Luminous Supersoft X-Ray Sources

We discuss possible evolution channels that lead to the formation of luminous supersoft X-ray sources, subclasses of which may be progenitors of type Ia supernovae. We carry out full evolution calculations from the zero-age main sequence to the supersoft source. A novel feature of our calculations is the inclusion of thermohaline mixing after mass transfer during binary evolution. The main effect of this is to produce secondaries of non-solar composition. Candidate initial progenitors are intermediate-mass donors of about 7 Mwith companions in the range 1.5−3.0 M� . We concentrate on early case-C evolution, which means that the primary fills its Roche lobe when it ascends the Early Asymptotic Giant Branch while its core is highly evolved and massive enough to form a CO white dwarf. A crucial role, established by observations in this part of HR diagram, is played by mass loss in winds and we treat winds with a new approach. Since common-envelope evolution (CE) is generally invoked to explain the formation of close binaries with one or two degenerate components, we assume that the progenitors undergo severe mass and angular momentum loss through such a phase. We further study how the configurations of the post- CE systems, composed of a massive white dwarf and a 1.5−3.0 Mcompanion, depend on the parameters of CE-evolution and mass-loss rates in various phases of evolution. Under these general assumptions a new path for the formation of SSSs is found which differs from that of the, usually assumed, solar composition donors. Our results may explain supersoft systems with enhanced helium abundances such as U Sco and very luminous extragalactic supersoft sources such as CAL 83 in the LMC and possibly the CHANDRA source (N1) in M 81.

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