Modelling The Irradiated Upper Atmospheres Of Unevolved Companions In Precataclysmic Binaries (Pcb): Evidences On Thermal Instability

Abstract

We report on our results of modelling the physical processes in the uppermost layers of strongly irradiated atmospheres of low mass unevolved companions in Precataclysmic Binaries (PCBs). Reprocessing of Lyman continuum L c radiation from the hot subdwarf (sdw) primary is studied in detail. We solve explicitly a set of equations of hydrostatic, ionization and thermal equilibrium to calculate the intensity of the reprocessed emergent radiation in recombinations for an optically thin plasma. We consider contributions from free-free and bound-free transitions. Diffuse radiation is taken into account, but the effects of self-absorption of the re-emitted radiation are neglected. For a purely hydrogenic atmosphere and typical values of incident fluxes, densities, gas pressures for the irradiated upper atmospheric layers, we find that the L c radiation will be absorbed and re-emitted in recombinations within a column of effective thickness (106–108) cm of HII, depending on the electron density, the effective temperature of the sdw and the separation between the components. One of the non-trivial results of our model computations lies in the overheating of the uppermost layers of the irradiated atmosphere: the equilibrium temperature of the gas turns out to be considerably higher than the blackbody temperature following from the diluted incoming radiation of sdw. Energy balance considerations reveal that roughly 25–50% of the L c flux is expended on the ionization of H and is re-emitted subsequently in recombinations. The remaining portion of the L c flux is spent on readjustment of the uppermost layers of the irradiated atmosphere. One of the consequences is the onset of thermal instability in these layers.