Notes on the convection in the ATLAS9 model atmospheres.

The mixing-length theory for the convection, as it is used in the ATLAS9 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 between 4000K and 8500K, logg included between 2.5 and 4.5, and metallicities [M/H]=0.0 and [M/H]=-3.0. We found that the Kurucz solar model (SUNK94) with the ``overshooting'' option switched on reproduces more observations than that without ``overshooting''. In the H<sub>gamma</sub>_ and H<sub>beta</sub>_ regions no solar model is able to reproduce the level of the true continuum deduced from high-resolution observations absolutely calibrated. At 486 nm the computed continuum is about 6.6% higher than that inferred from the observed spectrum. We found that the largest effect of the ``approximate overshooting'' on the model structure occurs for models with T_eff_>6250K and it decreases with decreasing gravity. The differences in (b-y), (B-V), and (V-K) indices computed from models with the ``overshooting'' option switched on and off, correspond to T_eff_ differences which may amount up to 180K, 100K, 60K respectively. The differences in T_eff_ from Balmer profiles may amount up to 340K and they occur also for T_eff_<6250K down to about 5000K. The c_1_ index yields gravity differences {DELTA}logg as a function of logg which, for each T_eff_, grow to a maximum value. The maximum {DELTA}logg decreases with increasing temperatures and ranges, for solar metallicity, from 0.7 dex at logg=0.5 and T_eff_=5500K to 0.2dex at logg=4.5 and T_eff_=8000K. This behaviour does not change for [M/H]=-3.0. Comparisons with the observations indicate that model parameters 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 logg from NOVER models are closer to the parameters derived from model independent methods than are T_eff_ and logg 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 ``overshooting'' option is switched on or off. Independently of the convection option, the largest differences in T_eff_ derived with different methods are of the order of 200K for Procyon and 150K for the Sun.