NON-LTE MODEL CHROMOSPHERES OF ZETA-AURIGAE STARS

Plane-parallel, one-component, non-local thermodynamic equilibrium (LT E), semiempirical model chromospheres have been constructed for the pr imary stars in the zeta Aurigae systems HR 6902 and 22 Vul. This has b een accomplished by means of radiative transfer calculations undertake n with a non-LTE multilevel radiative transfer code, MULTI, and curves of growth applied to chromospheric eclipse spectra. The former are th e subject of this paper, which is divided into two parts. The first br iefly describes the observations and derivation of basic stellar param eters, while the second concerns the radiative transfer calculations a nd conclusions which follow from them. In particular, the calculated m odel photospheres plus chromospheres are used to investigate the relat ive contribution of primary and secondary stars to the overall radiati on field. The radiation field of the hot secondary is approximated by use of an appropriate Kurucz model atmosphere. Although the hot compan ion is wholly responsible for the ionization of the metals, this is no t the case as far as excitation is concerned. Use of a 47-level Fe ato mic model demonstrates the intimate convolution between atomic and atm ospheric physics. The chromospheres calculated by radiative transfer m ethods are compared with those derived empirically by curves of growth and are found to be comparable in the line-forming region of Ca II H & K, Mg II h & k and the many Fe II lines. Finally, owing to the effec ts of radiative transitions from high-lying levels which are enhanced by the radiation field of the hot companion, it is found that lines re sulting from transitions from the a(4)F, a(4)D and a(4)P levels of Fe II may not be used to derive an inner wind temperature for 22 Vul. Thi s is the first time that zeta Aurigae chromospheres have been the subj ect of such intensive radiative transfer calculations.