Hydrogen‐Accreting Carbon‐Oxygen White Dwarfs of Low Mass: Thermal and Chemical Behavior of Burning Shells

Abstract

Numerical experiments have been performed to investigate the thermal behavior of a cooled-down white dwarf of initial mass MWD=0.516 Msolar that accretes hydrogen-rich matter with Z=0.02 at the rate M=10-8 Msolar yr-1, typical for a recurrent hydrogen shell flash regime. The evolution of the main physical quantities of a model during a pulse cycle is examined in detail. From selected models in the mass range MWD=0.52-0.68 Msolar, we derive the borders in the MWD-M plane of the steady state accretion regime when hydrogen is burned at a constant rate as rapidly as it is accreted. The physical properties during a hydrogen shell flash in white dwarfs accreting hydrogen-rich matter with metallicities Z=0.001 and Z=0.0001 are also studied. For a fixed accretion rate, a decrease in the metallicity of the accreted matter leads to an increase in the thickness of the hydrogen-rich layer at outburst and a decrease in the hydrogen-burning shell efficiency. In the MWD-M plane, the borders of the steady state accretion band are critically dependent on the metallicity of the accreted matter: on decreasing the metallicity, the band is shifted to lower accretion rates and its width in M is reduced.