THE EFFECT OF IRRADIATION IN ALGOL-TYPE BINARIES - METALLICITY AND POSITION IN THE HR DIAGRAM

We present a study of the most relevant effects of irradiation in the secondary components of Algol-type binaries. We have used the so-called Uppsala Model Atmosphere (UMA) code to simulate the cool secondary stars while the external fluxes of the hotter primaries were computed using the models by Kurucz interpolated for the characteristic wavelengths of the UMA program. Theoretical bolometric albedos have been found to be in good agreement with observations. A comparison between the spectral energy distribution of model atmospheres with and without irradiation, for the same T(eff) and log g, denotes clear differences mainly in the depth of the absorption lines. Furthermore, we have found that the computed irradiated spectra are very similar, in some spectral regions, to the equivalent model atmosphere without irradiation but a lower metallicity. This means that, if observed atmospheric distributions of flux in close binaries are not conveniently corrected for the effect of irradiation, systematic errors may be introduced in their interpretation, in particular with respect to the metal content and effective temperatures of the secondaries. On the other hand, irradiation produces an increase in log - T(eff) due to the heating of the upper layers. We argue that this shift could be responsible, at least partly, for the observed over-luminosity of the secondaries of classical Algols.