Surface hydrogen-burning modeling of supersoft X-ray binaries: Are they type Ia supernova progenitors?

Nova explosions occur on the white dwarf (WD) component of a cataclysmic variable stellar system that is accreting matter lost by a companion. A Type Ia supernova (SN Ia) explosion is thought to result when a WD, in a similar binary configuration, grows in mass to the Chandrasekhar limit. Here, we present calculations of accretion of solar matter, at a variety of mass accretion rates, onto hot (2.3 × 10⁵ K), luminous (30 L_⊙), massive (1.25, 1.35 M_⊙) carbon-oxygen WDs. In contrast to our nova simulations, where the WD has a low initial luminosity and a thermonuclear runaway (TNR) occurs and ejects material, these simulations do not eject material (or only a small fraction of the accreted material), and the WD grows in mass. A hydrogen TNR does not occur because hydrogen fuses to helium in the surface layers, and we call this process surface hydrogen burning (SHB). As the helium layer grows in mass, it gradually fuses either to carbon and oxygen or to more massive nuclei, depending on the WD mass and mass accretion rate. If such a WD were to explode in a SN Ia event, therefore, it would show neither hydrogen nor helium in its spectrum as is observed. Moreover, the luminosities and effective temperatures of our simulations agree with the observations of some of the supersoft X-ray binary sources, and therefore, our results strengthen previous speculation that some of them (CAL 83 and CAL 87, for example) are probably progenitors of SN Ia explosions. Finally, we have achieved SHB for values of the mass accretion rate that almost span the observed values of the cataclysmic variables.