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Astron. Astrophys. 328, 349-360 (1997)

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1. Introduction

The gross properties of a star, such as broad-band colours and flux distributions are significantly influenced by the microscopic effects of convection in stars later than mid A-type. Consequently, our treatment of convection in stellar atmosphere models can significantly alter our interpretation of observed phenomena. The Kurucz (1979a) ATLAS6 model atmospheres have generally had considerable success in the interpretation of stellar fluxes and spectra of O, B, A, F, and G stars. However, small systematic errors were found in the colours calculated for late-A and F stars. Relyea & Kurucz (1978) discussed several possible reasons for discrepancies between theoretical and observational Strömgren (1963, 1966) uvby colours, including the effects of missing opacity and convection.

Model atmosphere fluxes enable us to calculate uvby colours by using suitable filter passbands. Details of this procedure can be found in Relyea & Kurucz (1978). In all three cases of models considered here, the colours have been calculated with the routines used by Kurucz (1993). All the model grids have been normalized to agree for Vega ([FORMULA] Lyr), the usual procedure in normalizing theoretical colours. The normalization was the same as that given in Kurucz (1993), with Vega represented by an ATLAS9 model with [FORMULA] = 9550 K, [FORMULA] = 3.95, [M/H] = -0.5 and a microturbulence of 2 km s-1 (Castelli & Kurucz 1994). In this way, the grids used here all agree at one point. This does not in any way bias the convection study, since the atmosphere of Vega can be considered as totally radiative and thus unaffected by changes in the treatments of convection. Though the Schwarzschild stability criterion predicts an instability of the atmospheric stratification against convection within a small layer (where the ionization of hydrogen takes place), all convection models investigated in this paper give convective fluxes for Vega which are several orders of magnitudes less than the usual deviations from flux constancy obtained for "converged" ATLAS model atmospheres. Consequently, an entirely radiative ATLAS model atmosphere of Vega cannot be distinguished from its convective counterparts in terms of observed fluxes and colours. Thus, our normalization to Vega allows us to compare only the differences due to treatment of convection.

In this paper we present a discussion of the effects of different treatments of convection on the uvby colours. The effects of mixing-length theory, with and without approximate overshooting, on [FORMULA] and [FORMULA] were discussed in detail by Castelli et al. (1997). In this paper we concentrate on the comparison between mixing-length theory and the turbulent convection theory of Canuto & Mazzitelli (1991, 1992).

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998