Chromospheric models for late A-type stars

Abstract

The chromospheres of alpha Aql (Altair) and alpha Cep (Alderamin) are investigated. Different semi-empirical model atmospheres are constructed, and the predicted spectra are compared to the observations of the hydrogen Lyman-alpha 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 Lalpha 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 Lalpha and C ii lines. Provisional reference models are proposed for alpha Aql. The lower emission of alpha Cep with respect to alpha Aql is better explained by a difference of column mass than by a difference of temperature. The chromospheric column mass ratio (alpha Cep / alpha 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. 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.