The role of convection on the uvby colours of A, F, and G stars ?

We discuss the effects of convection on the theoret- ical uvby colours of A, F, and G stars. The standard mixing- length theory atlas9 models of Kurucz (1993), with and with- out approximate overshooting, are compared to models using the turbulent convection theory proposed by Canuto & Mazz- itelli (1991, 1992) and implemented by Kupka (1996a). Comparison with fundamental Teff and logg stars reveals that the Canuto & Mazzitelli models give results that are gen- erally superior to standard mixing-length theory (MLT) with- out convective overshooting. MLT models with overshooting are found to be clearly discrepant. This is supported by com- parisons of non-fundamental stars, with Teff obtained from the Infrared Flux Method and logg from stellar evolutionary mod- els for open cluster stars. The Canuto & Mazzitelli theory gives values of (b y)0 and c0 that are in best overall agreement with observations. Investigations of the m0 index reveal that all of the treat- ments of convection presented here give values that are signif- icantly discrepant for models with Teff < 6000 K. It is unclear as to whether this is due to problems with the treatment of con- vection, missing opacity, or some other reason. None of the models give totally satisfactory m0 indices for hotter stars, but theCanuto&Mazzitellimodelsareinclosestoverallagreement above 7000 K. Gridsofuvbycolours,basedontheCMtreatmentofconvec- tion, are presented. These grids represent an improvement over the colours obtained from models using the mixing-length the- ory. The agreement with fundamental stars enables the colours to be used directly without the need for semi-empirical adjust- ments that were necessary with the earlier colour grids.