Effective Gravity-Darkening of Close Binary Systems

Abstract

From a quantitative analysis of the observed ellipticity effect, empirical values of the exponent of effective gravity-darkening have been deduced for the components of close binary systems whose physical parameters are accurately known. The result of analysis indicate: (1) for the MS components of detached systems the empirical exponents are consistent with the existing theories of radiative and convective stellar atmospheres, (2) for the Roche-lobe filling secondary components of semi-detached systems, the empirically deduced effective gravity-darkening is seriously greater than the unity which suggests an excessive darkening due to the mass-loss, and (3) for contact systems, an excess of gravity-darkening, though lesser degree, can be also seen and the relationship between the exponent of effective gravity-darkening and the mass ratio is clearly different between W UMa systems and early-type contact ones, indicating different atmospheric conditions of common envelopes.