Chemical abundances of $β$ Cephei stars from low- and high-resolution UV spectra

Abstract

In early type stars the ultraviolet spectral region is important for several reasons. Firstly, since the majority of the total flux is emitted here, it provides a rather sensitive indicator of a photospheric temperature and luminosity. Secondly, the UV spectra are dominated by lines of the iron-group elements, which cause the origin of the opacity bump at temperature $2 \times 10^5$ K, where the classical $κ$ mechanism drives pulsations of $β$ Cep stars. Consequently, the position of the instability domain in the HR diagram strongly depends on the metal abundance (Pamyatnykh 1999). We derive stellar parameters (metallicity, [m/H], effective temperature, $T_{\rm eff}$, stellar diameter, $θ$) and interstellar extinction, $E(B-V)$, for all $β$ Cep stars observed by {\it International Ultraviolet Explorer} satellite. The parameters are derived by means of an algorithmic procedure of fitting theoretical flux distributions to the low-resolution IUE spectra and optical spectrophotometric observations. The errors are estimated by using the bootstrap method (Press et al. 1992). We also show some examples of high-resolution HST/GHRS spectra for one $β$ Cep star: $γ$ Peg.