• EWOCS – Extended Westerlund One Chandra (and JWST) Survey
At a distance of 3.9 kpc, Westerlund 1 (3-5 Myrs, but its age is still debated and recent studies determined older ages) is the closest starburst cluster to the Sun. What is a starburst cluster? While the typical star forming regions in our Galaxy today form hundreds to a few thousand stars, in starburst star forming regions tens of thousand to million of stars form very quickly, often in a single event of star formation. Compared to the typical star forming regions in our Galaxy today, thus, starburst regions represent by far a more extreme star forming environment in terms of both stellar density and local energetic (UV) radiation field, the latter emitted by a rich population of massive stars.
Starburst clusters represent a window open on a star forming environment typical of galaxies experiencing intense epochs of star formation. In fact, despite being rare in our Galaxy today, starburst regions are common in the early Universe and in interacting galaxies, in which stars form at a rate which is tens to hundreds times higher than in the Milky Way. The study of Westerlund 1 will thus help us better understanding the star and planet formation processes and the early stellar evolution in a star-forming environment which is typical of the early Universe and of past epochs of the evolution of our own Galaxy.
Left Panel: Hubble Space Telescope RGB image of Westerlund 1 (red F160W; green F139M; blue F125W); Right panel: 1 Msec combined and smoothed ACIS-I image of Westerlund 1, with soft (0.5-1.2 keV) photons marked in blue, medium (1.2-2 keV) photons in green, and hard (2-7.9 keV) photons in red. The length of the segment in the upper right corner of the images is equal to 0.5 arcmin.
The EWOCS (Extended Westerlund One Chandra Survey) project is based on new observations of Westerlund 1 (a 1Msec Chandra/ACIS-I observation and a JWST/MIRI and NIRCAM observation of 21.6 hours), that will be implemented with other existing observations at high spatial resolution (HST, GeMS/GSAOI), which is required to resolve single faint objects in the densely packed central region of the cluster.
The Chandra/ACIS-I observations will provide:
- a selection of low mass stars belonging to Westerlund 1 down to about 1 solar mass, thanks to the typical intense emission of X-rays from pre-main sequence low mass stars. This will be crucial in the outskirt of the cluster, where the contamination from unrelated stars is more critical;
- an insight on the mechanisms responsible for the production of X-rays in the rich population of massive stars of the cluster;
- an important view on the properties of the magnetar CXO J164710.20-455217 and the possibility for searching for other compact objects in the cluster.
The JWST observations are instead designed in order to:
- Select stars with disks in the cluster thanks to photometry in infrared broad bands;
- study disks activity thanks to observations in specific narrow bands;
- select the brown dwarfs population of Westerlund 1.
The main objectives of the EWOCS projects:
- The evolution of protoplanetary disks in starburst clusters. In stellar clusters characterized by intense local UV fields or high stellar density, protoplanetary disks can be rapidly dispersed by several phenomena. We want to verify which fraction of planetary disks of Westerlund 1 are destroyed by the starburst environment before they can form planets.
- Planetary formation in starburst clusters. We want to understand if the starburst environment affects the final mass accretion of planets, and the chemical properties of their atmospheres. Together with the previous point, EWOCS will provide important information on how planets formation occurs in galaxies during intense star formation phases.
- Formation of low-mass stars in starburst. We want to verify whether the mass spectrum of the stars born in starburst is the same of that of the low-mass star-forming environments in the Galaxy today, or whether starburst clusters form preferentially massive or low-mass stars, down to the brown dwarf regime.
- The formation mechanism and evolution of starburst clusters. We want to understand whether the formation and evolution of starburst clusters is different than that of low-mass clusters, and also to understand whether starburst clusters are doomed to disperse in a few million years as most of the low-mass clusters or whether if their intense gravitational field bound these clusters for billion of years as in globular clusters.
- Formation of Intermediate Mass Black Holes in starburst. The dense stellar environment provided by starburst clusters can allow the formation by coalescence of intermediate mass black holes, which are responsible for most of the gravitational waves detected by LIGO and VIRGO, as suggested by theoretical works. We will search for X-ray sources which can be candidate of being Intermediate Mass Black Holes;
- Mechanisms responsible of X-ray emission from the unique ensemble of massive stars of this starburst clusters. Westerlund 1 hosts some of the most massive stars in the Universe and also several massive stars in transient evolutionary phases. X-rays can be produced by different mechanisms in stars in different evolutionary phases or in binary systems with different properties. We want to provide an exhaustive analysis of the X-ray emission mechanisms working on these massive stars.
- Transient source CXOU J164710.2-455216. Westerlund 1 hosts a magnetar, demonstrating that supernova explosions already occurred in the cluster. We will study the spectral properties of the pulsar during its quiescent phase in order to better understand the properties of its magnetospheres and verify whether the previously observed outbursts have modified its quiescent properties.
The team of the project counts to date 34 astronomers from Europe, US and Argentina (the full list of the astronomers involved in EWOCS can be found here).