1998A&A...338..191S

We present the first simultaneous multi-line fitting of a semi-empirical atmospheric model with a chromosphere and transition region to the HI and CaII 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 CaII K and two of the CaII 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 6000K 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 CaII IR³ lines are best fit by a model in which T_min is cooler by 300K 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 T_min region, whereas the other diagnostics arise from layers well above T_min, 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 [A/H]=-0.4 provide a somewhat better fit to all the lines considered.