Stellar and Wind Properties of LMC WC4 stars - A metallicity dependence for Wolf-Rayet mass-loss rates

Abstract

We use ultraviolet space-based (FUSE, HST) and optical/IR ground-based (2.3m MSSSO, NTT) spectroscopy to determine the physical parameters of six WC4-type Wolf-Rayet stars in the Large Magellanic Cloud. Stellar parameters are revised significantly relative to Grafener et al. (1998) based on improved observations and more sophisticated model atmosphere codes, which account for line blanketing and clumping. We find that stellar luminosities are revised upwards by up to 0.4 dex, with surface abundances spanning a lower range of 0.1<C/He<0.35 (20-45% carbon by mass) and O/He<0.06 (<10% oxygen by mass). Relative to Galactic WC5-8 stars at known distance, and analysed in a similar manner, LMC WC4 stars possess systematically higher stellar luminosities, 0.2dex lower wind densities, yet a similar range of surface chemistries. We illustrate how the classification CIII 5696 line is extremely sensitive to wind density, such that this is the principal difference between the subtype distribution of LMC and Galactic early-type WC stars. Temperature differences do play a role, but carbon abundance does not affect WC spectral types. We illustrate the effect of varying temperature and mass-loss rate on the WC spectral type for HD32257 (WC4, LMC) and HD156385 (WC7, Galaxy) which possess similar abundances and luminosities. Using the latest evolutionary models, pre-supernova stellar masses in the range 11-19 Mo are anticipated for LMC WC4 stars, with 7-14 Mo for Galactic WC stars with known distances. These values are consistent with pre-cursors of bright Type-Ic supernovae such as SN1998bw (alias GRB 980425) for which a minimum total mass of C and O of 14Mo has been independently derived.