SPECTRAL ANALYSES OF 25 GALACTIC WOLF-RAYET STARS OF THE CARBON SEQUENCE

We present a grid of helium-carbon models for Wolf-Rayet (WR) stars of the carbon sequence (WC) with beta(c) = 0.2 (carbon mass fraction), t hus extending our previously released grid with beta(c) = 0.6 to a dif ferent 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 ca rbon are represented by detailed model atoms, especially concerning th e ions C III and C IV. Using the model grids 25 Galactic WC stars of i ntermediate subtype (WC 5 to WC 8) are analyzed. Subsequently we perfo rm fine analyses by calculating several individual models for each of the program stars. Temperatures, radii, mass-loss rates and terminal v elocities are determined together with the carbon to helium ratio. The analyzed WC stars are found to form two groups, which can be distingu ished by the strength of their emission lines. Stars with weak lines ( WC-w) have effective temperatures close to 50 kK and their winds are r elatively thin, forming the continuous spectrum in regions with small expansion velocities. WC stars with strong lines (WC-s) have higher ef fective 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 10(4.7) and 10(5.5) L(.) and mass-l oss rates from 10(-4.8) to 10(-3.9) M(.) yr(-1). The carbon mass fract ion varies from 0.2 to 0.6. No correlation is found between the carbon abundance and any of the stellar parameters (e.g. temperature, lumino sity) or the spectral subtype. The evolution of WR stars is discussed by comparing the results of our analyses with evolutionary tracks.