A New Spectral Classification System for the Earliest O Stars: Definition of Type O2

Abstract

High-quality, blue-violet spectroscopic data are collected for 24 stars that have been classified as type O3 and that display the hallmark N iv and N v lines. A new member of the class is presented; it is the second known in the Cyg OB2 association, and only the second in the northern hemisphere. New digital data are also presented for several of the other stars. Although the data are inhomogeneous, the uniform plots by subcategory reveal some interesting new relationships. Several issues concerning the classification of the hottest O-type spectra are discussed, and new digital data are presented for the five original O3 dwarfs in the Carina Nebula, in which the N iv, N v features are very weak or absent. New spectral types O2 and O3.5 are introduced here as steps toward resolving these issues. The relationship between the derived absolute visual magnitudes and the spectroscopic luminosity classes of the O2-O3 stars shows more scatter than at later O types, at least partly because some overluminous dwarfs are unresolved multiple systems, and some close binary systems of relatively low luminosity and mass emulate O3 supergiant spectra. However, it also appears that the behavior of He ii 4686, the primary luminosity criterion at later O types, responds to other phenomena in addition to luminosity at spectral types O2-O3. There is evidence that these spectral types may correspond to an immediate pre-WN phase, with a correspondingly large range of luminosities and masses. A complete census of spectra classified into the original O3 subcategories considered here (not including intermediate O3/WN types or O3 dwarfs without N iv, N v features) totals 45 stars; 34 of them belong to the Large Magellanic Cloud and 20 of the latter to 30 Doradus.

An MK-like system of spectral classification for hot subdwarfs

“…He-normal (strong hydrogen, metallic lines or both HeI and HeII present): it is for the stellar spectra shown in Fig. 1 that we see variations very similar to those for MK luminosity class V. We chose BD+28 4211 as the standard for spectral class sdO2, since it shows weak lines of NV, OIV and CIV, but no trace of HeI at this resolution, which is consistent with the definition of spectral class O2 given by Walborn et al (2002). From O2 through B1, the principal criterion for the spectral class is HeI λ 4471/HeII λ 4541.…”

Massive runaway stars in the Large Magellanic Cloud

“…The discovery of a bow shock produced by BI 237 lends strong support to the idea that this and other isolated massive stars in the field of the LMC are runaway stars (Walborn et al 2002;Massey et al 2005;Evans et al 2006Evans et al , 2010. The young ages (∼2 Myr) of BI 237 and other O2-type field stars decidedly argue that their peculiar velocities cannot be explained by supernova explosions in binary systems (Blaauw 1961); the massive companion (primary) stars would simply have no time to end their lives in supernovae.…”

“…The details of these stars (listed in order of their RA) are summarized in Table 1. For the first four stars, we give their peculiar radial velocities, while the transverse velocities are listed for the remaining two, inferred under the assumption that both stars were ejected ∼2 Myr ago from R136 (Walborn et al 2002). Massey et al (2005).…”

Herschelfar-infrared observations of the Carina Nebula complex

“…Among the 65 known O-type member stars (Smith 2006) are some of the most massive (M > 100 M ) and luminous stars in our Galaxy. These include the famous Luminous Blue Variable η Carinae (see Corcoran et al 2004;Smith 2006), the O2 supergiant HD 93129A (see Walborn et al 2002) with about 120 M , and also four Wolf-Rayet stars (see Crowther et al 1995;Smith & Conti 2008). Most of the very massive stars are gathered in several open clusters, including A&A 549, A67 (2013) Trumpler 14-16.…”