Spectral analyses of the Galactic Wolf-Rayet stars: hydrogen-helium abundances and improved stellar parameters for the WN class

W.-R. Hamann, L. Koesterke and U. Wessolowski

Institut für Theoretische Physik und Sternwarte der Universitäat, Olshausenstraße 40, D-24118 Kiel, Germany

Almost all known Galactic single WN stars have been analyzed applying non-LTE models for spherically expanding atmospheres. While in a previous study we performed "coarse" spectral analyses based essentially on helium-line equivalent widths compared to a grid of models, we now present improved results from 25 detailed "fine" analyses, i.e. from fitting the line profiles with individual model calculations. The non-LTE models applied now account for a helium-hydrogen composition, and corresponding abundances are determined for each star. 53% of the studied Galactic WN stars (33 out of 62) are hydrogen-free, populating a luminosity range from 10^4.6 to 10^5.7 Lsun. The occurrence of hydrogen is restricted to those WN stars with lowest stellar temperatures (30 ... 35 kK, with few exceptions), comprising most stars of late subtype (WNL) and part of the weak-lined early-type WN stars (WNE-w). But all WN stars are definitely hydrogen-deficient, compared to the solar value. Most WN stars showing hydrogen exhibit mass fractions beta_ H between 10% and 30%. They cover the whole luminosity range of the the hydrogen-free WN stars, but extend also to higher values: a group of ten WNL stars with hydrogen is brighter than 10^5.7 Lsun, i.e. brighter than any hydrogen-free WN star. Only three WN stars are found with hydrogen mass fractions as small as approximately 5%. A small group of four WN7 stars with high luminosities >10^5.6 Lsun) has outstandingly high hydrogen mass fractions (40 ... 53%). The gradual switching from the high mass-loss rates of the hydrogen-free WN stars to the much lower mass-loss rates of typical OB stars is found to be correlated with the hydrogen abundance.

Astron. Astrophys. 299, 151 (1995)

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