SPECTRAL ANALYSES OF WC STARS IN THE LMC

Spectra of six Wolf-Rayet stars in the Large Magellanic Cloud (LMC) of the subtype WC 4, the dominant WC spectral type in the LMC, are taken with the Faint Object Spectrograph (FOS) on board the Hubble Space Te lescope (HST). They are analyzed by means of non-LTE models for spheri cally expanding atmospheres computed using standard assumptions. Compl ex model atoms of helium, carbon and oxygen are taken into account. We find stellar luminosities in the range from 10(5.1) L-. to 10(5.6) L- ., and mass loss rates of approximately 10(-4) M-. yr(-1). The stellar temperature T, defined as the effective temperature related to the s tellar core radius, is of the order of 100 kK. The atmospheric composi tions show carbon mass fractions of about 0.4, oxygen mass fractions i n the range 0.1-0.3 and no indications of nitrogen. Stellar parameters obtained this way build an empirical basis for the understanding of t he late evolutionary phases of massive stars. The C/He and O/C abundan ce ratios are sensitive discriminates of stellar evolutionary models, since they depend, for example, on the adopted mass loss relation, the amount of overshooting, the C-12 (alpha, gamma) O-16 reaction rate, a nd the metallicity of the progenitor. We compare our results to evolut ionary calculations and to a galactic sample of WC stars. The deduced abundances agree with some evolutionary calculations, but are in disag reement with others. The derived mass loss rates are found to be in ge neral agreement with common M-L-relations. Systematic differences betw een the galactic and the LMC sample exist, possibly reflecting a depen dence of WC mass loss on metallicity.