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[17.11] Constraints on Evolutionary Models of the Ultracompact Binary 4U~1820-30 from Type I X-Ray Bursts

A. Cumming (University of California, Santa Cruz)

Evolutionary models for ultracompact binaries such as 4U~1820-30 predict either a pure helium or hydrogen-poor (mass fraction of hydrogen X~0.1) secondary. For 4U~1820-30, I discuss the expected properties of Type I X-ray bursts in each case, and compare to observations. I show that the presence of hydrogen significantly affects the burst recurrence time when X>0.03, and CNO mass fraction >1/10 solar. The conclusions are sensitive to the accretion rate: the observed 2-4 hour recurrence times are reproduced for a pure helium companion if \dot M=8-11\times 10^-9\ M_\odot\ {\rm yr^-1}, or a hydrogen poor companion if \dot M=4-7\times 10^-9\ M_\odot\ {\rm yr^-1}. I compare this result to evolutionary models, and go on to discuss the expected carbon production and superburst properties in each case.

Bulletin of the American Astronomical Society, 35#2
© 2003. The American Astronomical Soceity.