Pulsation data for Cephei stars are an important source of information about the considered objects. These data consist of periods of oscillation and nonadiabatic observables, which are related to the eigenvalues and eigenfunctions of nonadiabatic oscillations, respectively. The nonadiabatic observables, and , change markedly with , Z and depend on the stellar opacity data used, cf. Cugier et al. (1994). In this paper, an attempt was made to calibrate these data using both time-series photometric data and spectro-photometric observations corresponding to the mean stellar state of Cet.
We found that the model calculated with the OPAL opacities for log = 4.347, log g = 3.73 and Z = 0.02 describes very well the observed properties of the analysed star. This model, obtained from the analysis of time-series of the multicolour uvby photometric data, describes also very well the observed UV and visual energy flux distribution, as well as line profiles taken in the high-resolution mode. In other words, this model is located well within error boxes derived from "classical" spectro-photometric analysis of Cet. Models based on OP opacities predict too high an effective temperature and fit markedly worse these observations.
Having established chemical composition and opacity data, nonadiabatic observables offer determination of the effective temperature and luminosity with high precision, viz., log and log for Cet. Thus the data of individual Cep stars may be used for precise determination of the distances of these objects. At the moment, this solution must be treated with caution, because the calibration procedure should first be extended for a number of Cep stars. A more realistic estimation is therefore 4.04 log 4.16, which is consistent with both OPAL and OP opacities. This estimation is also insensitive to moderate chemical composition changes and is not significantly influenced by uncertainties in description of the convective overshooting effect.
New aspects also arise for spectroscopic studies. Determination of the chemical composition of Cephei stars is an important point, due to the large sensitivity of the oscillation parameters to the stellar opacities. We found that the calculated spectra fit very well the observed C III, Si III and Si IV line profiles of Cet, assuming solar abundance of carbon and silicon. This is in agreement with the conclusion derived from the nonadiabatic observables (cf. Sect. 2). This is also consistent with recent theoretical results (cf. Dziembowski & Pamyatnykh 1993 and Gautschy & Saio 1993) showing that the metal abundance parameter Z = 0.02 is sufficient for driving pulsations in Cephei stars. Finally, an analysis of the H I line reveals the hydrogen interstellar column density toward Cet equal to log .
© European Southern Observatory (ESO) 1997
Online publication: October 15, 1997