EWOCS ongoing project: The Wolf-Rayet stars

Westerlund 1 hosts an impressive population of massive stars, counting at least 166 stars with initial masses between ∼25 M⊙ and ∼50 M⊙.


  • 1 Luminous Blue Variable (LBV, Wd1-243; Clark & Negueruela 2003)
  • 24 Wolf–Rayet stars (WR; Clark+2005; Crowther+2006; Groh+2006; Fenech+2018)
  • 6 yellow hypergiants (YHGs) with spectral type ranging from A5Ia+ to F8Ia+  (YHG; Clark+2005, 2010)
  • 4 red supergiants (RSG, Clark+2020)
  • more than 100 supergiants dominated by spectral types O9-B1 (Clark+2019)
  • 7 blue hypergiants (BHG, Clark+2020)


Westerlund 1 hosts examples of every transitional evolutionary phase between H-rich OB supergiants and H-depleted WRs (e.g. BHG, LBV, sgB[e], YHG, and RSG). This sample is a treasure chest for studies of the evolutionary paths of massive stars, mainly involving transitional phases and rare objects.


In particular, the existence in the cluster of Wolf Rayet (WR) stars both isolated and in binary systems is hard to be explained according to our current knowledge of the evolutionary path of massive stars. Any of the existing models for single stars can reproduce the variety of massive stars type observed in Westerlund 1: Binary interaction, mass-transfer and stripping of the outer layers are required (Ritchie+2010; Clark+2014, 2018; de Mink+2014; Schneider+2014a). The study of the massive binaries in the cluster can also help us better understanding formation and evolution of massive stars, and the resulting remnant.


The first target analyzed in the framework of this project is WR star W44/L, which has been observed with the optical spectrographs VLT/FLAMES and VLT/UVES from 2004 to 2016, providing a very good orbital solution for the system. The spectra also provided some useful information on the nature of the primary star, which likely is a WN10-11 or an extreme B0 Ia+ hypergiant with an effective temperature of about 24000 K, which is very low for a WR star.


The combined study of the spectroscopic and X-ray data will provide stringent results on the properties of the system, its dynamic, and the geometry of the colliding winds emitted by the two stars. Timing and spectral analysis is ongoing, led by G. L. Israel and E. Flaccomio, respectively.


Chandra/ACIS-I (left) and HST F125W image of W44


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