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 \mathrm { { } ^ { 12 } C } + \mathrm { { } ^ { 12 } C } 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 \mathrm { { } ^ { 12 } C } + \mathrm { { } ^ { 12 } C } triggers superbursts .
We then examine the \mathrm { { } ^ { 12 } C } + \mathrm { { } ^ { 12 } C } fusion rate .
The reaction cross-section is experimentally unknown at astrophysically relevant energies , but resonances exist in the \mathrm { { } ^ { 12 } C } + \mathrm { { } ^ { 12 } C } 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 E ^ { \mathrm { pk } } \approx 1.5 \mathrm { M } \mathrm { eV } .
For such a hypothetical 1.5 \mathrm { M } \mathrm { eV } resonance , we derive both a fiducial value and upper limit to the resonance strength ( \omega \gamma ) _ { \mathrm { 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 \mathrm { { } ^ { 12 } C } ignition at a column depth 10 ^ { 12 } \mathrm { g } { \mathrm { c } \mathrm { m } } ^ { -2 } from 6 \times 10 ^ { 8 } \mathrm { K } to 5 \times 10 ^ { 8 } \mathrm { K } .
A resonance at 1.5 \mathrm { M } \mathrm { eV } would not strongly affect the ignition density of Type Ia supernovae , but it would lower the temperature at which \mathrm { { } ^ { 12 } C } ignites in massive post–main-sequence stars .
Determining the existence of a strong resonance in the Gamow window requires measurements of the \mathrm { { } ^ { 12 } C } + \mathrm { { } ^ { 12 } C } cross-section down to a center-of-mass energy near 1.5 \mathrm { M } \mathrm { eV } , which is within reach of the proposed DUSEL facility .