The gravity-brightening effect and stellar atmospheres - II Results for illuminated models with 3 700 K < T-eff < 7 000 K

The influence of the so-called "reflection effect" (mutual illumination in a close binary) on the gravity-brightening exponent (beta) is studied using the UMA (Uppsala Model Atmosphere) code. The model is applied to convectiv e grey tin the sense of continuum-only-opacity) and non-grey (line-blankete d) atmospheres with 3 700K < T-eff, < 7000K. illuminated by grey and non-gr ey fluxes. The results for grey atmospheres illuminated by grey or non-grey fluxes are very similar. In this case beta mostly depends on the amount of incident energy and on the illumination direction. apart from the dependen ce on the effective temperature already discussed for nonilluminated models in a previous work (Alencar & Vat 1997. Thr existence of a maximum in the; 3(T-eff) relation is due to the interplay between the convection and opacit y properties of the models. The external illumination increases the values of 3, that is, the larger the amount of incident fur the larger the value o f the exponent. This effect is caused by the "quenching" of convection as t he external illumination heats the surface layers of the illuminated star, thus bringing it closer to radiative equilibrium, where beta is close to un ity. We provide a polynomial fit to the variation of 3 with the fundamental parameters, in order to make it possible to easily account for the effect in light curve synthesis programs. For line-blanketed illuminated atmospheres then is an additional dependence on the effective temperature of the incident Bull (the heating temperature ). This is related to the overall wavelength dependence of the spectral lin e opacity. particularly in the UV, the lint opacity is so strong that it pr events a significant amount of the incident flux from penetrating to the co ntinuum formation layers. The quenching of convection by the external illum ination and the related increase of 3 are thus partly prevented.