We show that burning of a small mass fraction of carbon in a neutron starocean is thermally unstable at low accumulated masses when the ocean containsheavy ashes from the hydrogen burning rapid proton (rp) process. The key toearly unstable ignition is the low thermal conductivity of a heavy elementocean. The instability requires accretion rates in excess of one-tenth theEddington limit when the carbon mass fraction is 0.1 or less. The unstableflashes release 10^{42} to 10^{43} ergs over hours to days, and are likely thecause of the recently discovered large Type I X-ray bursts (so-called``superbursts'') from six Galactic low mass X-ray binaries. In addition toexplaining the energetics, recurrence times, and durations of the superbursts,these mixed carbon/heavy element flashes have an accretion rate dependence ofunstable burning similar to that observed. Though the instability is present ataccretion rates near Eddington, there is less contrast with the accretionluminosity there, explaining why most detections are made at accretion ratesbetween 0.1 and 0.3 Eddington. Future comparisons of time dependentcalculations with observations will provide new insights into the rp process.
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