On the Very Long Term Evolutionary Behavior of Hydrogen-accreting Low-Mass CO White Dwarfs

Hydrogen-rich matter has been added to a CO white dwarf of initial mass 0.516\msun at the rates $10^{-8}$ and $2\times 10^{-8}$ \msun \yrm1, and results arecompared with those for a white dwarf of the same initial mass which accretespure helium at the same rates. For the chosen accretion rates, hydrogen burnsin a series of recurrent mild flashes and the ashes of hydrogen burning buildup a helium layer at the base of which a He flash eventually occurs. Inprevious studies involving accretion at higher rates and including initiallymore massive WDs, the diffusion of energy inward from the H shell-flashingregion contributes to the increase in the temperature at the base of the heliumlayer, and the mass of the helium layer when the He flash begins issignificantly smaller than in a comparison model accreting pure helium; the Heshell flash is not strong enough to develop into a supernova explosion. Incontrast, for the conditions adopted here, the temperature at the base of theHe layer becomes gradually independent of the deposition of energy by H shellflashes, and the mass of the He layer when the He flash occurs is a functiononly of the accretion rate, independent of the hydrogen content of the accretedmatter. When the He flash takes place, due to the high degeneracy at the baseof the He layer, temperatures in the flashing zone will rise without acorresponding increase in pressure, nuclear burning will continue until nuclearstatistical equilibrium is achieved; the model will become a supernova, but notof the classical type Ia variety.Comment: 14 pages and 3 Postscript figures, Accepted for publication on ApJ Letter