The Wolf-Rayet binaries of the nitrogen sequence in the Large Magellanic Cloud: spectroscopy, orbital analysis, formation, and evolution
Shenar, T.; Sablowski, D.P.; Hainich, R.; Todt, H.; Moffat, A.F.J.; Oskinova, L.M.; Ramachandran, V.; Sana, H,; Sander, A.A.C.; Schnurr, O.; St-Louis, N.; Vanbeveren, D.; Goetberg, Y.; Hamann, W.-R.
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). With spectroscopy of
WR binaries so-far largely avoided due to its complexity, our study
focuses on the 44 WR binaries / binary candidates of the Large
Magellanic Cloud (LMC, metallicity Z~0.5 Zsun), identified on the basis
of radial velocity variations, composite spectra, or high X-ray
luminosities. 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 sub-solar metallicity, and
constraining the impact of binary interaction in forming them.
Spectroscopy is performed using the Potsdam Wolf-Rayet (PoWR) code and
cross-correlation techniques. Disentanglement is performed using the
code Spectangular or the shift-and-add algorithm. Evolutionary status is
interpreted using the Binary Population and Spectral Synthesis (BPASS)
code, exploring binary interaction and chemically-homogeneous evolution.
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.