Sedimentation and Type I X‐Ray Bursts at Low Accretion Rates

Neutron stars, with their strong surface gravity, have interestingly short timescales for the sedimentation of heavy elements. Motivated by observations of Type I X-ray bursts from sources with extremely low persistent accretion luminosities, L X < 10 36 ergs s -1 (≃0.01 L Edd ), we study how sedimentation affects the distribution of isotopes and the ignition of H and He in the envelope of an accreting neutron star. For local mass accretion rates ≲ 10 -2 Edd (for which the ignition of H is unstable), where Edd = 8.8 × 10 4 g cm -2 s -1 , the helium and CNO elements sediment out of the accreted fuel before reaching a temperature at which H would ignite. Using one-zone calculations of the thermonuclear burning, we find a range of accretion rates for which the unstable H ignition does not trigger unstable He burning. This range depends on the emergent flux from reactions in the deep neutron star crust; for F = (0.1 MeV) ( / m u ), the range is 3 × 10 -3 Edd ≲ ≲ 10 -2 Edd . We speculate that sources accreting in this range would build up a massive He layer that would later produce an energetic and long X-ray burst. At mass accretion rates lower than this range, we find that the H flash leads to a strong mixed H/He flash. Surprisingly, even at accretion rates ≳ 0.1 Edd , although the H and He do not completely segregate, the H abundance at the base of the accumulated layer is still reduced. While following the evolution of the X-ray burst is beyond the scope of this introductory paper, we note that the reduced proton-to-seed ratio favors the production of 12 C—an important ingredient for subsequent superbursts.