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 starswithTeff included between 4000Kand 8500K, log g 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 Hγ and Hβ 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 Teff>6250 K 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 Teff differences whichmay amount up to 180K, 100K, 60K respectively. The differences in Teff from Balmer profiles may amount up to 340 K and they occur also for Teff< 6250 K down to about 5000 K. The c1 index yields gravity differences ∆ log g as a function of log gwhich, for each Teff , grow to amaximumvalue. The maximum ∆ log g decreases with increasing temperatures and ranges, for solar metallicity, from 0.7 dex at log g=0.5 and Teff=5500 K to 0.2 dex at log g=4.5 and Teff=8000 K. 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, Teff and log g from NOVER models are closer to the parameters derived from model independent methods than are Teff 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 Procyonwith a unique Teff , regardless of whether the “overshooting” option is switched on or off. Independently of the convection option, the largest differences Send offprint requests to: F. Castelli in Teff derived with different methods are of the order of 200 K for Procyon and 150 K for the Sun.