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Session 76 - Cataclysmic Variables and Accretion Disks.
Display session, Wednesday, January 15
High Li abundances have been reported in the late type secondaries of three soft X-ray transients (SXTs), V404 Cyg, A0620-00, 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 depletion in close binaries. SXTs in the quiescent state have been shown to contain hot advection-dominated accretion flows (ADAFs). The ion temperature in these flows is essentially virial and is high enough for Li production via \alpha-\alpha spallation. We show that quiescent SXTs can produce sufficient Li via spallation to explain the observations in V404 Cyg and A0620-00. Depending on the Li depletion time scale in the secondary, which may range between 10^7-10^9 yr, the model requires \sim1-0.01% of the accreted mass to be ejected isotropically after undergoing Li production. In the case of Cen X-4, we can explain the observed Li only if the mass accretion rate is \sim 10^-3 times the Eddington rate coupled with 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. However, the estimate of the Li yield is somewhat uncertain since it is not known exactly when during outburst the accretion occurs as an ADAF. We calculate the expected luminosity in gamma-ray lines due to the production of excited Li and Be nuclei, but conclude that the line cannot be detected with current instruments.
Program listing for Wednesday