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Fundamental Parameters of Four Massive Eclipsing Binaries in Westerlund 1

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Fundamental Parameters of Four Massive Eclipsing Binaries in Westerlund 1 A&A 547, A30 (2012) Astronomy DOI: 10.1051/0004-6361/201219465 & c ESO 2012 Astrophysics Fundamental parameters of four massive eclipsing binaries in Westerlund 1 E. Koumpia1 and A. Z. Bonanos2 1 SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen; Kapteyn Institute, University of Groningen, The Netherlands e-mail: [email protected] 2 National Observatory of Athens, Institute of Astronomy, Astrophysics, Space Applications & Remote Sensing, I. Metaxa & Vas. Pavlou St., Palaia Penteli, 15236 Athens, Greece e-mail: [email protected] Received 24 April 2012 / Accepted 11 September 2012 ABSTRACT Context. Only a small number of high mass stars (>30 M) have fundamental parameters (i.e. masses and radii) measured with high enough accuracy from eclipsing binaries to constrain formation and evolutionary models of massive stars. Aims. This work aims to increase this limited sample, by studying the four massive eclipsing binary candidates discovered by Bonanos in the young massive cluster Westerlund 1. Methods. We present new follow-up echelle spectroscopy of these binaries and models of their light and radial velocity curves. Results. We obtain fundamental parameters for the eight component stars, finding masses that span a range of 10−40 M, and con- tributing accurate fundamental parameters for one additional very massive star, the 33 M component of W13. WR77o is found to have a ∼40 M companion, which provides a second dynamical constraint on the mass of the progenitor of the magnetar known in the cluster. We also use W13 to estimate the first, direct, eclipsing binary distance to Westerlund 1 and therefore the magnetar and find it to be at 3.7 ± 0.6 kpc. Conclusions. Our results confirm previous evidence for a high mass for the progenitor of the magnetar. In addition, the availabil- ity of eclipsing binaries with accurate parameters opens the way for direct, independent, high precision eclipsing binary distance measurements to Westerlund 1. Key words. stars: fundamental parameters – binaries: eclipsing – open clusters and associations: individual: Westerlund 1 – stars: early-type – stars: Wolf-Rayet – stars: massive 1. Introduction young massive “super star cluster” Westerlund 1 (Clark et al. 2005) in search of massive eclipsing binary candidates, as it is Improving stellar structure, formation and evolution models of one of the most massive young clusters known in the Local massive stars is limited by the accuracy with which their fun- Group, with an age of 4.5−5Myr(Crowther et al. 2006). Wd 1 is damental parameters (masses and radii) are known. Andersen of great interest, as it contains an assortment of rare evolved mas- (1991) reviewed the importance of accurate parameters from sive stars, such as blue, yellow and red supergiants, including a eclipsing binaries, while Torres et al. (2010) revisited the sub- rare supergiant B[e] star (Clark et al. 2005; Negueruela et al. ject, reviewing the advances made possible by the continuous 2010), 24 confirmed Wolf-Rayet stars (Crowther et al. 2006), improvements in observational accuracy. They also compiled a a luminous blue variable (Clark & Negueruela 2004), and a mag- catalog of detached systems with measurements accurate to ±3% netar (Muno et al. 2006), thereby making possible the determi- from the literature, which consists of 95 systems and extends nation of fundamental parameters of massive stars at different the mass range from 0.2 M to 27 M. The earliest-type star in evolutionary stages. The photometric variability search resulted the sample was an O7 star with a mass of 27.3 M . Bonanos in four massive eclipsing binary candidates (WDEB, W13, W36, (2009) found that the less stringent accuracy criterion of 10% is and WR77o; Bonanos 2007), which are the subject of this work. only fulfilled by 15 eclipsing binaries with masses above 30 M, In the following sections, we present follow-up spectroscopy this number having only increased to 17 (with the components and modeling of these four binaries, and the resulting component of Cyg OB2-17, measured by Stroud et al. 2010)inthelast parameters. We present the observations in Sect. 2, the radial three years. This astounding lack of accurate measurements of velocity measurements in Sect. 3, the models for each eclipsing the fundamental parameters of the highest mass stars, combined binary in Sect. 4, the