Journal Article

Spectral classification of O2–3.5 If*/WN5–7 stars

Paul A. Crowther and Nolan R. Walborn

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 416, issue 2, pages 1311-1323
Published in print September 2011 | ISSN: 0035-8711
Published online September 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.19129.x
Spectral classification of O2–3.5 If*/WN5–7 stars

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An updated classification scheme for transition O2–3.5 If*/WN5–7 stars is presented, following recent revisions to the spectral classifications for O and WN stars. We propose that O2–3.5 If*, O2–3.5 If*/WN5–7 and WN5–7 stars may be discriminated using the morphology of Hβ to trace increasing wind density as follows: purely in absorption for O2–3.5 If* stars in addition to the usual diagnostics from Walborn et al.; P Cygni for O2–3.5 If*/WN5–7 stars; purely in emission for WN stars in addition to the usual diagnostics from Smith et al. We also discuss approximate criteria to discriminate between these subtypes from near-infrared spectroscopy. The physical and wind properties of such stars are qualitatively discussed together with their evolutionary significance. We suggest that the majority of O2–3.5 If*/WN5–7 stars are young, very massive hydrogen-burning stars, genuinely intermediate between O2–3.5 If* and WN5–7 subtypes, although a minority are apparently core helium-burning stars evolving blueward towards the classical WN sequence. Finally, we reassess classifications for stars exhibiting lower ionization spectral features plus Hβ emission.

Keywords: stars: early-type; stars: evolution; stars: fundamental parameters; stars: massive; stars: Wolf–Rayet

Journal Article.  7204 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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