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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
Received 29 May 1997 / Accepted 21 May 1999
The chromospheres of Aql (Altair) and Cep (Alderamin) are investigated. Different semi-empirical model atmospheres are constructed, and the predicted spectra are compared to the observations of the hydrogen Lyman- line (IUE) and carbon (both neutral and ionized) multiplets (GHRS-HST). It is found that chromospheric models at low temperature ( 10000 K) which fit the L line, produce C II lines in absorption, contrary to observations. Among the models investigated, only those with a mean temperature of 20000 K are able to fit simultaneously the L and C II lines. Provisional reference models are proposed for Aql. The lower emission of Cep with respect to Aql is better explained by a difference of column mass than by a difference of temperature. The chromospheric column mass ratio ( Cep / 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
Online publication: July 16, 1999