Double-degenerate semidetached binaries with helium secondaries: cataclysmic variables, supersoft X-ray sources, supernovae and accretion-induced collapses

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Abstract
We model the galactic population of ultrashort-period binaries with helium white dwarf (WD) donors and carbon-oxygen WD accretors. Their total number is ∼ 108. Our model suggests that ∼ 10 systems of this kind, with V≤ 13.0 mag, 6≾Porb≾50 min and 0.03 ≾M2/M⊙≾ 0.15, may be observed. The rate of accretion-induced collapses in all systems with helium donors is a strong function of the efficiency of the orbital energy dissipation in the common envelope stages preceding their formation, and may be as high as ∼ 10−5 yr−1. The total galactic rate of supernova-scale events induced by helium shell flashes on accreting white dwarfs in binary systems with degenerate and non-degenerate helium donors may be as high as ∼ 5 ×10−3 yr−1 comparable to the estimates of the galactic rate of type Ia supernovae. We estimate that among semidetached double-degenerate systems with helium donors up to ∼ 100 may be luminous supersoft X-ray sources which arise as a result of stationary helium burning at the surface of an accreting dwarf and/or of the presence of compact hot discs. Typical orbital periods of these systems are ∼ 2 –4 min.

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