EWOCS ongoing project: Massive Stars spectral analysis

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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.

Most of these massive stars are bright X-ray sources, with a variety of mechanisms producing X-rays in act. Given the large binary fraction among the massive population of the cluster, several systems are present with colliding winds resulting in the emission of hard X-ray photons. In other systems with complex circumstellar environment, the interaction between the wind and the surrounding medium can also affect the X-ray spectra.

For the comprehension of the physics of massive stars, it is crucial to analyze in details the X-ray spectra of these sources, together with a temporal analysis of the X-ray emission. Results will also be compared to what it is known from optical and near-infrared spectroscopy to obtain a complete view of the properties of these stars.