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The Wolf–Rayet Binaries of the Nitrogen Sequence in the Large Magellanic Cloud Spectroscopy, Orbital Analysis, Formation, and Evolution

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The Wolf–Rayet Binaries of the Nitrogen Sequence in the Large Magellanic Cloud Spectroscopy, Orbital Analysis, Formation, and Evolution A&A 627, A151 (2019) Astronomy https://doi.org/10.1051/0004-6361/201935684 & c ESO 2019 Astrophysics The Wolf–Rayet binaries of the nitrogen sequence in the Large Magellanic Cloud Spectroscopy, orbital analysis, formation, and evolution T. Shenar1, D. P. Sablowski2, R. Hainich3, H. Todt3, A. F. J. Moffat4, L. M. Oskinova3, V. Ramachandran3, H. Sana1, A. A. C. Sander5, O. Schnurr6, N. St-Louis4, D. Vanbeveren7, Y. Götberg8, and W.-R. Hamann3 1 Institute of Astrophysics, KU Leuven, Celestijnlaan 200D, 3001 Leuven, Belgium e-mail: [email protected] 2 Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany 3 Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany 4 Département de physique and Centre de Recherche en Astrophysique du Québec (CRAQ), Université de Montréal, 6128, Succ. Centre-Ville, H3C 3J7 Montréal, Québec, Canada 5 Armagh Observatory, College Hill, Armagh BT61 9DG, UK 6 Cherenkov Telescope Array Observatory gGmbH, Via Piero Gobetti 93/3, 40126 Bologna, Italy 7 Astronomy and Astrophysics Research Group, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium 8 The Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA Received 12 April 2019 / Accepted 10 May 2019 ABSTRACT Context. Massive Wolf–Rayet (WR) stars dominate the radiative and mechanical energy budget of galaxies and probe a critical phase in the evolution of massive stars prior to core collapse. It is not known whether core He-burning WR stars (classical WR; cWR) form predominantly through wind stripping (w-WR) or binary stripping (b-WR). Whereas spectroscopy of WR binaries has so-far largely been avoided because of its complexity, our study focuses on the 44 WR binaries and binary candidates of the Large Magellanic Cloud (LMC; metallicity Z ≈ 0:5 Z ), which were identified on the basis of radial velocity variations, composite spectra, or high X-ray luminosities. Aims. Relying on a diverse spectroscopic database, we aim to derive the physical and orbital parameters of our targets, confronting evolution models of evolved massive stars at subsolar metallicity and constraining the impact of binary interaction in forming these stars. Methods. Spectroscopy was performed using the Potsdam Wolf–Rayet (PoWR) code and cross-correlation techniques. Disentangle- ment was performed using the code Spectangular or the shift-and-add algorithm. Evolutionary status was interpreted using the Binary Population and Spectral Synthesis (BPASS) code, exploring binary interaction and chemically homogeneous evolution. Results. Among our sample, 28/44 objects show composite spectra and are analyzed as such. An additional five targets show pe- riodically moving WR primaries but no detected companions (SB1); two (BAT9999 and 112) are potential WR + compact-object candidates owing to their high X-ray luminosities. We cannot confirm the binary nature of the remaining 11 candidates. About two-thirds of the WN components in binaries are identified as cWR, and one-third as hydrogen-burning WR stars. We establish metallicity-dependent mass-loss recipes, which broadly agree with those recently derived for single WN stars, and in which so-called WN3/O3 stars are clear outliers. We estimate that 45 ± 30% of the cWR stars in our sample have interacted with a companion via mass transfer. However, only ≈12 ± 7% of the cWR stars in our sample naively appear to have formed purely owing to stripping via a companion (12% b-WR). Assuming that apparently single WR stars truly formed as single stars, this comprises ≈4% of the whole LMC WN population, which is about ten times less than expected. No obvious differences in the properties of single and binary WN stars, whose luminosities extend down to log L ≈ 5:2 [L ], are apparent. With the exception of a few systems (BAT9919, 49, and 103), −1 the equatorial rotational velocities of the OB-type companions are moderate (veq . 