Astronomy and Astrophysics, volume 299, 151-162 (1995/7-1)
Spectral analyses of the Galactic Wolf-Rayet stars: hydrogen-helium abundances and improved stellar parameters for the WN class.
HAMANN W.-R., KOESTERKE L. and WESSOLOWSKI U.
Abstract (from CDS):
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 104.6 to 105.7L☉. The occurrence of hydrogen is restricted to those WN stars with lowest stellar temperatures (30...35kK, 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 βH 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 105.7L☉, i.e. brighter than any hydrogen-free WN star. Only three WN stars are found with hydrogen mass fractions as small as ≃5%. A small group of four WN7 stars with high luminosities (>105.6L☉) 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.
stars: abundances - stars: atmospheres - stars: fundamental parameters - Hertzsprung-Russell (HR) diagram - stars: mass loss - stars: Wolf-Rayet