Modeling the Irradiated Atmospheres of Unevolved Companions in Precataclysmic Binaries (PCB)

Abstract

We report results of modeling physical processes in the uppermost layers of strongly irradiated atmospheres of low mass companions in PCBs where L c radiation coming from the hot subdwarf (sdw) primary is reprocessed. PCB-s are a small group of detached close binary systems, which are observed at centres of PN, consisting of a hot white dwarf or a sdw accompanied by an essentially unevolved low mass cool companion M 2 ≤ lM⊙(see, for instance, Bond et al 1991). We have re-evaluated the extant observational methods of analysis applied to PCB-s (see Pustynski, Pustylnik 2000, Pustylnik, Pustynski, Kubat 2001 (PPK 2001)). We used the approximate model of the uppermost atmosphere where the incident L c flux is fully reprocessed. Since the effective temperatures of the primary correspond to absolutely black body temperatures of some tens thousands Kelvins, a significant portion of the incoming flux energy is concentrated in L c. We considered the sdw to radiate like absolutely black body with temperature T le ff, both stars supposed to have spherical shapes. We adopted “on the spot” approximation for model atmospheres (Osterbrock 1974). We elaborated a theoretical model of the upper atmosphere of the secondary (for more details see (PPK 2001)). We suppose upper non-LTE layers to be purely hydrogenic and solve the system of three equations: the equations of ionization balance, of thermal and of hydrostatic equilibrium to find the equilibrium temperature T eq(R).