Binary degenerate dwarfs : theory andobservationsBy

Systems containing two white dwarfs can descend in less than a Hubble time from wide or close binary stars with initial masses of components exceeding 0:8 M. The degenerate components may be composed of helium, of mixtures of carbon and oxygen, or of mixtures of oxygen and neon (with traces of sodium and magnesium). They may be detached or semidetached. We present a numerical model of this population in which the total Galactic formation rate of binary degenerate dwarfs is 0:1 yr ?1 in close binaries and 0:006 yr ?1 in wide ones. The estimate of the Galactic birthrate of binaries containing a neutron star or a black hole accompanied by a white dwarf is 0:001 yr ?1. The main model parameters of close binary degenerates are in reasonable agreement with the parameters of their currently observed counterparts. Mergers of components of double degenerates under the innuence of angular momentum loss via gravitational wave radiation can result in type Ia supernova explosions (0:003 yr ?1) or in the formation of semidetached binaries with degenerate helium donors (0:01 yr ?1), some of which may perhaps manifest themselves as AM CVn stars, helium nondegenerate subdwarfs (0:014 yr ?1), and hydrogen-deecient giants (0:001 yr ?1).