Model Atmospheres and Spectral Analyses of Wolf-Rayet Stars

Abstract

Stratified non-LTE models for expanding atmospheres have become available in the recent years. They are based on the idealized assumptions of spherical symmetry, stationarity and radiative equilibrium. The satisfactory agreement between calculated and observed Wolf-Rayet spectra suggests that this “standard model” is basically adequate for describing real WR atmospheres and hence can be applied for their quantitative spectral analyses. By the application of these models, the fundamental parameters have been determined meanwhile for the majority of the known Galactic WR stars. Most WN stars populate a vertical strip in the Hertzspung-Russell diagram at effective temperatures of ≈35 kK, the luminosities ranging from 10^4.5 to 10^5.9 L_⊙. Only three WN stars of earliest subtype, other early-type WN stars if they have strong lines, and the WC stars are hotter. The chemical compositions of the WR atmospheres correspond to nuclear-processed material (WN: hydrogen burning in the CNO cycle; WC: helium burning). Hydrogen is depleted but still detectable in the cooler members of the WN subclass. Quantitatively, the hydrogen abundances show an interesting correlation with the luminosity which can be compared with the predictions from evolutionary calculations.