On the so-called WO stars

Abstract

The Wolf-Rayet WO spectral sequence was introduced by Barlow & Hummer (1982) for four Pop I WR stars with very strong O VI 381 nm doublet emission. At the time, the evolution of very massive stars was poorly known, and their classification was based on phenomenological grounds. Recently, efforts to improve the WO spectral subtype classification are increasing (Kingsburgh et al. 1995; Crowther et al. 1998).