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Investigating the Origin of the Spectral Line Profiles of the Hot Wolf–Rayet

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Investigating the Origin of the Spectral Line Profiles of the Hot Wolf–Rayet View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Caltech Authors MNRAS 484, 5834–5844 (2019) doi:10.1093/mnras/stz411 Advance Access publication 2019 February 11 Investigating the origin of the spectral line profiles of the Hot Wolf–Rayet Star WR 2 1,2,3‹ 4 4 5 6 A.-N. Chene´ , N. St-Louis , A. F. J. Moffat, O. Schnurr, P. A. Crowther, G. Downloaded from https://academic.oup.com/mnras/article-abstract/484/4/5834/5315800 by California Institute of Technology user on 10 April 2019 A. Wade,7 N. D. Richardson ,8 C. Baranec,9 C. A. Ziegler,10 N. M. Law,10 R. Riddle ,11 G. A. Rate,6 E.´ Artigau4 and E. Alecian12 BinaMIcS collaboration 1Gemini Observatory, Northern Operations Center, 670 North A’ohoku Place, Hilo, HI 96720, USA 2Departamento de F´ısica y Astronom´ıa, Universidad de Valpara´ıso, Av. Gran Bretana˜ 1111, Playa Ancha, Casilla 5030, Chile 3Departamento de Astronom´ıa, Universidad de Concepcion,´ Casilla 160-C, Chile 4Departement´ de Physique, Universite´ de Montreal,´ C. P. 6128, succ. centre-ville, Montreal´ (Qc) H3C 3J7 Centre de Recherche en Astrophysique du Quebec,´ Canada 5Leibniz-Institut fur¨ Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany 6Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK 7Department of Physics, Royal Military College of Canada, PO Box 17000, Stn Forces, Kingston, K7K 7B4, Canada 8Ritter Observatory, Department of Physics and Astronomy, The University of Toledo, Toledo, OH 43606-3390, USA 9Institute for Astronomy, University of Hawai‘i at Manoa,¯ Hilo, HI 96720-2700, USA 10Dunlap Institute for Astronomy and Astrophysics, University of Toronto, Ontario M5S 3H4, Canada 11Division of Physics, MathematicsAstronomy, California Institute of Technology, Pasadena, CA 91125, USA 12Universite´ Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France Accepted 2019 January 30. Received 2019 January 28; in original form 2018 October 6 ABSTRACT The hot WN star WR 2 (HD 6327) has been claimed to have many singular characteristics. To explain its unusually rounded and relatively weak emission line profiles, it has been proposed that WR 2 is rotating close to break-up with a magnetically confined wind. Alternatively, the line profiles could be explained by the dilution of WR 2’s spectrum by that of a companion. In this paper, we present a study of WR 2 using near-infrared AO imaging and optical spectroscopy and polarimetry. Our spectra reveal the presence of weak photospheric absorption lines from a B 2.5–4V companion, which however contributes only 5–10 per cent to the total light, suggesting that the companion is a background object. Therefore, its flux cannot be causing any significant dilution of the WR star’s emission lines. The absence of intrinsic linear continuum polarization from WR 2 does not support the proposed fast rotation. Our Stokes V spectrum was not of sufficient quality to test the presence of a moderately strong organized magnetic field but our new modelling indicates that to confine the wind the putative magnetic field must be significantly stronger than was previously suggested sufficiently strong as to make its presence implausible. Key words: stars: individual: WR 2 – stars: magnetic fields – stars: rotation – stars: winds, outflows – stars: Wolf–Rayet. former is commonly subdivided into early (WN2–6) and late (WN7– 1 INTRODUCTION 11) types, depending upon the degree of ionization of helium and Classical Wolf–Rayet (WR) stars are the chemically evolved de- nitrogen (Smith, Shara & Moffat 1996). Early-type WN stars in the scendants of O stars, possessing a unique spectroscopic signature Milky Way exhibit either strong, broad Gaussian line profiles (e.g. of strong, broad emission lines, arising within their fast, dense winds WR6, WN4b), or weak, narrow triangular profiles [e.g. WR152, (Crowther 2007). Their two main subclasses, WN and WC, reflect WN3(h)], with one notable exception: WR 2 (WN2b), the subject the products of core hydrogen and helium burning, respectively. The of this study. WR 2 (HD 6327) is the only known WN2 star in our Galaxy. Its spectrum displays relatively weak ‘bowler-hat’-shaped broad E-mail: [email protected] C 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Origin of the profile of WR 2 spectral lines 5835 emission lines, in stark contrast with the more normal strong 2005; ud-Doula et al. 2013), and its asymmetry could be detected (Gaussian) or weak (triangular) profiles of other early-type WN in linear spectropolarimetry. stars. Considering that WR 2 has an extreme combination of a small This study presents our investigation of the origin of the rounded radius, fairly low mass-loss rate and high temperature (Hamann, emission line