THE ROLE OF CONVECTION ON THE UVBY COLORS OF A-STARS, F-STARS, AND G-STARS

We discuss the effects of convection on the theoretical uvby colours o f A, F, and G stars. The standard mixing-length theory ATLAS9 models o f Kurucz (1993), with and without approximate overshooting, are compar ed to models using the turbulent convection theory proposed by Canuto & Mazzitelli (1991, 1992) and implemented by Kupka (1996a). Comparison with fundamental T-eff and log g stars reveals that the Canuto & Mazz itelli models give results that are generally superior to standard mix ing-length theory (MLT) without convective overshooting. MLT models wi th overshooting are found to be clearly discrepant. This is supported by comparisons of non-fundamental stars, with T-eff obtained from the Infrared Flux Method and log g from stellar evolutionary models for op en cluster stars. The Canuto & Mazzitelli theory gives values of (b - y)(0) and c(0) that are in best overall agreement with observations. I nvestigations of the m(0) index reveal that all of the treatments of c onvection presented here give values that are significantly discrepant for models with T-eff < 6000 K. It is unclear as to whether this is d ue to problems with the treatment of convection, missing opacity, or s ome other reason. None of the models give totally satisfactory m(0) in dices for hotter stars, but the Canuto & Mazzitelli models are in clos est overall agreement above 7000 K. Grids of uvby colours, based on th e CM treatment of convection, are presented. These grids represent an improvement over the colours obtained from models using the mixing-len gth theory. The agreement with fundamental stars enables the colours t o be used directly without the need for semi-empirical adjustments tha t were necessary with the earlier colour grids.