Properties of WNh stars in the Small Magellanic Cloud: evidence for homogeneous evolution^*

¹ GRAAL – CNRS, Université Montpellier II – UMR 5024, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France e-mail: martins@graal.univ-montp2.fr
² Department of Physics and Astronomy, University of Pittsburgh, 3941 O'Hara street, Pittsburgh, PA 15260, USA
³ LAM – CNRS, Université de Provence – UMR 6110, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
⁴ Las Cumbres Observatory, 6740 Cortona Dr, suite 102, 93117 Goleta, CA, USA
⁵ LAOG – CNRS, Observatoire de Grenoble, Université Joseph Fourier – UMR 5571, BP 53, 38041, Grenoble Cedex 9, France
⁶ Department of Physics and Astronomy, Western Kentucky University, Bowling Green, KY, USA
⁷ Département de Physique, Université de Montréal, succursalle Centre-ville, Montréal, QC H3C 3J7, Canada

Received: 22 September 2008
Accepted: 13 November 2008

Abstract

Aims. We aim to assess the nature of three WNh stars in the SMC, to constrain stellar evolution beyond the main sequence at low metallicity, and to investigate the metallicity dependence of the clumping properties of massive stars.

Methods. We compute atmosphere models to derive the stellar and wind properties of the three WNh targets. A FUV/UV/optical/near-infrared analysis gives access to temperatures, luminosities, mass loss rates, terminal velocities and stellar abundances.

Results. We find that all stars still have a large hydrogen mass fraction in their atmosphere, and show clear signs of CNO processing in their surface abundances. One of the targets can be accounted for by normal stellar evolution. It is a star with an initial mass around 40-50 in, or close to, the core He burning phase. The other two objects likely follow a peculiar evolution, governed by fast rotation. In particular, one object is likely evolving homogeneously due to its position blue-ward of the main sequence and its high H mass fraction. The clumping factor of one star is found to be 0.15 ± 0.05. This is comparable to values found for Galactic Wolf-Rayet stars, indicating that within the uncertainties, the clumping factor does not seem to depend on metallicity.