Notes on the convection in the ATLAS9 model atmospheres.
Abstract
The mixinglength 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_{gamma}_ and H_{beta}_ regions no solar model is able to reproduce the level of the true continuum deduced from highresolution 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 (by), (BV), and (VK) 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.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 February 1997
 Bibcode:
 1997A&A...318..841C
 Keywords:

 STARS: ATMOSPHERES;
 STARS: FUNDAMENTAL PARAMETERS;
 STARS: INDIVIDUAL: PROCYON;
 SUN: GENERAL;
 CONVECTION