THE GRAVITY-BRIGHTENING EFFECT AND STELLAR ATMOSPHERES .1. RESULTS FOR MODELS WITH 3700K-LESS-THAN-T-EFF-LESS-THAN-7000K

The gravity-brightening exponent (beta) is studied using the UMA (Upps ala Model Atmosphere) code in nonilluminated convective grey (in the s ense of ''continuum only'' opacity) and non-grey (line blanketed) atmo spheres, for 3700 K < T-eff < 7000 K. The value beta=0.32 proposed by Lucy was confirmed only for T-eff similar to 6500 K. beta depends upon T-eff (an analytical expression is given), being rather insensitive t o variations of the mixing length parameter, of the stellar mass and t o the use of grey or non-grey atmospheres. A comparison with empirical determinations and a preliminary test of the use of the proposed beta values in eclipsing binary light curve analysis are presented. A depe ndence of beta on external illumination (''reflection effect'') for co nvective models is evidenced for the first time, the larger the amount of incident flux the larger the beta exponent.