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

Neutron stars, with their strong surface gravity, have interestingly shorttimescales for the sedimentation of heavy elements. Motivated by observationsof Type I X-ray bursts from sources with extremely low persistent accretionluminosities, $L_X < 10^{36}\usp\ergspersecond (\simeq0.01\ensuremath{L_{\mathrm{Edd}}}$), we study how sedimentation affects thedistribution of isotopes and the ignition of H and He in the envelope of anaccreting neutron star. For local mass accretion rates $\mdot \lesssim10^{-2}\medd$ (for which the ignition of H is unstable), where $\medd =8.8\times 10^{4}\nsp\gpscps$, the helium and CNO elements sediment out of theaccreted fuel before reaching a temperature where H would ignite. Usingone-zone calculations of the thermonuclear burning, we find a range ofaccretion rates for which the unstable H ignition does not trigger unstable Heburning. This range depends on the emergent flux from reactions in the deepneutron star crust; for $F = 0.1\nsp\MeV(\dot{m}/\mb)$, the range is $3\times10^{-3}\medd\lesssim\mdot\lesssim 10^{-2}\medd$. We speculate that sourcesaccreting in this range will build up a massive He layer that later produces anenergetic 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, evenat accretion rates $\mdot \gtrsim 0.1\medd$, although the H and He do notcompletely segregate, the H abundance at the base of the accumulated layer isstill reduced. While following the evolution of the X-ray burst is beyond thescope of this introductory paper, we note that the reduced proton-to-seed ratiofavors the production of \iso{12}{C}--an important ingredient for subsequentsuperbursts.Comment: 15 pages, 14 figures, submitted to ApJ, revised versio