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

Abstract

The stellar properties of Sk 41 (AB4, WN5h), the only known single Wolf-Rayet star in the SMC, are derived from ultraviolet (IUE), optical (AAT) and near-IR (NTT) spectroscopy. Contrary to expectations for low metal environments, the stellar properties of Sk 41 are typical of equivalent WN stars in the Galaxy and LMC, with T_* ~ 42kK, log(L/L_sun)=5.7, v_infty =1300 km s^-1, H/He ~ 2 by number, and dot {M}/sqrt {f}=3x10^-5 M_sunyr^-1, where f is the volume filling factor. The stellar luminosity of Sk 41 is 50% below the minimum value predicted by single star evolutionary models at the metallicity of the SMC, suggesting a deficiency in present evolutionary models at low metallicity. Emission line luminosities of He II lambda 4686 and C IV lambda lambda 5801-12 in SMC WR stars are not systematically lower than their Galactic and LMC 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 in young starburst galaxies. Synthetic WN models are calculated with identical parameters except that metal abundances are varied. Following the Smith et al. WN classification scheme, CNO equilibrium models reveal that earlier spectral types are predicted at lower metallicity, i.e. WN3-4 at 0.04Z_sun versus WN6 at 1.0Z_sun. This provides an explanation for the trend towards earlier WN spectral types at low metallicity. Based, in part, on observations collected at the European Southern Observatory, La Silla, Chile (Proposal No. 61.D--0680 and 63.H--0683)