Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...274..397h&link_type=abstract
Astronomy and Astrophysics, Vol. 274, NO. 2/JUL(II), P. 397, 1993
Astronomy and Astrophysics
Astrophysics
109
Stars: Atmospheres, Stars: Fundamental Parameters, Stars: Mass-Loss, Stars: Wolf-Rayet
Scientific paper
The vast majority of the known Galactic single WN stars is analyzed, and the stellar parameters are determined. The analyses are based on a grid of non-LTE models for spherically expanding pure-helium atmospheres. Additionally, the presence or absence of hydrogen is qualitatively assessed.
The results reveal that the WN stars of "early" subtype and with weak lines (WNE-w subclass) continue the WNL subclass ("late subtypes") towards lower luminosities, while both subclasses have effective temperatures in the narrow range from 30 to 40 kK. The WNE-s stars ("early" subtypes with strong lines) form a separate group with higher temperatures. Only these stars have "extended" atmospheres in the sense that the continuum radiation emerges from rapidly expanding layers. Hence in WNE-s stars the wind acceleration mainly happens below the photosphere, while in WNE-w and WNL stars the continuum becomes transparent at small expansion velocities. Mass-loss rates from most of the WNE-s and part of the WNL stars are not correlated with the luminosity, indicating that radiation pressure is not the only relevant mechanism determining these rates. However, radiation pressure on lines is probably sufficient to explain the wind acceleration outside the photosphere.
Existing theoretical tracks for post red supergiant evolution do not agree with our results quantitatively, but there are qualitative similarities indicating that this scenario might be basically adequate. Hydrogen is detected only in the coolest WNL and WNE-w stars, confirming that the evolution proceeds towards higher temperatures. However, the effective temperatures and, most severe, the luminosities are generally much lower observed than predicted.
Hamann Wolf-Rainer
Koesterke Lars
Wessolowski Ulf
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