EWOCS-III: JWST observations of the supermassive star cluster Westerlund 1

¹ Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
³ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisbon, Portugal
⁴ Lockheed Martin, 3251 Hanover St, Palo Alto, CA 94304, USA
⁵ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁶ Department of Physics and Chemistry, University of Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁷ Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21218, USA
⁸ Department of Astronomy, University of Florida, PO Box 112055, Gainesville, FL 32611-2055, USA
⁹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
¹⁰ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹¹ Departamento de Astrofísica, Centro de Astrobiología, (CSIC-INTA), Ctra. Torrejón a Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
¹² Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Alicante, Carretera de San Vicente s/n, 03690 San Vicente del Raspeig, Spain
¹³ Department of Physics and Astronomy, Embry-Riddle Aeronautical University, 3700 Willow Creek Rd., Prescott, AZ 86301, USA
¹⁴ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
¹⁵ University of Split, Faculty of Science, Department of Physics, Ruđera Boškovića 33, 21000 Split, Croatia
¹⁶ Gemini Observatory/NSF’s NOIRLab, 950 North Cherry Avenue, Tucson, AZ 85719, USA
¹⁷ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
¹⁸ Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astrofisico di Catania, Via Santa Sofia 78, 95123 Catania, Italy
¹⁹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, 06300 Nice, France
²⁰ Astrophysics Group, Keele University, Keele, Staffordshire ST5 5BG, UK
²¹ AURA for the European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

^★ Corresponding author; mario.guarcello@inaf.it

Received: 6 September 2024
Accepted: 28 October 2024

Abstract

Context. The typically large distances, extinction, and crowding of Galactic supermassive star clusters (stellar clusters more massive than 10⁴ M_⊙) have so far hampered the identification of their very low mass members, required to extend our understanding of star and planet formation, and early stellar evolution, to the extremely energetic star-forming environment typical of starbursts. This situation has now evolved thanks to the James Webb Space Telescope (JWST), and its unmatched resolution and sensitivity in the infrared.

Aims. In this paper, the third of the series of the Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS), we present JWST/NIRCam and JWST/MIRI observations of the supermassive star cluster Westerlund 1. These observations are specifically designed to unveil the cluster members down to the brown dwarf mass regime, and to allow us to select and study the protoplane-tary disks in the cluster and to study the mutual feedback between the cluster members and the surrounding environment.

Methods. Westerlund 1 was observed as part of JWST GO-1905 for 23.6 hours. The data have been reduced using the JWST calibration pipeline, together with specific tools necessary to remove artifacts, such as the 1 /f random noise in NIRCam images. Source identification and photometry were performed with DOLPHOT.

Results. The MIRI images show a plethora of different features. Diffuse nebular emission is observed around the cluster, which is typically composed of myriads of droplet-like features pointing toward the cluster center or the group of massive stars surrounding the Wolf–Rayet star W72/A. A long pillar is also observed in the northwest. The MIRI images also show resolved shells and outflows surrounding the M-type supergiants W20, W26, W75, and W237, the sgB[e] star W9 and the yellow hypergiant W4. Some of these shells have been observed before at other wavelengths, but never with the level of detail provided by JWST. The color-magnitude diagrams built using the NIRCam photometry show a clear cluster sequence, which is marked in its upper part by the 1828 NIRCam stars with X-ray counterparts. NIRCam observations using the F115W filter have reached the 23.8 mag limit with 50% completeness (roughly corresponding to a 0.06 M0 brown dwarf).