8th HEAD Meeting, 8-11 September, 2004
Session 25 X-ray Binaries and White Dwarfs
Poster, Friday, September 10, 2004, 9:00am-10:00pm

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[25.14] Heavy Element Sedimentation and X-ray Bursts

F. Peng (University of Chicago), E.F. Brown (Michigan State University), J.W. Truran (University of Chicago)

There are a few X-ray bursts detected from sources with extremely low persistent luminosities, LX < 1036\,{\rm ergs\,s-1}. At such implied low-mass accretion rates (\dot{M} < 10-10\,{\rm M\odot\,yr-1}), the sedimentation velocity of heavier elements is comparable to the downward flow velocity in the accumulating atmosphere. Motivated by these observations, we study in detail how elemental sedimentation affects the distribution of isotopes in the atmosphere of an accreting neutron star and the ignition of H and He. We find that for \dot{M} < 2\times 10-10\,{\rm M\odot\,yr-1} (for which the ignition of H is unstable), the helium and CNO elements sediment and separate from hydrogen before reaching a temperature where H would ignite. Although the downward flow velocity is faster than the differential sedimentation velocity at higher accretion rates (for which the H burning is stable and the burst is triggered by He ignition), there can still be an effect: the H and He do not completely separate, but the H abundance at the base of the accumulated layer is reduced. This changes the proton-to-seed ratio for the rp-process. In the absence of mixing, the partial stratification will change the composition of the ashes and in particular enhance the abundance of {\rm 12 C}.

This work is supported by the Department of Energy under grant B341495 to the Center for Astrophysical Thermonuclear Flashes at the University of Chicago.

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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.