1999A&A...348..198G

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