profiles of WR 2. In Section 2, we search for any Grafener¨ & Liermann 2006), a certain line dilution due to the manifestation of a companion star and investigate how its presence Baldwineffect(vanGentetal.2001) is expected. However, as can lead to round emission lines. We search for evidence of rapid we will present below, this does not seem to be sufficient to explain stellar rotation in Section 3, where we present new linear broadband Downloaded from https://academic.oup.com/mnras/article-abstract/484/4/5834/5315800 by California Institute of Technology user on 10 April 2019 the line profiles. polarimetry as well as spectropolarimetry to assess the symmetry Two scenarios have been proposed to explain this unusual char- of WR 2’s wind. We look for evidence of the presence of a strong acteristic. The first involves a companion star, where intrinsically magnetic field in Section 4, where we discuss our attempt to detect a strong Gaussian lines appear weak owing to significant line dilution global magnetic field in WR 2 using the Zeeman effect (Section 4.1). from a companion (Crowther 1993). However, convincing evidence Finally, all our results are summarized in Section 5, where we of such a companion has not been reported in the literature to date. present what we have learned from these observations about the The second scenario involves the intrinsic Gaussian/triangular lines true nature of WR 2. being modified by rapid rotation. The Potsdam group has analysed WR 2 using the latest version of their model atmosphere code (Hamann et al. 2006), and remarkably the model spectrum failed to 2 SEARCH FOR A COMPANION STAR reproduce its line profiles, unless it was folded with a rotation profile Before testing any extreme stellar parameter of WR 2, we first verify near break-up velocity, with an equatorial rotation speed of 1900 km the possibility that its lines are diluted by the flux of a companion. s−1. Of course, as mentioned by the authors themselves, it is a purely mathematical solution, as a mere solid rotation is not physical and many complex interactions are expected when such a fast rotation 2.1 Observations speed occurs. More recently, Shenar, Hamann & Todt (2014)have 2.1.1 Spectroscopy developed a more detailed atmospheric model which proposes a much slower, and realistic surface rotation speed (200 km s−1), but This study is based on a series of published and unpublished, a very strong magnetic field of 40 kG at the surface to force rigid archival and new spectra from different observatories. This includes rotation of the wind up to a radius 10 R∗. spectra from the 1.6m telescope of the Observatoire du Mont Four more ‘round-lined’ WR stars are known in the LMC, BAT99 Megantic´ (OMM), the 4.2m William Herschel Telescope (WHT) 7, 51, 88, and 94 (Ruhling¨ 2008; Shenar et al. 2014), but this and the 1.85m telescope of the Dominion Astrophysical Obser- type of line profile still remains a very rare oddity. As WR 2 vatory (DAO) obtained over nearly 10 yr. We add to that a high- has an apparent magnitude that is at least 4 mag brighter than resolution spectrum obtained with the ESPaDOnS spectropolarime- the more distant LMC stars, it is a perfect proxy to study the ter at the 3.6m Canada–France–Hawaii Telescope (CFHT) that we phenomenon. present in more detail in Section 3.2, and high S/N spectra obtained If WR 2’s claimed rapid rotation were confirmed, it would using GMOS at the 8.2m Gemini-North telescope (originally ob- make it a prime long-soft gamma-ray burst (LGRB) progenitor tained to search for line profile variations). Finally, we also include candidate. LGRBs are currently best explained by the ‘collap- 2.5m Isaac Newton Telescope (INT) spectrophotometry which is sar’ model (Woosley 1993; MacFadyen & Woosley 1999;Mac- used for modelling in Section 2.4. When series of spectra were Fadyen, Woosley & Heger 2001), involving a rapidly rotating, fully obtained, all were combined into nightly averages to optimize the hydrogen-depleted WR star undergoing core collapse. Classical WR S/N. The details of the spectroscopic observations are summarized stars are the helium-burning cores of evolved massive stars in their in Table 1. last visible evolutionary phase before they explode as type Ib/c core-collapse supernovae (ccSNe). Direct observational evidence of a type-Ic–LGRB connection (Kelly, Kirshner & Pahre 2008) 2.1.2 Direct imaging lends considerable weight to the collapsar model, and might imply In an effort to spatially isolate a putative companion of WR 2, we that LGRB progenitors are merely the rapidly rotating tail in the first acquired visible-light adaptive optics imaging of WR 2 using distribution of type Ic progenitors. The mere presence of rapid the Robo-AO system (Baranec et al. 2014) on the 1.5m Telescope at rotation can in principle be revealed quite easily by linear spec- Palomar Observatory on 2014 November 9.
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