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

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7. Conclusion and future work

A discussion of the effects of the treatment of convection on the theoretical uvby colours of A, F, and G stars has been presented. The standard mixing-length theory ATLAS9 models of Kurucz (1993), with and without approximate overshooting, were compared to models using the turbulent convection theory proposed by Canuto & Mazzitelli (1991, 1992).

Comparison with fundamental [FORMULA] and [FORMULA] stars reveals that the CM models yield results that are generally superior to standard mixing-length theory without convective overshooting (MLT noOV). Models with overshooting (MLT OV) are found to be clearly discrepant. This is supported by comparisons to non-fundamental stars with [FORMULA] obtained from the Infrared Flux Method and [FORMULA] from stellar evolutionary models for open cluster stars.

Investigations of the [FORMULA] index have revealed that all three treatments of convection produce values that are significantly discrepant for models with [FORMULA] [FORMULA] 6000 K. It is unclear as to whether this is due to problems with the treatment of convection, missing atomic or molecular opacity, or due to some other reason. None of the models give totally satisfactory [FORMULA] indices for hotter stars, but the CM models are in good agreement above 7000 K.

Several models with a local treatment of convection had been implemented and tested before the colour grids discussed here were computed (among them the CM model and the CGM model of Canuto et al. 1996). They revealed rather small differences in observed fluxes and colours for early G stars. If the discrepancies between observations and model predictions for the [FORMULA] index of stars with [FORMULA] [FORMULA]  K are indeed mainly induced by convection, only a fully non-local model of convection can be expected to provide a sufficiently large quantitative jump to bring both into better agreement. A non-local model based on a one-point Reynolds stress closure was already derived in Canuto (1993) who also gives many references on earlier work on this field. An improved version of this model has recently passed a severe test provided by helioseismology which requires the size of the under-shooting below the solar convection zone to be [FORMULA] 0.05 [FORMULA] (see Basu 1997; Basu & Antia 1997).

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998