Spectral analyses of 25 Galactic Wolf-Rayet stars of the carbon sequence

L. Koesterke and W.-R. Hamann

Institut für Theoretische Physik und Sternwarte der Universität,
D-24098 Kiel, Federal Republic of Germany,
e-mail: (L.K.) pas19@rz.uni-kiel.d400.de

We present a grid of helium-carbon models for Wolf-Rayet (WR) stars of the carbon sequence (WC) with betac =0.2 (carbon mass fraction), thus extending our previously released grid with betac=0.6 to a different chemical composition. The WR model atmospheres are based on the so-called standard assumptions. The calculations account for non-LTE radiation transfer in spherically expanding atmospheres. Helium and carbon are represented by detailed model atoms, especially concerning the ions C III and C IV. Using the model grids 25 Galactic WC stars of intermediate subtype (WC 5 to WC 8) are analyzed. Subsequently we perform fine analyses by calculating several individual models for each of the program stars. Temperatures, radii, mass-loss rates and terminal velocities are determined together with the carbon to helium ratio. The analyzed WC stars are found to form two groups, which can be distinguished by the strength of their emission lines. Stars with weak lines (WC-w) have effective temperatures close to 50 kK and their winds are relatively thin, forming the continuous spectrum in regions with small expansion velocities. WC stars with strong lines (WC-s) have higher effective temperatures (60 to 100 kK, referring to the core radius) and thick winds. Thus there is a strong analogy to the distribution of the early-type WN stars (WNE -w and WNE -s, respectively). For the WC stars we determine luminosities between 104.7 and 105.5 Lsun and mass-loss rates from 10-4.8 to 10-3.9 Msun/yr. The carbon mass fraction varies from 0.2 to 0.6. No correlation is found between the carbon abundance and any of the stellar parameters (e.g. temperature, luminosity) or the spectral subtype.

The evolution of WR stars is discussed by comparing the results of our analyses with evolutionary tracks.

Keywords: stars: Wolf-Rayet-stars: atmospheres-stars: abundances - stars: mass-loss

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