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Astron. Astrophys. 318, 841-869 (1997)


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Notes on the convection in the ATLAS9 model atmospheres

F. Castelli 1, R.G. Gratton 2 and R.L. Kurucz 3

1 CNR-Gruppo Nazionale Astronomia and Osservatorio Astronomico, Via G.Tiepolo 11, I-34131 Trieste, Italy
2 Osservatorio Astronomico di Padova, Vicolo dell' Osservatorio 5, I-35122 Padova, Italy
3 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received 9 April 1996 / Accepted 20 June 1996

Abstract

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 [FORMULA] included between 4000 K and 8500 K, [FORMULA] 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 [FORMULA] and [FORMULA] 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 [FORMULA] [FORMULA] 6250 K and it decreases with decreasing gravity. The differences in ([FORMULA]), ([FORMULA]), and ([FORMULA]) indices computed from models with the "overshooting" option switched on and off, correspond to [FORMULA] differences which may amount up to 180 K, 100 K, 60 K respectively. The differences in [FORMULA] from Balmer profiles may amount up to 340 K and they occur also for [FORMULA] [FORMULA]  6250 K down to about 5000 K. The [FORMULA] index yields gravity differences [FORMULA] as a function of [FORMULA] which, for each [FORMULA], grow to a maximum value. The maximum [FORMULA] decreases with increasing temperatures and ranges, for solar metallicity, from 0.7 dex at [FORMULA] =0.5 and [FORMULA] =5500 K to 0.2 dex at [FORMULA] =4.5 and [FORMULA] =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, [FORMULA] and [FORMULA] from NOVER models are closer to the parameters derived from model independent methods than are [FORMULA] and [FORMULA] 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 [FORMULA], regardless of whether the "overshooting" option is switched on or off. Independently of the convection option, the largest differences in [FORMULA] derived with different methods are of the order of 200 K for Procyon and 150 K for the Sun.

Key words: stars: atmospheres – stars: fundamental parameters – stars: individual: Procyon – Sun: general – convection

Send offprint requests to: F. Castelli

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998

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