comparison with evolutionary models in with the large parameter space of metallicity and evolutionary Sect. 5, an estimation of the distance in Sect. 6, and a discussion stage that they span, which is further complicated by multiplic- of our results and conclusions in Sect. 7. ity and in particular binary evolution, serves as the motivation for this work. 2. Observations and data reduction To improve the situation, a systematic survey of the most massive stars in the Local Group has been undertaken (see Multi-epoch spectroscopy of the four eclipsing binaries was ob- Bonanos 2010, for an overview). As part of this broader sur- tained during seven nights in 2007−2008 and one night in 2011 vey, Bonanos (2007) conducted the first variability survey of the using the MIKE spectrograph (Bernstein et al. 2003)atthe Article published by EDP Sciences A30, page 1 of 10 A&A 547, A30 (2012) Table 1. Log of Magellan spectroscopic observations. Table 2. Radial velocity measurements. Binary HJD Phase RV ± σ (O−C) RV ± σ (O−C) UT date Instru- Resolu- Disper- Total exposure time (s)/S/N 1 RV 1 1 2 RV 2 2 − −1 −1 −1 −1 ment tion sion 2 450 000 φ (km s )(kms)(kms)(kms) − ± − ± − (λ/Δλ)(Å/pix) W W36 W13 WR77o WDEB 4167.87328 0.42 188 35 19 105 11 16 DEB ± − ± − 01/03/2007 MIKE 25 500 0.05 . 1800/14 1440/30 4800/7 4168.86327 0.64 95 35 56 165 11 17 − ± − ± − 08/03/2007 MIKE 24 200 0.10 3600/22 2700/38 1440/50 4800/12 4620.60505 0.22 205 35 40 63 11 54 5805.52158 0.66 130 ± 35 75 −165 ± 11 −6 09/03/2007 MIKE 24 200 0.10 3600/20 . 1440/55 3300/14 ± − ± − 15/03/2007 MIKE 24 200 0.10 . 2700/42 1260/49 3600/14 W36 4160.79354 0.90 100 32 27 200 16 16 4167.79030 0.10 −170 ± 32 −25 110 ± 16 4 28/06/2007 MOE 21 000 0.25 . 1800/31 ... 4168.78891 0.41 −160 ± 32 −33 95 ± 16 −10 29/06/2007 MOE 21 000 0.25 . 2400/20 2400/35 ... 4174.78780 0.30 −200 ± 32 −3 170 ± 16 −24 30/06/2007 MOE 21 000 0.25 . 3600/17 ... 4280.76847 0.61 100 ± 32 25 −200 ± 16 1 01/07/2007 MOE 21 000 0.25 . 2400/35 . 2400/7 4282.66772 0.21 −180 ± 32 19 165 ± 16 −34 03/06/2008 MIKE 24 000 0.10 4800/13 900/14 1800/28 4800/7 4708.56044 0.09 . 100 ± 16 0 30/08/2008 MIKE 23 800 0.15 . 2700/56 1440/81 3600/27 4710.55897 0.72 100 ± 32 −27 −270 ± 16 5 01/09/2008 MIKE 23 800 0.15 . 2700/54 1440/80 3600/20 W13 4160.77158 0.88 . −155 ± 13 7 01/09/2011 MIKE 24 200 0.10 3600/32 ... ... ... 4167.76394 0.64 85 ± 23 −16 −190 ± 13 −20 4168.76923 0.74 145 ± 23 −2 −210 ± 13 −1 4174.76249 0.39 −180 ± 23 −713± 13 −7 4279.61631 0.71 . −230 ± 13 −28 6.5 m Magellan (Clay) telescope at Las Campanas Observatory, − ± × 4280.58040 0.81 . 180 13 17 Chile. This camera uses a 2048 4096 SITe CCD with a 4281.62310 0.92 . −125 ± 13 6 −1 −1 pixel size of 15 μmpixel and a pixel scale of 0. 13 pixel . 4708.48711 0.99 . −95 ± 13 −19 The 1.0 × 5.0 slit yielded a spectral resolution R ∼ 24 000 4710.48815 0.20 −250 ± 23 −11 75 ± 13 8 − (12 km s 1; the exact value depending on the binning used) WR77o 4160.84107 0.98 . −25 ± 30 58 at 7000 Å, as measured from the full width at half maxi- 4167.83559 0.97 . −15 ± 30 97 mum of the comparison lamp lines. Additional spectra were ob- 4168.82036 0.25 . 290 ± 30 −35 − ± tained over four consecutive nights in 2007 with the Inamori- 4174.82619 0.95 . 35 30 108 4282.49396 0.54 . −230 ± 30 −100 Magellan Areal Camera and Spectrograph (IMACS) multi- 4620.66407 0.61 . −340 ± 30 −74 object echelle (MOE) spectrograph (Dressler et al. 2011)onthe − ± − 4708.52114 0.57 . 250 30 61 6.5 m Magellan (Baade) telescope. The 0.6 slit yielded a re- 4710.51970 0.14 . 255 ± 30 13 solving power of R = 21 000 (14 km s−1). The IMACS spectra were reduced and extracted with the 3. Radial velocity analysis Carnegie Observatories System for MultiObject Spectroscopy (COSMOS) pipeline (version 2.15), while the MIKE spec- The radial velocities (RVs) were determined with a χ2 minimiza- tra with the MIKE python pipeline (version 2.5.4), written tion method, which compares the observed spectrum to a grid of by D. Kelson (Kelson et al. 2000; Kelson 2003). The extracted synthetic spectra. Each synthetic spectrum was constructed as a orders for each star were averaged, normalized, and merged, sum of two models, each shifted over a range of radial velocities yielding a wavelength range 5450−9400 Å for the MIKE and with a velocity step of 5 km s−1. We used TLUSTY models from 6380−8750 Å for the IMACS spectra, since the large amount the OSTAR2002 by Lanz & Hubeny (2003) and BSTAR2006 of reddening toward Westerlund 1 (AV ∼ 11 mag; Negueruela by Lanz & Hubeny (2007) grids for the components of each et al.
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