I. Yi et R. Narayan, LITHIUM PRODUCTION IN HOT ADVECTION-DOMINATED ACCRETION FLOWS IN SOFT-X-RAY TRANSIENTS, The Astrophysical journal, 486(1), 1997, pp. 363-371
High Li abundances have been reported in the late-type secondaries of
five soft X-ray transients (SXTs): V404 Cyg, A0620-00, GS 2000+25, Nov
a Mus 1991, and Cen X-4. Since Li is likely to be depleted in stars of
this type, the origin of the Li is puzzling. Li has not been seen in
similar secondaries of cataclysmic variables, which suggests that the
high Li abundance is not due to an anomalous suppression of Li depleti
on in close binaries. SXTs in the quiescent state have been modeled in
terms of hot advection-dominated accretion flows (ADAFs), in which th
e ions are essentially at virial temperature. At such temperatures, Li
production via a-a spallation is possible. We show that ADAFs can pro
duce sufficient Li via spallation to explain the observations in the b
lack hole SXTs V404 Cyg, A0620-00, GS 2000 + 25, and Nova Mus 1991. If
this explanation is correct, it is the first independent confirmation
that accretion flows in quiescent SXTs have a two-temperature plasma
with ion temperature much higher than electron temperature. Depending
on the Li depletion timescale in the secondary, which may range betwee
n 10(7)-10(9) yr, the model requires similar to 10(-4) to 10(-6) of th
e accreted mass to be intercepted by the secondary after undergoing Li
production and being ejected. In the case of the neutron star SXT, Ce
n X-4, we can explain the observed Li only if the mass-accretion rate
is similar to 10(-3) times the Eddington rate and if there is enhanced
ejection due to a propeller effect. We discuss possible observational
tests of this proposal. Li production during outbursts could be quite
important and may even dominate over the production during quiescence
, but the estimate of the Li yield is uncertain. We calculate the expe
cted luminosity in gamma-ray lines due to the production of excited Li
and Be nuclei, but conclude that the line cannot be detected with cur
rent instruments.