reactions, nucleosynthesis, abundances — stars: neutron — X-rays: bursts