HELIUM-ACCRETING DEGENERATE DWARFS AS PRESUPERNOVAE AND SCENARIOS FOR THE ULTRASOFT X-RAY SOURCES

Binaries with initial component masses in the range 7-10 M. can evolve into systems consisting of a massive degenerate dwarf which is accreting pure helium at the rate of approximately 10(-6) to 10(-5) M. yr-1 from a massive companion, whose precursor first fills its Roche lobe as a giant with a deep convective envelope. The mass-exchange rate in these ''helium Algols'' is precisely in the interval which allows the degenerate dwarf to burn the accreted helium in a quasi-stationary way. Thus, some massive dwarfs have a realistic possibility of reaching the Chandrasekhar limit and exploding as a supernova. In our Galaxy, the frequency of such supernovae may be approximately 1.7 x 10(-3) yr-1. During the phase of quasi-stationary helium shell burning, helium-accreting dwarfs are very bright (L congruent-to 10(37)-10(38) ergs s-1), stationary or flaring ultrasoft X-ray sources like those discovered recently in galaxies of the Local Group. Cataclysmic variable-like (CV-like) systems in which hydrogen-rich matter is transferred on the thermal timescale of the envelope of an evolved main-sequence donor can also contribute to the population of ultrasoft X-ray sources. We discuss the properties of such systems and conclude that further observational studies are needed to determine the relative contribution of helium Algols and of CV-like systems to the family of ultrasoft X-ray binaries.