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Register a new userA VLT/FLAMES survey for massive binaries in Westerlund 1 V. the X-ray selected blue stragglers Wd1-27 and -30a
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Observational studies suggest that many OB stars are found within binary systems which may be expected to interact during their lifetimes. Significant mass transfer or merger of both components will 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 outcomes of binary evolutionary channels are to be quantified. The massive cluster Westerlund 1 hosts a rich population of X-ray bright stars where the emission is thought to derive from the wind collision zones of massive binaries. Selected on this basis, we present the results of a multiwavelength analysis of the X-ray luminous O stars Wd1-27 and -30a. We find both 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 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. Analysis of Gaia proper motion and parallactic data indicates that both are cluster members; we also provide a membership list for Wd1 based on this analysis. The presence of hypergiants of spectral types O to M within Wd1 cannot be understood 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 (Abridged).
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We investigate the characteristics of two newly discovered short-period, double-lined, massive binary systems, VFTS 450 (O9.7$;$II--Ib$,$+$,$O7::) and VFTS 652 (B1$;$Ib$,+,$O9:$;$III:). We perform model-atmosphere analyses to characterise the photospheric properties of both members of each binary (denoting the `primary as the spectroscopically more conspicuous component). Radial velocities and optical photometry are used to estimate the binary-system parameters. We estimate $T_{rm eff}=27$ kK, $log{(g)}=2.9$ (cgs) for the VFTS 450 primary spectrum (34kK, 3.6: for the secondary spectrum); and $T_{rm eff} = 22$kK, $log{(g)}=2.8$ for the VFTS 652 primary spectrum (35kK, 3.7: for the secondary spectrum). Both primaries show surface nitrogen enrichments (of more than 1 dex for VFTS 652), and probable moderate oxygen depletions relative to reference LMC abundances. We determine orbital periods of 6.89d and 8.59d for VFTS 450 and VFTS 652, respectively, and argue that the primaries must be close to filling their Roche lobes. Supposing this to be the case, we estimate component masses in the range $sim$20--50M$_odot$. The secondary spectra are associated with the more massive components, suggesting that both systems are high-mass analogues of classical Algol systems, undergoing case-A mass transfer. Difficulties in reconciling the spectroscopic analyses with the light-curves and with evolutionary considerations suggest that the secondary spectra are contaminated by (or arise in) accretion disks.
We estimate physical parameters for the late-type massive stars observed as part of the VLT-FLAMES Tarantula Survey (VFTS) in the 30 Doradus region of the Large Magellanic Cloud (LMC). The observational sample comprises 20 candidate red supergiants (RSGs) which are the reddest (($B-V$) $>$ 1 mag) and brightest ($V$ $<$ 16 mag) objects in the VFTS. We use optical and near-IR photometry to estimate their temperatures and luminosities, and introduce the luminosity-age diagram to estimate their ages. We derive physical parameters for our targets, including temperatures from a new calibration of $(J-K_{rm s})_{0}$ colour for luminous cool stars in the LMC, luminosities from their $J$-band magnitudes (thence radii), and ages from comparisons with state-of-the-art evolutionary models. We show that interstellar extinction is a significant factor for our targets, highlighting the need to take it into account in analysis of the physical parameters of RSGs. We find that some of the candidate RSGs could be massive AGB stars. The apparent ages of the RSGs in the Hodge 301 and SL 639 clusters show a significant spread (12-24 Myr). We also apply our approach to the RSG population of the relatively nearby NGC 2100 cluster, finding a similarly large spread. We argue that the effects of mass-transfer in binaries may lead to more massive and luminous RSGs (which we call `red stragglers) than expected from single-star evolution, and that the true cluster ages correspond to the upper limit of the estimated RSG ages. In this way, the RSGs can serve as a new and potentially reliable age tracer in young star clusters. The corresponding analysis yields ages of 24$^{+5}_{-3}$ Myr for Hodge 301, 22$^{+6}_{-5}$ Myr for SL 639, and 23$^{+4}_{-2}$ Myr for NGC 2100.
We propose a formation mechanism for twin blue stragglers (BSs) in compact binaries that involves mass transfer from an evolved outer tertiary companion on to the inner binary via a circumbinary disk. We apply this scenario to the observed double BS system Binary 7782 in the old open cluster NGC 188, and show that its observed properties are naturally reproduced within the context of the proposed model. We predict the following properties for twin BSs: (1) For the outer tertiary orbit, the initial orbital period should lie between 220 days $lesssim$ P$_{rm out}$ $lesssim$ 1100 days, assuming initial masses for the inner binary components of $m_{rm 1} = 1.1$ M$_{odot}$ and $m_{rm 2} =$ 0.9 M$_{odot}$ and an outer tertiary mass of $m_{rm 3} = 1.4$ M$_{odot}$. After Roche-lobe overflow, the outer star turns into a white dwarf (WD) of mass 0.43 to 0.54,MSun. There is a correlation between the mass of this WD and the outer orbital period: more massive WDs will be on wider orbits. (3) The rotational axes of both BSs will be aligned with each other and the orbital plane of the outer tertiary WD. (4) The BSs will have roughly equal masses, independent of their initial masses (since the lower mass star accretes the most). The dominant accretor should, therefore, be more enriched by the accreted material. Hence, one of the BSs will appear to be more enriched by either He, C and O or by s-process elements, if the donor started Roche lobe overflow on, respectively, the red giant or asymptotic giant branch. (5) Relative to old clusters, twin BSs in close binaries formed from the proposed mechanism should be more frequent in the Galactic field and open clusters with ages $lesssim$ 4-6 Gyr, since then the donor will have a radiative envelope. (6) The orbit of the binary BS will have a small semi-major axis (typically $aplt 0.3$,au) and be close to circular ($e aplt 0.2$).
We present a number of notable results from the VLT-FLAMES Tarantula Survey (VFTS), an ESO Large Program during which we obtained multi-epoch medium-resolution optical spectroscopy of a very large sample of over 800 massive stars in the 30 Doradus region of the Large Magellanic Cloud (LMC). This unprecedented data-set has enabled us to address some key questions regarding atmospheres and winds, as well as the evolution of (very) massive stars. Here we focus on O-type runaways, the width of the main sequence, and the mass-loss rates for (very) massive stars. We also provide indications for the presence of a top-heavy initial mass function (IMF) in 30 Dor.
The VLT-FLAMES Survey of Massive Stars was an ESO Large Programme to understand rotational mixing and stellar mass-loss in different metallicity environments, in order to better constrain massive star evolution. We gathered high-quality spectra of over 800 stars in the Galaxy and in the Magellanic Clouds. A sample of this size is unprecedented, enabled by the first high-resolution, wide-field, multi-object spectrograph on an 8-m telescope. We developed spectral analysis techniques that, in combination with non-LTE, line-blanketed model atmospheres, were used to quantitatively characterise every star. The large sample, combined with the theoretical developments, has produced exciting new insights into the evolution of the most massive stars.
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