Testing convection theories using Balmer line profiles of A , F , and G stars ?

We consider the effects of convection on the Balmer line profiles (Hα and Hβ) of A, F, and G stars. The standard mixing-length theory (MLT) atlas9 models of Kurucz (1993), with and without overshooting, are compared toatlas9models based on the turbulent convection theory proposed by Canuto & Mazzitelli (1991, 1992) and implemented by Kupka (1996), and the improved version of this model proposed by Canuto et al. (1996) also implemented by Kupka. The Balmer line profiles are a useful tool in investigating convection because they are very sensitive to the parameters of convection used in the stellar atmosphere codes. The Hα and Hβ lines are formed at different depths in the atmosphere. The Hα line is formed just above the convection zone. The Hβ line, however, is partially formed inside the convection zone. We have calculated the Teff of observed stars by fitting Balmer line profiles to synthetic spectra and compared this to: (i) the Teff of the fundamental stars; (ii) the Teff of stars determined by the Infra-Red Flux Method and (iii) the Teff determined by Geneva photometry for the stars in the Hyades cluster. We find that the results from the Hα and Hβ lines are different, as expected, due to the differing levels of formation. The tests are inconclusive between three of the four models; MLT with no overshooting, CM and CGM models, which all give results in reasonable agreement with fundamental values. The results indicate that for the MLT theory with no overshooting it is necessary to set the mixing length parameter α equal to 0.5 for stars with Teff ≤ 6000 K or Teff ≥ 7000 K. However for stars with 6000 K≤ Teff ≤ 7000 K the required value for the parameter is α ≥ 1.25. Models with overshooting are found to be clearly discrepant, consistent with the results with uvby photometry by Smalley & Kupka (1997).