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*Astron. Astrophys. 328, 349-360 (1997)*
## 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 ( 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 = 9550 K,
= 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
and 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).
© European Southern Observatory (ESO) 1997
Online publication: March 24, 1998
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