@article{TramperStraalSanyaletal.2015,
  author    = {Tramper, F. and Straal, S. M. and Sanyal, D. and Sana, Hugues and de Koter, A. and Gr{\"a}fener, G. and Langer, N. and Vink, J. S. and de Mink, S. E. and Kaper, L.},
  title     = {Massive Wolf-Rayet stars on the verge to explode},
  series = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015},
  journal   = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-87786},
  pages     = {109 -- 112},
  year      = {2015},
  abstract  = {The enigmatic oxygen-sequence Wolf-Rayet stars represent a rare stage in the evolution of massive stars. Their properties can provide unique constraints on the pre-supernova evolution of massive stars. This work presents the results of a quantitative spectroscopic analysis of the known single WO stars, with the aim to obtain the key stellar parameters and deduce their evolutionary state.X-Shooter spectra of the WO stars are modeled using the line-blanketed non-local thermal equilibrium atmosphere code cmfgen. The obtained stellar parameters show that the WO stars are very hot, with temperatures ranging from 150 kK to 210 kK. Their chemical composition is dominated by carbon (>50\%), while the helium mass fraction is very low (down to 14\%). Oxygen mass fractions reach as high as 25\%. These properties can be reproduced with dedicated evolutionary models for helium stars, which show that the stars are post core-helium burning and very close to their eventual supernova explosion. The helium-star masses indicate initial masses or approximately 40 - 60M⊙.Thus, WO stars represent the final evolutionary stage of stars with estimated initial masses of 40 - 60M⊙. They are post core-helium burning and may explode as type Ic supernovae within a few thousand years.},
  language  = {en}
}
@misc{VinkHegerKrumholzetal.2012,
  author    = {Vink, Jorick Sandor and Heger, Alexander and Krumholz, Mark R. and Puls, Joachim and Banerjee, Shiladitya and Castro, Norberto and Chen, K.-J. and Chen{\`e}, A.-N. and Crowther, P. A. and Daminelli, A. and Gr{\"a}fener, G. and Groh, J. H. and Hamann, Wolf-Rainer and Heap, S. and Herrero, A. and Kaper, L. and Najarro, F. and Oskinova, Lida and Roman-Lopes, A. and Rosen, A. and Sander, A. and Shirazi, M. and Sugawara, Y. and Tramper, F. and Vanbeveren, D. and Voss, R. and Wofford, A. and Zhang, Y.},
  title     = {Very massive stars in the local universe},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {601},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-41522},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-415220},
  pages     = {29},
  year      = {2012},
  abstract  = {Recent studies have claimed the existence of very massive stars (VMS) up to 300 M⊙ in the local Universe. As this finding may represent a paradigm shift for the canonical stellar upper-mass limit of 150 M⊙, it is timely to discuss the status of the data, as well as the far-reaching implications of such objects. We held a Joint Discussion at the General Assembly in Beijing to discuss (i) the determination of the current masses of the most massive stars, (ii) the formation of VMS, (iii) their mass loss, and (iv) their evolution and final fate. The prime aim was to reach broad consensus between observers and theorists on how to identify and quantify the dominant physical processes.},
  language  = {en}
}