High-energy phenomena in the Wolf-Rayet stars in Westerlund 1

Massive stars live short and violent lives. Mainly during the later stages of their evolution, these stars undergo significant changes in their structure and lose a large fraction of their initial mass into the surrounding environment as stellar winds.

In particular, during the Wolf-Rayet phase, which lasts only a few hundred thousand years, massive stars eject their outer layers, which are rich in hydrogen, in fast winds that can reach terminal velocities of 2000 km/s, exposing their hot (more than 20000 K) inner layers rich in helium, nitrogen, carbon, and other heavy elements synthesized in their interiors.

Wolf-Rayet stars are objects of great interest for several reasons. For instance, despite its brevity, this evolutionary stage determines the star future evolution and the type of compact object it will form at the end of its life. Moreover, because of their hot surface temperatures, these stars produce intense UV radiation fields, which, together with their fast winds, have a significant impact on the surrounding environment.

 

Wolf-Rayet stars are rare, mainly due to the scarcity of massive stars compared to low-mass stars and the brevity of this evolutionary phase. In fact, the most up-to-date catalog of Wolf-Rayet stars, hosted by the University of Sheffield, contains 669 stars in the Milky Way and nearby galaxies. In our galaxy, the richest coeval sample of Wolf-Rayet stars consists of the 24 stars associated with the supermassive star cluster Westerlund 1, which is the target of the EWOCS (Extended Westerlund 1 and 2 Open Clusters Survey) project, led by astronomer M. G. Guarcello of INAF – Astronomical Observatory of Palermo.

 

The study of the 24 Wolf-Rayet stars in Westerlund 1 is the subject of the paper: EWOCS-II: X-ray properties of the Wolf-Rayet stars in the young Galactic super star cluster Westerlund 1‘, recently published in the journal Astronomy & Astrophysics. The study is led by astronomer K. Anastasopoulou of INAF – Astronomical Observatory of Palermo, now at the Harvard-Smithsonian Center for Astrophysics in Cambridge (US). The authors of the study analyzed the high-energy phenomena occurring in these stars using an X-ray observation obtained with the NASA satellite Chandra, which is 305 hours long.

In particular, the time analysis (which is the study of the variability in the radiation flux emitted by the sources over time) shows that several of these 24 stars exhibit the typical time behavior of binary systems, which are systems of two stars orbiting around each other. On the other hand, the spectral analysis, which examines the energy distribution of the X-ray photons detected from each star, revealed that all the Wolf-Rayet stars in Westerlund 1 show evidence of plasma hotter than 10 million degrees. Such hot plasma can form only in the region between two massive stars in a binary system, from a wind-wind collision where the large kinetic energy of the material moving at more than 1000 km/s is converted into thermal energy in the collision shock. It thus follows that all the Wolf-Rayet stars in Westerlund 1 are in binary systems with other massive stars, suggesting that such a configuration has played an important role in determining the evolutionary path of these stars and the formation of such a rich ensemble of massive stars.

 

The cover image (click here to view it in full) shows the X-ray images of the Wolf-Rayet stars in Westerlund 1. Each panel measures 10×10 arcseconds, which corresponds to 0.7 light years at the distance of Westerlund 1. The colors represent the energy of the X-ray photons detected from these sources: red indicates photons between 0.5 and 2 keV, green between 2.0 and 4.0 keV, and blue between 4.0 and 8.0 keV. The yellow circles mark the positions of the X-ray sources corresponding to the Wolf-Rayet stars, while the magenta curves delineate the areas from which the X-ray photons have been detected and whose properties have been measured.

 

Mario Giuseppe Guarcello  ( follow mariospiegacose) ( mariospiegacose) ( follow mariospiegacose)

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