Spectral analyses of WC stars in the LMC

G. Gräfener1, W.-R. Hamann1, D. J. Hillier2, L. Koesterke1,

1Institut für Theoretische Physik und Astrophysik der Universität Potsdam
2Department of Physics and Astronomy, University of Pittsburgh

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 Telescope (HST). They are analyzed by means of non-LTE models for spherically expanding atmospheres computed using standard assumptions. Complex model atoms of helium, carbon and oxygen are taken into account. We find stellar luminosities in the range from 105.1 Lsun to 105.6 Lsun, and mass loss rates of approximately 10-4Msun. The stellar temperature T*, defined as the effective temperature related to the stellar core radius, is of the order of 100 kK. The atmospheric compositions show carbon mass fractions of about 0.4, oxygen mass fractions in the range 0.1-0.3 and no indications of nitrogen. Stellar parameters obtained this way build an empirical basis for the understanding of the late evolutionary phases of massive stars. The C/He and O/C abundance ratios are sensitive discriminates of stellar evolutionary models, since they depend, for example, on the adopted mass loss relation, the amount of overshooting, the 12 C (alpha, gamma)16 O reaction rate, and the metallicity of the progenitor. We compare our results to evolutionary calculations and to a galactic sample of WC stars. The deduced abundances agree with some evolutionary calculations, but are in disagreement with others. The derived mass loss rates are found to be in general agreement with common massloss-luminosity-relations. Systematic differences between the galactic and the LMC sample exist, possibly reflecting a dependence of WC mass loss on metallicity.

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