Hydrogen-accreting carbon-oxygen white dwarfs of low mass:: Thermal and chemical behavior of burning shells

Numerical experiments have been performed to investigate the thermal behavior of a cooled-down white dwarf of initial mass M-WD = 0.516 M. that accretes hydrogen-rich matter with Z = 0.02 at the rate (M) over dot = 10(-8) M. 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 M-WD = 0.52-0.68 M., we derive the borders in the M-WD-(M) over dot 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 M-WD-(M) over dot 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) over dot is reduced.