We present initial results from a long-baseline radial velocity survey for massive binaries in the cluster Westerlund 1. Four systems are examined: the dust-producing WC binary W239, the double-lined eclipsing binary W13, and the single-lined B0 supergiants W43a and W3003. Finally, the evolutionary implications for the population of massive stars in Westerlund 1 are discussed

Clark, J. S.

Negueruela, I.

Ritchie, B. W.

English

arXiv

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThe massive binary population of the starburst clusterWesterlund 1Journal ItemHow to cite:Ritchie, B. W.; Clark, J. S. and Negueruela, I. (2010). The massive binary population of the starburst clusterWesterlund 1. Bulletin de la Societe Royales des Sciences de Lie`ge, 80 pp. 628–633.For guidance on citations see FAQs.c© Not knownVersion: Accepted ManuscriptLink(s) to article on publisher’s website:http://www.srsl-ulg.net/Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukarXiv:1010.6270v1  [astro-ph.SR]  29 Oct 2010The massive binary population of the starburst clusterWesterlund 1∗B.W. Ritchie1, J.S. Clark1 and I. Negueruela21 Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, U.K.2 Departamento de Fı´sica, Ingenierı´a de Sistemas y Teorı´a de la Sen˜al, Universidad de Alicante,Apdo. 99, 03080 Alicante, SpainAbstract: We present initial results from a long-baseline radial velocity survey for massive binaries in the clus-ter Westerlund 1. Four systems are examined: the dust-producing WC binary W239, the double-lined eclipsingbinary W13, and the single-lined B0 supergiants W43a and W3003. Finally, the evolutionary implications forthe population of massive stars in Westerlund 1 are discussed.1 IntroductionThe galactic starburst cluster Westerlund 1 (hereafter Wd1; Westerlund 1987; Clark et al. 2005)contains a rich, coeval population of massive stars that trace both the hot (OB supergiant, Wolf-Rayet)and cool (yellow hypergiant, red supergiant) phases of post-Main Sequence evolution. Motivated byX-ray, infra-red and radio observations that show Wd1 to be binary-rich (Crowther et al. 2006; Clarket al. 2008; Dougherty et al. 2010), we have undertaken an intensive multi-epoch radial velocity (RV)survey of Wd1 in order to obtain a census of massive binaries amongst both the highly-luminoustransitional supergiants and the lower-luminosity stars just evolving off the main sequence. In theseproceedings we discuss four massive binary systems identified by our survey, along with the possibleimplications of these objects for binary-mediated evolution in Wd1.2 ObservationsA list of targets discussed here is given in Table 1. With the exception of W3003, which is a newly-identified cluster member (Ritchie et al. 2009a), designations are from Westerlund (1987); alternateWR designations for the Wolf-Rayet population from Clark & Negueruela (2002) and Crowther etal. (2006) are also given in the text where appropriate. Data were obtained on 11 nights between20/06/2008 and 20/08/2009, using the FLAMES multi-object spectrograph on VLT UT2 Kueyenat Cerro Paranal, Chile. Setup HR21 was used to cover the 8484-9001A˚ range with a resolving∗Based on observations collected at the European Southern Observatory under programmes ESO 81.D-0324 and383.D-0633“The multi-wavelength view of hot, massive stars”; 39th Lie`ge Int. Astroph. Coll., 12-16 July 2010Table 1: List of targets.ID Spectral Type Period (days) RA (J2000) Dec (J2000) Ra Ia NotesbW13 B0.5 Ia++OB 9.27 16 47 06.45 -45 50 26.0 14.63 12.06 X, EW43a B0 Ia 16.27 16 47 03.54 -45 50 57.3 15.22 12.26 AW239 (F) WC9d 6.5 16 47 05.21 -45 52 25.0 15.39 12.90 X, AW3003 B0 Ib 11.12 16 47 11.60 -45 49 22.4 16.21 13.31 AaPhotometric R and I-band magnitudes are taken from Clark et al. (2005) or Bonanos (2007).bX-ray sources (Clark et al. 2008), Eclipsing or Aperiodic variables (Bonanos 2007).power ∼16200; target selection, data acquisition and reduction are described in detail in Ritchie et al.(2009a).Radial velocities were measured by fitting Gaussian profiles to the cores of strong absorptionand/or emission lines using the IRAF ngaussfit routines, with the derived velocity an error-weightedaverage of individual lines. The Paschen 11-15 lines were used to obtain radial velocities for W13,W43a and W3003, with the Pa16 λ8502 line that also falls within our coverage excluded due toblending with an adjacent C III λ8500 line that strengthens rapidly at B0.5 and earlier (Negueruela,Clark & Ritchie, 2010). In the case of the dusty Wolf-Rayet W239 (WR F; Clark & Negueruela 2002),strong C III λλ8500, 8664 emission lines were used for radial velocity measurement (see Clark et al.2010). A well-defined DIB at ∼8620A˚ provides a serendipitous check