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Astron. Astrophys. 346, 556-563 (1999)

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The gravity-brightening effect and stellar atmospheres

II. Results for illuminated models with 3700 K [FORMULA] [FORMULA] [FORMULA] 7000 K

S.H.P. Alencar 1, L.P.R. Vaz 1 and Å. Nordlund 2

1 Departamento de Física, ICEX-UFMG, C.P. 702, 30.123-970 Belo Horizonte, MG - Brazil (silvia, lpv@fisica.ufmg.br)
2 Niels Bohr Institute for Astronomy, Physics and Geophysics; Astronomical Observatory, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark (aake@astro.ku.dk)

Received 23 September 1998 / Accepted 18 February 1999

Abstract

The influence of the so-called "reflection effect" (mutual illumination in a close binary) on the gravity-brightening exponent ([FORMULA]) 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 [FORMULA] [FORMULA] [FORMULA] [FORMULA] [FORMULA], 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 [FORMULA] 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 1997). The existence of a maximum in the [FORMULA] relation is due to the interplay between the convection and opacity properties of the models. The external illumination increases the values of [FORMULA], 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 [FORMULA] is close to unity. We provide a polynomial fit to the variation of [FORMULA] 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 [FORMULA] are thus partly prevented.

Key words: stars: atmospheres – stars: binaries: close – stars: binaries: eclipsing – stars: fundamental parameters – stars: general

Send offprint requests to: L.P.R. Vaz

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© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999
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