On the nature of the galactic early-B hypergiants

Abstract

Aims. Despite their importance to a number of astrophysical fields, the lifecycles of very massive stars are still poorly defined. In order to address this shortcoming, we present a detailed quantitative study of the physical properties of four early-B hypergiants (BHGs) of spectral type B1-4 Ia + ; Cyg OB2 #12, ζ 1 Sco, HD 190603 and BP Cru. These are combined with an analysis of their long-term spectroscopic and photometric behaviour in order to determine their evolutionary status. Methods. Quantitative analysis of UV-radio photometric and spectroscopic datasets was undertaken with a non-LTE model atmosphere code in order to derive physical parameters for comparison with apparently closely related objects, such as B supergiants (BSGs) and luminous blue variables (LBVs), and theoretical evolutionary predictions. Results. The long-term photospheric and spectroscopic datasets compiled for the early-B HGs revealed that they are remarkably stable over long periods (≥40 yrs), with the possible exception of ζ 1 Sco prior to the 20th century; in contrast to the typical excursions that characterise LBVs. Quantitative analysis of ζ 1 Sco, HD 190603 and BP Cru yielded physical properties intermediate between BSGs and LBVs; we therefore suggest that BHGs are the immediate descendants and progenitors (respectively) of such stars, for initial masses in the range ∼30−60 M . Comparison of the properties of ζ 1 Sco with the stellar population of its host cluster/association NGC 6231/Sco OB1 provides further support for such an evolutionary scenario. In contrast, while the wind properties of Cyg OB2 #12 are consistent with this hypothesis, the combination of extreme luminosity and spectroscopic mass (∼110 M ) and comparatively low temperature means it cannot be accommodated in such a scheme. Likewise, despite its co-location with several LBVs above the Humphreys-Davidson (HD) limit, the lack of long term variability and its unevolved chemistry apparently excludes such an identification. Since such massive stars are not expected to evolve to such cool temperatures, instead traversing an O4-6Ia→O4-6Ia + →WN7-9ha pathway, the properties of Cyg OB2 #12 are therefore difficult to understand under current evolutionary paradigms. Finally, we note that as with AG Car in its cool phase, despite exceeding the HD limit, the properties of Cyg OB2 #12 imply that it lies below the Eddington limit -thus we conclude that the HD limit does not define a region of the HR diagram inherently inimical to the presence of massive stars.

A VLT/FLAMES survey for massive binaries in Westerlund 1

“…The proximity of Wd1-5 to the Eddington limit, together with influence of the wind/photosphere transition region translates into a relatively large uncertainty in our derived value at T eff = 21 900 +1500 −1200 K (cf. comparable modelling of the early BHGs Cyg OB2 #12, ζ 1 Sco and HD 190603;Clark et al 2012). This process is shown in detail in Fig.…”

“…Illustrative spectra of selected, closely related early-B hypergiants and WNVLh stars are also shown (with BP Cru and Wd1-13 overplotted in dashed cyan and green lines, respectively, to save space). Spectra of Wd1-13 and 44 were not available in the 5600−6000 Å window, while data on ζ 1 Sco and BP Cru were from Clark et al (2012) and Kaper et al (2006). Both Wd1-13 and -44 are SB2 binaries, although a precise spectral classification of their secondaries is uncertain at this time.…”

“…Given the spectroscopic similarity to galactic BHGs such as ζ 1 Sco and BP Cru (Figs. 2−4) we adopted the methodology employed by Clark et al (2012).…”

“…To estimate log g we made use of the Paschen lines in the highresolution and signal to noise (S/N) I-band spectrum, as they provide the best constraints for the surface gravity, especially the run of the line overlap among the higher members (see Clark et al 2012; Fig. 3).…”

Catalogue of particle-accelerating colliding-wind binaries

“…A multiplicity investigation is still lacking for this object, that is the lastest type star listed in the catalogue. This star presents properties reminiscent of LBV-like objects (Clark et al 2012). …”