for zero-point errors in ourdata, with spectra showing epoch-to-epoch variability of well under 1 km s−1.3 Results3.1 The double-lined eclipsing binary W13-300-200-100 0 100 200-0.2  0  0.2  0.4  0.6  0.8  1  1.2radial velocity (kms-1 )phaseabsorptionemissionFigure 1: RV curve for the double-lined eclipsing binary W13. T0 is at MJD=54643.080, whichcorresponds to the eclipse of the B0.5Ia+ emission-line star.W13 was identified as an eclipsing binary system by Bonanos (2007), with Ritchie et al. (2009a,2010) finding it to be a double-lined system consisting of a peculiar O9.5-B0.5 supergiant and anB0.5 Ia+ emission-line object. The strong similarities in spectral morphology between W13, theWN9h star W44 (WR L) and the WN10-11h star W5 (WR S) suggest that the B0.5 Ia+ star is an im-mediate evolutionary precursor to the Wolf-Rayet phase, although the weakness of the He I emissionlines and absence of N II λ6611 emission make it the least-evolved member of the WNL populationin Wd1.Results from our full FLAMES dataset presented by Ritchie et al. (2010) show W13 to have anorbital period of 9.2709±0.0015 days, with lower limits for the masses of the emission-line objectand supergiant companion of 21.4 ± 2.6M⊙ and 32.8 ± 4.0M⊙ respectively, rising to 23.2+3.3−3.0M⊙and 35.4+5.0−4.6M⊙ for our best-fit inclination 62+3−4 degrees. The evolved state, short orbital periodand near-contact configuration all suggest strong interaction during the evolution of the system, withcomparison with the evolutionary models of Petrovic, Langer & van der Hucht (2005) suggestinghighly non-conservative late-Case A/Case B mass transfer and an initial mass for the emission-lineobject of ∼40M⊙. This implies that the magnetar CXOU J164710.2-455216 formed from an evenmore massive progenitor, with close binary evolution apparently instrumental in shedding sufficientmass to avoid formation of a black hole (Clark et al. 2008; Ritchie et al. 2010).3.2 The WC9d binary W239 (WR F).The dust-forming WC9 star W239 (WR F) was noted by Ritchie et al. (2009a) as showing RVchanges consistent with binarity, and our full FLAMES dataset confirms a period of ∼6.5 days anda semi-amplitude of ∼45 km s−1 that is consistent with a WR+O binary viewed at i∼10–20◦. W239shows strong near-mid IR excess and dilute K-band spectrum, both indicative of hot circumstellardust (Crowther et al. 2006). However, the ∼6.5d orbital period is a factor of ∼5 shorter than anyother known dust-forming WC star, and implies likely Case A or contact evolution in a very closebinary (∼4 days) with subsequent wind-driven mass loss widening the orbit (Petrovic et al. 2005).W239 is examined in detail by Clark et al. (2010).3.3 The B0 supergiant binaries W43a and W3003.-160-140-120-100-80-60-40-20 0 20-0.2  0  0.2  0.4  0.6  0.8  1  1.2radial velocity (km/s)phaseradial velocity (km/s)Figure 2: RV curve for the single-lined binary W43a (B0 Ia), with T0 corresponding to the start of ourobservations at MJD=54646.185W43a (B0 Ia; Negueruela et al. 2010) displays an unremarkableR-band spectrum with weak windemission lines from Hα and C II, and He I in absorption. The star is not detected at X-ray or radiowavelengths (Clark et al. 2008; Dougherty et al. 2010), and no eclipses are apparent in the photometryof Bonanos (2007). Nevertheless, the I-band FLAMES data reveal significant RV changes on atimescale of a few days, although no indication of a companion is seen in the spectrum. A fit toeleven epochs of RV data gives a period of 16.266± 0.005 days, a semi-amplitude of 71 ± 6 km s−1and a systemic velocity of −67 ± 4 km s−1; the latter is somewhat blueshifted with respect to othermembers of Wd1, but this effect is seen also in the OB supergiant companion in W13 and discrepantsystemic velocities are commonly observed in early-type spectroscopic binaries (see Ritchie et al.2010 and refs. therein).The B0 Ib supergiant W3003 (Ritchie et al. 2009a), located to the north-east of the cluster, appearsa similar system to W43a. Our RV data find a period of 11.12 ± 0.01 days and a slightly eccentricorbit (e ∼< 0.05), with a systemic velocity of −39±8 km s−1 and a semi-amplitude of 38±5 km s−1.Once again, no indications of binarity are found in other observations3. These two supergiants aretherefore of interest as the first examples that previous estimates of the binary fraction of Wd1 basedon the signature of colliding-wind systems (in which both components must be sufficiently massiveto support a powerful stellar wind) are incomplete. In the case of W43a, assuming a ∼35M⊙ primaryand an inclination of ∼35–45◦ implies a main sequence secondary with a mass ∼15–21M⊙; lower in-clinations would suggest a higher-mass secondary that should be directly visible