Wind properties of Wolf-Rayet stars at low metallicity: Sk 41 (SMC)

The stellar properties of Sk41 (AB4, WN5h), the only known single Wolf-Raye t star in the SMC, are derived from ultraviolet (IUE), optical (AAT) and ne ar-IR (NTT) spectroscopy. Contrary to expectations for low metal environmen ts, the stellar properties of Sk41 are typical of equivalent WN stars in th e Galaxy and LMC, with T*similar to 42kK, log(L/L.)=5.7, upsilon(infinity)= 1300 km s(-1), H/He similar to 2 by number, and (M) over dot/root f=3 x 10( -5) M.yr(-1), where f is the volume filling factor. The stellar Luminosity of Sk 41 is 5046 below the minimum value predicted by single star evolution ary models at the metallicity of the SMC, suggesting a deficiency in presen t evolutionary models at low metallicity. Emission line luminosities of He II lambda 4686 and C rv lambda lambda 5801 -12 in SMC WR stars are not systematically lower than their Galactic and LM C counterparts. From 43 late-type and 59 early type WN stars, Log L-lambda( HeII)= 36.0 erg s(-1) and 35.8 erg s(-1), respectively, while log L-lambda( CIV)=36.5 erg s(-1). from 25 early-type WC stars. This new calibration has application in deriving WR populations ill young starburst galaxies. Synthetic WN models are calculated with identical parameters except that me tal abundances are varied. Following the Smith et al. WN classification sch eme, CNO equilibrium models reveal that earlier spectral types are predicte d at lower metallicity, i.e. WN3-4 at 0.04Z. versus WN6 at 1.0Z.. This prov ides an explanation for the trend towards earlier WN spectral types at low metallicity.