A VLT/FLAMES survey for massive binaries in Westerlund 1 V. the X-ray selected blue stragglers Wd1-27 and -30a

Abstract

Context. Recent observational studies indicate that a large number of OB stars are found within binary systems which may be expected to interact during their lifetimes. Significant mass transfer or indeed merger of both components is expected to modify evolutionary pathways, facilitating the production of exceptionally massive stars which will present as blue stragglers. Identification and characterisation of such objects is crucial if the efficiency of mass transfer is to be established; a critical parameter in determining the outcomes of binary evolutionary channels. Aims. The young and coeval massive cluster Westerlund 1 hosts a rich population of X-ray bright OB and Wolf-Rayet stars where the emission is attributed to shocks in the wind collision zones of massive binaries. Motivated by this, we instigated a study of the extremely X-ray luminous O supergiants Wd1-27 and -30a. Methods. We subjected a multi-wavelength and -epoch photometric and spectroscopic dataset to quantitative non-LTE model atmosphere and time-series analysis in order to determine fundamental stellar parameters and search for evidence of binarity. A detailed examination of the second Gaia data release was undertaken to establish cluster membership. Results. Both stars were found to be early/mid-O hypergiants with luminosities, temperatures and masses significantly in excess of other early stars within Wd1, hence qualifying as massive blue stragglers. The binary nature of Wd1-27 remains uncertain but the detection of radial velocity changes and the X-ray properties of Wd1-30a suggest that it is a binary with an orbital period ≤ 10 days. Analysis of Gaia proper motion and parallactic data indicates that both stars are cluster members; we also provide a membership list for Wd1 based on this analysis. Conclusions. The presence of hypergiants of spectral types O to M within Wd1 cannot be understood solely via single-star evolution. We suppose that the early-B and mid-O hypergiants formed via binary-induced mass-stripping of the primary and mass-transfer to the secondary, respectively. This implies that for a subset of objects massive star-formation may be regarded as a two-stage process, with binary-driven mass-transfer or merger yielding stars with masses significantly in excess of their initial 'birth' mass.

A VLT/FLAMES survey for massive binaries in Westerlund 1: VI. Properties of X-ray bright massive cluster members

“…Detailed classifications of the components are not possible due to the blending of the Paschen series lines but the absence of cool N i photospheric lines precludes a spectral type of B2 or later and by comparison to other spectra we suspect they are substantially earlier. By contrast Wd1-27 is similar to Wd1-30a, showing evidence for substantial mass accretion in a binary, leading to a classification of O7-8 Ia + and log(L bol /L ⊙ ) ∼ 5.97 +0.15 −0.10 (Clark et al 2018). No RV motion has been detected for this object; hence we cannot distinguish between e.g.…”

“…No RV motion has been detected for this object; hence we cannot distinguish between e.g. a binary with an extreme current mass ratio observed under an unfavourable inclination or the product of binary merger (Clark et al 2018).…”

“…With a spectral type of O4-5 Ia + it is the hottest OB star within Wd1 and has a luminosity of log(L bol /L ⊙ ) ∼ 5.87 +0.15 −0.10 , making it a factor of 5× more X-ray luminous than expected for a single star. Indeed the hardness of the emission implies an additional (dominant) contribution from a WCZ, with RV data indicating it still resides in a binary with a likely period of 10 days (Clark et al 2018).…”

“…2 and 3.2, Wd1-24 would also be included in this latter grouping if its binary nature were confirmed. (Clark et al 2018). Given this uncertainty and the quality of the X-ray spectra, re-analysis of extant data appears premature.…”

“…Unlike Wd1-9, detailed analysis of the dominant visible component of Wd1-30a is possible, and suggests that it is the massive, rejuvenated secondary in a post-interaction binary (Clark et al 2018). With a spectral type of O4-5 Ia + it is the hottest OB star within Wd1 and has a luminosity of log(L bol /L ⊙ ) ∼ 5.87 +0.15 −0.10 , making it a factor of 5× more X-ray luminous than expected for a single star.…”