Almost 6000 X-ray sources in the supermassive star cluster Westerlund 1 revealed by the EWOCS project
Stellar clusters are important targets for studying stellar evolution and, in the case of the youngest star clusters, the products of the star and planet formation processes. Our galaxy hosts a rich population of young stellar clusters, typically with masses of a few hundred solar masses. In the solar neighborhood, for instance, there are only a few young clusters with a mass of a few thousand solar masses, such as the Trapezium in Orion and NGC2264 in the Cone Nebula.
What are the most massive young clusters in the Milky Way? Currently, our Galaxy hosts only a few young star clusters with a mass exceeding ten thousand solar masses, such as the star clusters Westerlund 1 and 2, NGC3603, or the Arches and the Quintuplet Clusters, which are located near the center of our Galaxy. These rare “supermassive star clusters” are actually targets of great importance, as they represent the most extreme star-forming environments, characterized by intense fields of UV and X-ray radiation, and relativistic particles, produced by the compact and rich populations of massive stars in these clusters. Since this energetic radiation impacts every aspect of the star and planet formation process, stars and planets form differently in such massive star-forming environments compared to low-mass ones. Additionally, supermassive star clusters are common in galaxies experiencing intense epochs of star formation, such as interacting galaxies or in the early Universe. Even in the Milky Way, during major merging events, star formation typically occurred in supermassive star-forming environments.
The most massive young stellar cluster known in the Milky Way is Westerlund 1, with an actual mass exceeding 100000 solar masses, a distance from us of about 14000 light years (being the closest supermassive star cluster to the Sun), and an age of less than 10 million years. Given its characteristics, Westerlund 1 is the best target to study the products of star and planet formation processes in supermassive star-forming environments. Additionally, Westerlund 1 hosts the richest population of massive stars known in the Milky Way, along with some compact objects produced by past supernova explosions that occurred in the cluster.
This has motivated the project EWOCS (Extended Westerlund 1 and 2 Open Clusters Survey), led by the astronomer M. G. Guarcello (INAF – Astronomical Observatory of Palermo), which aims to study the two supermassive star clusters Westerlund 1 and 2. The project is primarily based on infrared observations with the Hubble Space Telescope and the James Webb Space Telescope, along with long X-ray observations (311 hours) using Chandra/ACIS. The X-ray observation is crucial for selecting the young stars in Westerlund 1, thanks to the high level of X-ray emission typical of young stars, and for studying the energetic phenomena occurring in these young stars, massive stars, compact objects, and within the cluster. The first study produced in the framework of EWOCS is titled “EWOCS-I: The catalog of X-ray sources in Westerlund 1 from the Extended Westerlund 1 and 2 Open Clusters Survey“, led by M. G. Guarcello and recently published in Astronomy & Astrophysics. The paper presents the analysis of the X-ray observations of Westerlund 1, as well as the selection and validation of 5963 X-ray sources, primarily stars within the cluster. The brightest X-ray source in Westerlund 1 is the pulsar CXO J164710.20-455217, a neutron star produced by a supernova explosion that occurred about 700000 years ago. The pulsar is followed by some massive stars and Wolf-Rayet stars in binary systems, where most of the X-rays are produced in the region where the winds of the massive stars collide at velocities of several hundred km per hour, reaching temperatures of millions of degrees. The catalog will be used in most of the upcoming EWOCS studies.
The figure (click here to view it in its entirety) displays two RGB images of Westerlund 1: one in X-ray produced with the Chandra/ACIS observations, and one in infrared produced with the Hubble Space Telescope (HST) observations.
Mario Giuseppe Guarcello ( follow mariospiegacose) ( mariospiegacose) ( follow mariospiegacose)
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