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

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

Jun 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999a%26a...346..556a&link_type=abstract

Astronomy and Astrophysics, v.346, p.556-563 (1999)

Astronomy and Astrophysics

Astrophysics

9

Stars: Atmospheres, Stars: Binaries: Close, Stars: Binaries: Eclipsing, Stars: Fundamental Parameters, Stars: General

Scientific paper

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 convective grey (in the sense of continuum-only-opacity) and non-grey (line-blanketed) atmospheres with 3 700 K < T_eff < 7 000 K, illuminated by grey and non-grey 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 dependence on the effective temperature already discussed for non-illuminated models in a previous work (Alencar & Vaz \cite{paper1}). The existence of a maximum in the beta (T_eff) relation is due to the interplay between the convection and opacity properties of the models. The external illumination increases the values of beta , that is, the larger the amount of incident flux the larger the value of the exponent. This effect is caused by the ``quenching'' of convection as the external illumination heats the surface layers of the illuminated star, thus bringing it closer to radiative equilibrium, where beta is close to unity. We provide a polynomial fit to the variation of beta 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 there is an additional dependence on the effective temperature of the incident flux (the heating temperature). This is related to the overall wavelength dependence of the spectral line opacity. Particularly in the UV, the line opacity is so strong that it prevents a significant amount of the incident flux from penetrating to the continuum formation layers. The quenching of convection by the external illumination and the related increase of beta are thus partly prevented.

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