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

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.