250 km s ) and challenge standard formalisms of angular-momentum accretion. For most objects, chemically homogeneous evolution can be rejected for the secondary, but not for the WR progenitor. Conclusions. No obvious dichotomy in the locations of apparently single and binary WN stars on the Hertzsprung-Russell diagram is apparent. According to commonly used stellar evolution models (BPASS, Geneva), most apparently single WN stars could not have formed as single stars, implying that they were stripped by an undetected companion. Otherwise, it must follow that pre-WR mass-loss/mixing (e.g., during the red supergiant phase) are strongly underestimated in standard stellar evolution models. Key words. stars: massive – stars: Wolf–Rayet – Magellanic Clouds – binaries: close – binaries: spectroscopic – stars: evolution 1. Introduction gy budget of their host galaxies. Among these stars, massive Wolf–Rayet (WR) stars define a spectral class of stars with Through their stellar winds, intense radiation, and supernova emission-dominated spectra that are physically characterized (SN) explosions, massive stars (Mi & 8 M ) dominate the ener- by strong, radiatively driven winds (see Crowther 2007, for a Article published by EDP Sciences A151, page 1 of 68 A&A 627, A151 (2019) review). They are subdivided in three main flavors: the nitrogen T*/kK sequence (WN), the carbon sequence (WC), or the very rare oxy- 200 150 100 60 40 20 10 gen sequence (WO), depending on whether their atmospheres are LMC N-rich (CNO cycle products) or C/O-rich (He-burning products). Born Most known WR stars are classical WR stars1 (cWR), defined as this evolved, core He-burning (or rarely C-burning) WR stars. How- way ms-WR ever, very massive stars can already appear as WR stars on the 100 M main sequence (de Koter et al. 1997). As immediate progen- ⊙ 6.0 itors of black holes (BHs) and neutron stars, the attributes of Single WR stars largely determine observed properties of SN explosions wind stripped and gravitational-wave (GW) detections arising from the merg- (Conti scenario) w-WR ing of compact objects. Studying WR stars is hence essential ) Binary for understanding the evolution of massive stars (e.g., Hamann wb-WR L et al. 2006; Tramper et al. 2015; Shenar et al. 2016; Sander et al. / L 50 M 2019), the energy budget of galaxies (e.g., Doran et al. 2013; ⊙ 5.5 wind+binary Ramachandran et al. 2018), the upper-mass limit of stars (e.g., log ( Bestenlehner et al. 2011; Shenar et al. 2017; Tehrani et al. 2019), stripped Binary Single and the properties of compact objects and SNe (e.g., Woosley et al. 2002; Langer 2012; de Mink et al. 2014; Marchant et al. binary b-WR 2016; Hainich et al. 2018). Despite this, their formation, espe- stripped cially in the context of binary interaction, is still considered poorly understood. pre WR phase We discern among four distinct formation channels for WR WN, 0 < XH < 0.74 5.0 non-WR 25 M WN, X = 0 ⊙ H stars. These formation channels are illustrated in a Hertzsprung- spectrum ZAMS WC/WO phase zero age main Russell diagram (HRD) in Fig.1 using evolution tracks cal- sequence RLOF culated with the BPASS2 (Binary Population and Spectral Synthesis) code V2.0 (Eldridge et al. 2008; Eldridge & Stanway 5.5 5.0 4.5 4.0 3.5 2016), and are defined as follows: log (T*/K) 1. Main-sequence WR stars (ms-WR; “born this way”) are Fig. 1. Illustration of the ms-WR, w-WR, wb-WR, and w-WR for- core H-burning WR stars. They typically exhibit WN spec- mation channels of WR stars. Shown are evolution tracks calculated tra already on the main sequence by virtue of their very at a metallicity Z = 0:008 (≈ZLMC) with the BPASS code for sin- large masses (&60 M at solar metallicity) and correspond- gle stars with initial masses Mi = 100; 50; and 25 M (upper, mid- ingly strong winds (de Koter et al. 1997; Crowther & dle, and dashed lower tracks, respectively). Also plotted are BPASS Walborn 2011). Spectroscopically, they are associated with binary evolution tracks with Mi = 50 M ; Pi = 25 d, qi = 0:7 (upper), weaker wind “slash WR stars” (/WN), hydrogen-rich WN and Mi = 25 M ; Pi = 25 d, qi = 0:7 (lower). The colors corre- stars (WNh), and luminous blue variables (LBVs). Examples spond to surface hydrogen mass fractions (blue: 0:4 < XH < 0:74, red: include WR 24 in the Galaxy (WN6h), the two components 0:2 < XH < 0:4, orange: 0:05 < XH < 0:2, green: XH < 0:05). Empiri- of BAT99119 (WN6h + O3.5 If/WN7) in the Large Magel- cally, stars found in the dashed region are not expected to show a WR spectrum (see Sect. 6.1.3). lanic Cloud (LMC), and probably the two components of HD 5980 (WN6h + WN6-7h) in the Small Magellanic Cloud (SMC). binary WR 139 (V444 Cyg, WN5 + O6) likely started its life 2. Wind-stripped WR stars (w-WR) are cWR stars that formed with M 30 M and was partly stripped by the secondary through intrinsic mass loss, i.e., stellar winds or eruptions i & star (Vanbeveren et al. 1998b), making it a good candidate (Conti 1976; Smith 2014). Only stars that are sufficiently for a wb-WR star. Other examples include most confirmed massive can become w-WR stars. The minimum initial mass WR binaries in the SMC (Shenar et al. 2016). M is a strong function of the metallicity Z. It is esti- i,w-WR 4. Binary-stripped WR stars (b-WR) are cWR stars that could mated to be ≈20−30 M at solar metallicity, ≈30−60 M at only form as a result of binary interaction. That is, a b-WR LMC metallicity (≈1=3 Z ), and 45−100 M at SMC metal- star would not become a WR star without a companion.
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