Mon. Not. R. Astron. Soc., 416, 1311-1323 (2011/September-2)
Spectral classification of O2–3.5 If*/WN5–7 stars.
CROWTHER P.A. and WALBORN N.R.
Abstract (from CDS):
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.
2011 The Authors Monthly Notices of the Royal Astronomical Society2011 RAS