WN stars in the LMC: parameters and atmospheric abundances

W.-R. Hamann, L. Koesterke

Lehrstuhl Astrophysik der Universität Potsdam, Postfach 601553, D-14415 Potsdam, Germany

The spectra of 18 WN stars in the Large Magellanic Cloud (LMC) are quantitatively analyzed by means of "standard" Wolf-Rayet model atmospheres, using the helium and nitrogen lines as well as the spectral energy distribution. The hydrogen abundance is also determined. Carbon is included for a subset of 4 stars. The studied sample covers all spectral subtypes (WN2 ... WN9) and also includes one WN/WC transition object.

The luminosities of the program stars span a wide range ( L/Lsun = 5.0 ... 6.5). Due to the given LMC membership, these results are free from uncertainties inferred from the distance. 50 % of the studied stars (both, late and early WN subtypes) have rather low luminosity (L/Lsun < 5.5). This puts tough constraints on their evolutionary formation. If coming from single stars, it provides evidence for strong internal mixing processes.

The empirical mass-loss rates are scaled down by a factor of about two due to the impact of clumping, compared to previous studies adopting homogeneous winds. There is no obvious strong correlation between the mass-loss rates and other parameters like luminosity, temperature and composition.

The stellar parameters for the present LMC sample are not systematically different from those of the Galactic WN stars studied previously with the same techniques, in contrast to the expected metallicity effects.

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