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Astron. Astrophys. 326, 257-262 (1997)

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4. Conclusions

The gravity-brightening law, Eq. (1), is valid to a high degree for all models generated in this work with a modified version of the UMA code (plane-parallel atmosphere model in both hydrodynamic and local thermodynamic equilibrium). The [FORMULA] exponents for convective atmospheres were calculated theoretically by forcing the entropy at the bottom of the atmosphere to be equal in models that should represent the same star. Our model reproduces the mean result of Lucy (1967) for the convective regime.

We extended the result of Lucy (1967, for three values of [FORMULA]) to the range [FORMULA] [FORMULA] [FORMULA] [FORMULA] [FORMULA] and found that [FORMULA] practically only depends on [FORMULA], being rather insensitive to the value of the mixing length parameter, to the use of grey or non-grey atmospheres, and to the model total mass. A third order polynomial, [FORMULA], was given as a convenient way to use the results in the analysis of EBLC.

The influence of the "reflection effect" on the gravity-brightening exponent for convective atmospheres is evidenced for the first time: the external illumination increases the values of [FORMULA] significantly, indicating that in close systems, where the reflection effect is prominent, the standard [FORMULA] values might be too low. A detailed analysis including illuminated models and an extension of the present work to higher effective temperatures are in progress.

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© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998