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Astron. Astrophys. 348, 198-210 (1999)


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Chromospheric models for late A-type stars *

P. Gouttebroze 1, R. Freire Ferrero 2, E. Marilli 3 and S. Catalano 3

1 Institut d'Astrophysique Spatiale, Unité Mixte CNRS-Université Paris XI, bât 121, F-91405 Orsay Cedex, France
2 Observatoire Astronomique, 11 rue de l'Université, F-67000 Strasbourg, France
3 Osservatorio Astrofisico di Catania, Viale A. Doria 6, I-95125 Catania, Italy

Received 29 May 1997 / Accepted 21 May 1999

Abstract

The chromospheres of [FORMULA] Aql (Altair) and [FORMULA] Cep (Alderamin) are investigated. Different semi-empirical model atmospheres are constructed, and the predicted spectra are compared to the observations of the hydrogen Lyman-[FORMULA] line (IUE) and carbon (both neutral and ionized) multiplets (GHRS-HST). It is found that chromospheric models at low temperature ([FORMULA] 10000 K) which fit the L[FORMULA] line, produce C II lines in absorption, contrary to observations. Among the models investigated, only those with a mean temperature of [FORMULA] 20000 K are able to fit simultaneously the L[FORMULA] and C II lines. Provisional reference models are proposed for [FORMULA] Aql. The lower emission of [FORMULA] Cep with respect to [FORMULA] Aql is better explained by a difference of column mass than by a difference of temperature. The chromospheric column mass ratio ([FORMULA] Cep / [FORMULA] Aql) is estimated to about 0.8. For both stars, the relatively narrow dips of the C II emission feature cannot be of stellar origin, owing to the fast rotation, but of interstellar absorption nature.

Key words: line: profiles – stars: atmospheres – stars: chromospheres – stars: Hertzsprung – Russel (HR) and C-M diagrams – ISM: atoms, ions

* Based on observations by the IUE satellite collected at Villafranca ESA IUE Observatory and at the GSFC NASA IUE Observatory. Also based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive, at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract NAS 5-26555.

Send offprint requests to: P. Gouttebroze

© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999

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