HYDROGEN AND HELIUM SHELL FLASHES ON MASSIVE ACCRETING WHITE-DWARFS

A semianalytical model of double shell thermonuclear burning on accreting white dwarfs has been developed. A plane-parallel geometry is assumed and the white dwarf core is supposed to be in a steady state, as time scales for external evolution are much shorter than those corresponding to the interior. Direct as well as indirect accretion of helium are modelled. In the last case, it is the burning of hydrogen directly accreted which produces the helium finally incorporated onto the white dwarf. The simplifications inherent to this model allow the study of a large set of situations, excluding those that lead to strong flashes with previsible mass loss. Simultaneous treatment of hydrogen and helium flashes, when hydrogen-rich matter is transferred at nonproblematic rates, shows the interaction between both shells. Some properties, as recurrence periods and violence of outbursts, are analysed. In particular, average properties of burning shells along many cycles are obtained for different sets of model parameters. These average values of some physical magnitudes might be used as boundary conditions for the study of internal evolution. Their influence in this evolution deserves further attention, since it probably changes some of the conclusions relative to the fate of accreting white dwarfs.