Hydrogen-accreting carbon-oxygen white dwarfs

Matter of solar system composition has been added to the surfaces of two initially cool carbon-oxygen (GO) white dwarfs of masses 0.5 M-. and 0.8 M-. at rates in the range 10(-8) to 10(-6) M-. yr(-1). Four different regimes are encountered. (1) At the highest accretion rates, models become red giants after the accretion of only a very small amount of mass. As the accretion rate is decreased, models are encountered that (2) burn hydrogen at the same rate at which it is accreted, (3) experience a series of nondynamical hydrogen shell flashes followed eventually by a powerful helium shell flash, and, finally, (4) experience nova-like hydrogen shell flashes. Although all of the regimes have been explored, special attention has been given to models that experience recurrent mild hydrogen-burning pulses or burn hydrogen at a stationary rate. For lower accretion rates, the helium flash is so powerful that the convective layer forced by helium burning penetrates deeply into the hydrogen-rich envelope; this penetration may lead to the ejection of external layers even if the helium flash would not of itself have become dynamical. For higher accretion rates, even when convection does not penetrate into hydrogen-rich layers, the helium layer expands, and much, if not most, of the accreted matter is lost during the event because of the interaction of the expanded envelope with the companion star. Analysis of the results suggests that it is unlikely that, in the real world, a hydrogen-accreting CO white dwarf with a typical initial mass will attain the Chandrasekhar mass. Dynamical helium-burning flashes are probable.