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L. A. Nelson (Bishop's University), S. A. Rappaport (M.I.T.)
We examine various evolutionary scenarios for the recently discovered class of accretion-powered binary millisecond pulsars and conclude that their present-day properties can be naturally explained by invoking the standard Roche-lobe overflow model. The actual properties of the donors (i.e., their metallicity, state of chemical evolution, and initial mass), and the exact mode of orbital angular momentum losses during the binary's evolution do not have to be individually fine tuned in order to reproduce the observed properties of these systems. However, the donor stars in at least two of these systems (XTE J0929-314 and XTE J1751-305) must have been sufficiently (chemically) evolved at the onset of mass transfer or have evolved significantly during the mass-transfer phase in order to ensure that the binaries could evolve to the measured ultrashort orbital periods (43.6 and 42.4 minutes, respectively). A probability analysis based on the measured mass functions reveals that the donors must be significantly hydrogen depleted, and that their masses are very likely to be less than 0.03 solar masses. Our best estimates for the donors' masses based on the evolutionary models suggest that they are likely to be between 0.015 to 0.023 solar masses, and that their surface hydrogen abundances (as a mass fraction) are almost certainly less than 10%. Our evolutionary models also indicate that the donors' radii should be at least 5% larger than the corresponding zero-temperature values and that they are much more likely to be a factor of 10 - 30% larger. The relationship between these systems and low-mass X-ray binaries is also explored.
This research was supported in part by NASA under ATP grants NAG5-7479 and NAG5-4057 and by the Natural Sciences and Engineering Research Council (NSERC) of Canada.
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.