New very massive stars in Cygnus OB2

Abstract

Context. The compact association Cygnus OB2 is known to contain a large population of massive stars, but its total mass is currently a matter of debate. While recent surveys have uncovered large numbers of OB stars in the area around Cyg OB2, detailed study of the optically brightest among them suggests that most are not part of the association. Aims. We observed an additional sample of optically faint OB star candidates, with the aim of checking if more obscured candidates are correspondingly more likely to be members of Cyg OB2. Methods. Low resolution spectra of 9 objects allow the rejection of one foreground star and the selection of four O-type stars, which were later observed at higher resolution. In a subsequent run, we observed three more stars in the classification region and three other stars in the far red. Results. We identify five (perhaps six) new evolved very massive stars and three main sequence O-type stars, all of which are likely to be members of Cyg OB2. The new findings allow a much better definition of the upper HR diagram, suggesting an age ∼2.5 Myr for the association and hinting that the O3−5 supergiants in the association are blue stragglers, either younger or following a different evolutionary path from other cluster members. Though the bulk of the early stars seems to belong to an (approximately) single-age population, there is ample evidence for the presence of somewhat older stars at the same distance. Conclusions. Our results suggest that, even though Cyg OB2 is unlikely to contain as many as 100 O-type stars, it is indeed substantially more massive than was thought prior to recent infrared surveys.

The eclipsing, double-lined, Of supergiant binary Cygnus OB2-B17

“…However, given that it appears significantly less massive than the WN6ha + WN6ha binary WR20a (m 1 = m 2 = 83 M , Rauw et al 2004;Bonanos et al 2004) we suspect that it will instead evolve to resemble Cyg OB2 #5 and hence to a configuration similar to GCIRS 16SW, composed Negueruela et al (2008), based on published spectral classes and 2MASS JHK S photometry and a DM of 11.3. The continuous lines are non-rotating isochrones for log t = 6.2, 6.3 and 6.4 from Schaller et al (1992) and the dashed line is the log t = 6.4 isochrone in the high-rotation models from Meynet & Maeder (2003).…”

“…As a check on consistency we have calculated a preliminary bolometric magnitude for the components, following a similar procedure the that employed by Negueruela et al (2008). Assuming that both components are of similar colour 6 , we adopted the 2MASS value for (J − K S ) used in Negueruela et al (2008), the effective temperature T eff and bolometric correction (BC) calibrations of Martins et al (2005), and the intrinsic (J−V) 0 and (K−V) 0 colour calibrations of Wegner (1994). Using the 2MASS observed (J − K S ), we derived E(J − K S ).…”

“…Comerón et al (2002) observed the system as part of a near-infrared spectroscopic survey and found the spectrum presented Brγ emission, confirming it to be an evolved massive star. Follow up observations were made by Negueruela et al (2008), who found it to stand out from the rest of the members due to its variability and strong emission lines. They classified it as an Ofpe star and suggested it was a strong binary candidate.…”

The little-studied cluster Berkeley 90

“…We note that the luminosity difference between dwarfs and supergiants is relatively small at the earliest spectral types. Assuming that LS III +46 12 is slightly evolved, we can perhaps give an age ∼2 Ma, but luminous O-type stars in Cygnus OB2 with spectral types O3-O5 are significantly hotter than the main-sequence turn-off (Negueruela et al 2008). By analogy, an age of up to 3 Ma is possible.…”

The 2.35 year itch of Cygnus OB2 #9

“…As suggested by its name, this star belongs to the Cyg OB2 association, one of the richest OB associations of our Galaxy since it contains about one hundred O-stars (Knödlseder 2000;Negueruela et al 2008;Wright et al 2010). As for other members of this association, Cyg OB2 #9 is strongly extinguished, which renders its study quite difficult (E[B − V] = 2.11, Massey & Thompson 1991).…”