NOTES ON THE CONVECTION IN THE ATLAS9 MODEL ATMOSPHERES

The mixing-length theory for the convection, as it is used in the ATLA S9 code (Kurucz, 1993a), is summarized and discussed. We investigated the effect of the modification called ''approximate overshooting'' on the model structure of the Sun and of stars with T-eff included betwee n 4000 K and 8500 K, log g included between 2.5 and 4.5, and metallici ties [M/H] = 0.0 and [M/H] = -3.0. We found that the Kurucz solar mode l (SUNK94) with the ''overshooting'' option switched on reproduces mor e observations than that without ''overshooting''. In the H-gamma, and H-beta regions no solar model is able to reproduce the level of the t rue continuum deduced from high-resolution observations absolutely cal ibrated. At 486 nm the computed continuum is about 6.6% higher than th at inferred from the observed spectrum. We found that the largest effe ct of the ''approximate overshooting'' on the model structure occurs f or models with T-eff > 6250 K and it decreases with decreasing gravity . The differences in (b - y), (B - V), and (V - K) indices computed fr om models with the ''overshooting'' option switched on and off, corres pond to T-eff differences which may amount up to 180 K, 100 K, 60 K re spectively. The differences in T-eff from Balmer profiles may amount u p to 340 K and they occur also for T-eff < 6250 K down to about 5000 K . The c(1) index yields gravity . differences Delta log g as a functio n of log g which, for each T-eff, grow to a maximum value. The maximum a log g decreases with increasing temperatures and ranges, for solar metallicity, from 0.7 dex at log g = 0.5 and T-eff = 5500 K to 0.2 dex at log g = 4.5 and T-eff = 8000 K. This behaviour does not change for [M/H] = -3.0. Comparisons with the observations indicate that model p arameters derived with different methods are more consistent when the ''overshooting'' option is switched off (NOVER models), except for the Sun. In particular for Procyon, T-eff and log g from NOVER models are closer to the parameters derived from model independent methods than are T-eff and log g derived from the Kurucz (1995) grids. However, no model is able to explain the whole observed spectrum of either the Sun or Procyon with a unique T-eff, regardless of whether the ''overshoot ing'' option is switched on or off. Independently of the convection op tion, the largest differences in T-eff derived with different methods are of the order of 200 K for Procyon and 150 K for the Sun.