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

TL;DR

This paper introduces an updated spectral classification scheme for transition O2-3.5If*/WN5-7 stars, based on Hbeta morphology and spectral diagnostics, and discusses their physical properties, evolution, and near-IR criteria.

Contribution

It proposes a new classification method for transition stars using Hbeta features and discusses their evolutionary status and physical properties.

Findings

Transition stars can be discriminated by Hbeta morphology.

Most are young, massive hydrogen-burning stars.

Some are evolving blueward as core helium-burning stars.

Abstract

An updated classification scheme for transition O2-3.5If*/WN5-7 stars is presented, following recent revisions to the spectral classifications for O and WN stars. We propose that O2-3.5If*, O2-3.5If*/WN5-7 and WN5-7 stars may be discriminated using the morphology of Hbeta to trace increasing wind density as follows: purely in absorption for O2-3.5If* stars in addition to the usual diagnostics from Walborn et al.; P Cygni for O2-3.5If*/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-IR spectroscopy. The physical and wind properties of such stars are qualitatively discussed together with their evolutionary significance. We suggest that the majority of O2-3.5If*/WN5-7 stars are young, very massive hydrogen-burning stars, genuinely intermediate between O2-3.5If* 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 Hbeta emission.