The chromosphere of II Pegasi: multi-line modelling of an RS Canum Venaticorum star

We present the first simultaneous multi-line fitting of a semi-empirical atmospheric model with a chromosphere and transition region to the H I and Ca II spectra of an RS CVn star (II Peg). The static component of the Hα emission core, the line profile of Hβ, the apparent absence of Hγ and Hδ, and the emission core profiles of Ca II K and two of the Ca II IR3 lines are all approximately fitted by a static 1D model with the following properties: logm at the onset of the transition region is ≈ −2.85, and a 6000 K plateau in the upper chromosphere that spans about a decade in logm. In particular, the model is able to reproduce the unusually steep Balmer decrement (compared to the dMe stars), in which Hα is strongly in emission and Hβ is in absorption, without recourse to extra-atmospheric material. The Ca II IR3 lines are best fit by a model in which Tmin is cooler by 300 K and shallower by over half a decade in logm than that which best fits the optical lines. The emergent flux in the IR3 line cores arises from the Tmin region, whereas the other diagnostics arise from layers well above Tmin, and it is postulated that this may be the cause of the discrepancy. There has very recently been evidence from other investigators that the metallicity of II Peg may be sub-solar, and we find that models with [ H ] = −0.4 provide a somewhat better fit to all the lines considered.