in our spectra, whilehigher inclinations would result in a detectable eclipsing system. Similarly, the low semi-amplitude ofW3003 suggests either a very low inclination and/or a low-mass companion. The intrinsic X-ray lu-minosity of individual OB supergiants in Wd1 (Lx ∼< 1032 erg s−1; Clark et al. 2008) is insufficient fordirect detection, and unequal-mass OB supergiant+main sequence binaries such as W43a and W3003will lack the strong wind interaction required to significantly raise their X-ray luminosities. Neithersystem is expected to have begun strong binary interaction, but once shell burning commences theprimary will rapidly lose its Hydrogen envelope via Case B mass transfer, leaving a WR+O binarywith an orbital period of a few weeks (Petrovic et al. 2005).4 Evolutionary implications 5.5 5.6 5.7 5.8 5.9 6 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2log (L/Lo)log TM0IK0IG0IA0I40Mo40Mo (rot)25Mo (rot)243 (LBV)16a42657,3323726RSGslog (L/Lo)log (L/Lo)Figure 3: Location of luminous hypergiants and red supergiants in Wd1 compared to evolutionarytracks with and without rotation (Meynet & Maeder 2003).The OB supergiant population of Wd1 suggests a single burst of star formation lasting less than1 Myr and a cluster age of ∼5 Myr (Negueruela et al. 2010). The ∼40M⊙ main sequence mass of theemission-line object in W13 and lack of evidence for significant non-coevality in the cluster thereforesuggests that the ten highly-luminous B5-F8 Ia+ hypergiants in Wd1 all evolved from progenitorswith Mini ∼>35M⊙. Although these objects appear to be in good agreement with evolutionary tracks3Both W43a and W3003 are identified as aperiodic variables in the photometry of Bonanos (2007), but this likelyreflects pulsational instability seen in all stars later than ∼B0 in Wd1 (Ritchie et al. 2009a).including rotation (Meynet & Maeder 2003), these models do not predict further evolution to thered supergiant (RSG) phase for stars in this mass range. In contrast, the transitional population inWd1 also contains four RSGs (see Figure 3), while the early-A LBV W243 (Ritchie et al. 2009b)also displays nitrogen enrichment and oxygen and carbon depletion suggestive of CNO-processedmaterial ‘dredged up’ during a previous RSG phase.Lack of contemporaneous spectroscopy and photometry means that the luminosities of the Wd1RSGs are somewhat uncertain, although the spectral types, derived from TiO bandhead strengths,are secure. Estimates of the luminosity of the RSG W26 (M1–6 Ia) suggest log(L/L⊙)∼5.8, and aconsequent radius possibly as large as ∼2000 R⊙, while non-LTE modelling of the LBV W243 yieldsR ∼ 450(d/4.5kpc) R⊙. Such objects are clearly incompatible with close binary evolution channelsin which stars are separated by ∼< 100 R⊙, and the distribution of Wolf-Rayets and cool hypergiantsin Wd1 therefore hints at a split evolutionary sequence in which the close binary population undergostrong interaction as the primary evolves off the main sequence, becoming WR+O binaries like W13and W239, while isolated (or long-period binary) stars become B–F hypergiants en route to the RSGphase. Further observations of the transitional hypergiant, RSG and Wolf-Rayet populations willallow this hypothesis to be tested directly, and this topic is explored further in Clark et al. (2010).AcknowledgementsWe thank the referee for a thorough reading of this manuscript and helpful comments. JSC acknowl-edges support from an RCUK fellowship. IN has been funded by grants AYA2008-06166-C03-03 andConsolider-GTC CSD-2006-00070 from the Spanish Ministerio de Ciencia e Innovacio´n (MICINN).ReferencesBonanos, A.Z., 2007, AJ, 133, 2696Clark J.S. & Negueruela, I., 2002, A&A, 396, L25Clark J.S., Negueruela I., Crowther P.A. & Goodwin, S.P., 2005, A&A, 434, 949Clark J.S., Muno M.P., Negueruela I., et al., 2008, A&A, 477, 147Clark J.S., Ritchie B.W., Negueruela I., et al., 2010, A&A, submittedCrowther P.A., Hadfield L.J, Clark J.S., et al., 2006, MNRAS, 372, 1407Dougherty S.M., Clark J.S., Negueruela I., et al., 2010, A&A, 511, A58Meynet, G. & Maeder, A., 2003, A&A, 404, 975Negueruela I., Clark J.S. & Ritchie B.W., 2010, A&A, 516, A78Petrovic J., Langer N. & van der Hucht, K.A., 2005, A&A, 435, 1013Ritchie B.W., Clark J.S., Negueruela, I. & Crowther, P.A., 2009a, A&A, 507, 1585Ritchie B.W., Clark J.S., Negueruela, I. & Najarro, F., 2009b, A&A, 507, 1597Ritchie B.W., Clark J.S., Negueruela, I. & Langer, N., 2010, A&A, 520, A48Westerlund, B.E., 1987, A&AS, 70, 311

The massive binary population of the starburst cluster Westerlund 1

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The massive binary population of the starburst cluster Westerlund 1

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