## Notes on the convection in the ATLAS9 model atmospheres
^{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
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 included between 4000 K and 8500 K, 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 and 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 6250 K and it decreases with decreasing gravity. The differences in (), (), and () indices computed from models with the "overshooting" option switched on and off, correspond to differences which may amount up to 180 K, 100 K, 60 K respectively. The differences in from Balmer profiles may amount up to 340 K and they occur also for 6250 K down to about 5000 K. The index yields gravity differences as a function of which, for each , grow to a maximum value. The maximum decreases with increasing temperatures and ranges, for solar metallicity, from 0.7 dex at =0.5 and =5500 K to 0.2 dex at =4.5 and =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, and from NOVER models are closer to the parameters derived from model independent methods than are and 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 , regardless of whether the "overshooting" option is switched on or off. Independently of the convection option, the largest differences in derived with different methods are of the order of 200 K for Procyon and 150 K for the Sun.
## Contents- 1. Introduction
- 2. Convection in ATLAS9
- 3. Codes and models
- 4. The solar model and the value of the mixing-length
- 5. The problem of the level of the continuum in the Sun
- 6. The convective flux as a function of and
- 7. from colour indices (), (), and ()
- 8. Gravity from Strömgren index
- 9. The effect of convection on Balmer lines
- 10. An example: the model parameters for Procyon
- 10.1. from colour indices
- 10.2. from spectrophotometry
- 10.3. from Balmer profiles
- 10.4. from the ionization equilibria
- 10.5. The final parameters of Procyon
- 11. Comments
- 12. Conclusions
- Acknowledgements
- References
© European Southern Observatory (ESO) 1997 Online publication: July 3, 1998 |