Non-LTE model chromospheres of zeta Aurigae stars

Abstract

Plane-parallel, one-component, non-local thermodynamic equilibrium (LTE), semi-empirical model chromospheres have been constructed for the primary stars in the zeta Aurigae systems HR 6902 and 22 Vul. This has been accomplished by means of radiative transfer calculations undertaken with a non-LTE multilevel radiative transfer code, MULTI, and curves of growth applied to chromospheric eclipse spectra. The former are the subject of this paper, which is divided into two parts. The first briefly describes the observations and derivation of basic stellar parameters, while the second concerns the radiative transfer calculations and conclusions which follow from them. In particular, the calculated model photospheres plus chromospheres are used to investigate the relative contribution of primary and secondary stars to the overall radiation field. The radiation field of the hot secondary is approximated by use of an appropriate Kurucz model atmosphere. Although the hot companion is wholly responsible for the ionization of the metals, this is not the case as far as excitation is concerned. Use of a 47-level Fe atomic model demonstrates the intimate convolution between atomic and atmospheric physics. The chromospheres calculated by radiative transfer methods are compared with those derived empirically by curves of growth and are found to be comparable in the line-forming region of CaII H & K, MgII h & k and the many FeII lines. Finally, owing to the effects of radiative transitions from high-lying levels which are enhanced by the radiation field of the hot companion, it is found that lines resulting from transitions from the a^4F, a^4D and a^4P levels of FeII may not be used to derive an inner wind temperature for 22 Vul. This is the first time that zeta Aurigae chromospheres have been the subject of such intensive radiative transfer calculations.