Carbon Flashes in the Heavy-Element Ocean on Accreting Neutron Stars
Abstract
We show that the burning of a small mass fraction X12 of 12C in a neutron star ocean is thermally unstable at low accumulated masses when the ocean contains heavy ashes from the hydrogen-burning rapid proton process. The key to early unstable ignition is the decreased thermal conductivity of a heavy-element ocean. The instability requires accretion rates, M, in excess of 1/10 the Eddington limit when X12<0.1. Lower M's will stably burn a small mass fraction of 12C. The unstable flashes release ~1042-1043 ergs over hours to days and are likely the cause of the recently discovered large type I bursts (the so-called superbursts) from six Galactic low-mass X-ray binaries. In addition to explaining the energetics, recurrence times, and durations of the superbursts, these mixed 12C flashes also have an M dependence of unstable burning similar to that observed. Although the instability is present at accretion rates ~MEdd, the flashes provide less of a contrast with the accretion luminosity there, thus explaining why detection is easier when M~(0.1-0.3)MEdd.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 2001
- DOI:
- 10.1086/323937
- arXiv:
- arXiv:astro-ph/0107213
- Bibcode:
- 2001ApJ...559L.127C
- Keywords:
-
- Accretion;
- Accretion Disks;
- Nuclear Reactions;
- Nucleosynthesis;
- Abundances;
- Stars: Neutron;
- X-Rays: Bursts;
- Astrophysics
- E-Print:
- Submitted to Astrophysical Journal Letters (6 pages, 3 figures)