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W. Boland, H. Woerden (1985)
Birth and evolution of massive stars and stellar groups; Proceedings of the Symposium, Dwingeloo, Netherlands, September 24-26, 1984
H. Lamers, C. Loore (1987)
Instabilities in luminous early type stars, 136
Wolf-Rayet (W-R) stars are hot, luminous objects with strong, broad ( '" 103 km s -I ) emission lines in the optical region ( 189) due to a substantial stellar wind. The W-R phase is a normal stage of evolution for all stars initially more massive than ",40 M0' and these extreme Population I stars are the topic of this review. A W-R phase is also present in some central stars of planetary nebulae, which we do not discuss here. Some 160 indi vidual stars of this class are known in our own Galaxy, and many others have been identified among galaxies of the Local Group. W-R stars come in two major subtypes (22): the WN, in which lines of helium and nitrogen ions are seen; and the we, which contain lines of carbon and oxygen along with the helium ions. An additional minor subtype (WO) with strong 0 VI lines has also recently been identified ( 19). The main characteristic of the optical spectrum of W-R stars is the dominance of emission lines. In a few WN stars, hydrogen appears to be present (52, 159), but generally the spectra of W-R stars are notable for the absence
Annual Review of Astronomy and Astrophysics – Annual Reviews
Published: Sep 1, 1987
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