The circumstellar environment and evolutionary state of the supergiant B[e] star Wd1-9

Abstract

Context. Historically, supergiant (sg)B[e] stars have been difficult to include in theoretical schemes for the evolution of massive OB stars. Aims. The location of Wd1-9 within the coeval starburst cluster Westerlund 1 means that it may be placed into a proper evolutionary context and we therefore aim to utilise a comprehensive multiwavelength dataset to determine its physical properties and consequently its relation to other sgB[e] stars and the global population of massive evolved stars within Wd1. Methods. Multi-epoch R-and I-band VLT/UVES and VLT/FORS2 spectra are used to constrain the properties of the circumstellar gas, while an ISO-SWS spectrum covering 2.45-45 μm is used to investigate the distribution, geometry and composition of the dust via a semi-analytic irradiated disk model. Radio emission enables a long term mass-loss history to be determined, while X-ray observations reveal the physical nature of high energy processes within the system. Results. Wd1-9 exhibits the rich optical emission line spectrum that is characteristic of sgB[e] stars. Likewise its mid-IR spectrum resembles those of the LMC sgB[e] stars R66 and 126, revealing the presence of equatorially concentrated silicate dust, with a mass of ∼10 −4 M . Extreme historical and ongoing mass loss ( 10 −4 M yr −1 ) is inferred from the radio observations. The X-ray properties of Wd1-9 imply the presence of high temperature plasma within the system and are directly comparable to a number of confirmed short-period colliding wind binaries within Wd1. Conclusions. The most complete explanation for the observational properties of Wd1-9 is that it is a massive interacting binary currently undergoing, or recently exited from, rapid Roche-lobe overflow, supporting the hypothesis that binarity mediates the formation of (a subset of) sgB[e] stars. The mass loss rate of Wd1-9 is consistent with such an assertion, while viable progenitor and descendent systems are present within Wd1 and comparable sgB[e] binaries have been identified in the Galaxy. Moreover, the rarity of sgB[e] stars -only two examples are identified from a census of ∼68 young massive Galactic clusters and associations containing ∼600 post-Main Sequence stars -is explicable given the rapidity (∼10 4 yr) expected for this phase of massive binary evolution.

Resolving the clumpy circumstellar environment of the B[e] supergiant LHA 120-S 35

“…Recently, the LHA 120-S 35 membership to the cluster SL 482 has been discussed by Bitsakis et al (2017), who were able to determine the members of the star cluster using a robust decontamination technique, providing a probability of more than 92% that LHA 120-S 35 is member of the cluster. The previously reported B[e]SGs associated to open clusters are: LHA 120-S 111, which belongs to the compact cluster NGC 1994 of the LMC (Melgarejo et al 2001;Lortet & Testor 1988) and Wd1-9, which is a member of the massive Galactic stellar cluster Westerlund 1 (Clark et al 2013b;Fenech et al 2017). The small number of B[e]SGs found in stellar clusters is reasonable taking into account that their lifetime in this post-main-sequence phase is relatively short.…”

“…Inspection of the profiles of the He i λ 5876 line, which seems to be the only unblended absorption line in the optical spectrum of LHA 120-S 35 reveals that this line also shows variability in its profile that might originate from stellar pulsations. Alternatively, even though evidence indicative of binarity has not yet been found in LHA 120-S 35, a Roche lobe overflow phase in an interacting binary or a colliding-wind binary system could also be suitable scenarios to analyse the complex circumstellar ring-like (and possibly spiral) structures of LHA 120-S 35, as for example in RY Scuti (Smith et al 2011) or Wd1-9 (Clark et al 2013b). However, emission in key lines attributed to binarity as N iii λλ 4634-42 and He ii λ 4686 is not seen, conversely to what was detected in B[e]SGs known to be binaries like LHA 115-S 18 (Clark et al 2013a) and HD 38489 (Massey et al 2014).…”

A VLT/FLAMES survey for massive binaries in Westerlund 1

“…In particular we highlight the binary supergiant B[e] star Wd1-9, which currently appears to be undergoing the rapid case A evolution we hypothesise for Wd1-5 ( Fig. 10; Clark et al 2013). …”

Multiwavelength observations of NaSt1 (WR 122): equatorial mass loss and X-rays from an interacting Wolf–Rayet binary

“…The presence of strong He ii emission in NaSt1, however, suggests a significantly hotter ionization source than that of Wd1-9, which does not exhibit He ii emission. Clark et al (2013) interpreted the double-peaked line profiles of Wd1-9 as being the result of quasi-Keplerian rotation in a massive disk or torus. This interpretation has also been invoked to explain the double-peaked line profiles detected in the LBV binary MWC 314 (Lobel et al 2013; also shown in Figure 9).…”