Mario Giuseppe Guarcello

Context . Investigating substellar populations in young massive clusters offers crucial insights into the formation of brown dwarfs (BDs) and the role of environmental conditions in shaping their properties. Westerlund 2 (Wd2), as one of the nearest dense and massive clusters in the Milky Way, represents an ideal laboratory for studying the effects of high stellar density and ionizing radiation from massive stars on BD formation. Aims . This paper presents deep JWST/NIRCam observations of Wd2 in a large number of filters between 1.15 and 4.1 µm. Our analysis focuses on identifying and characterizing the BD population within the cluster. Methods . We carried out point spread function (PSF) photometry on deep JWST/NIRCam data obtained in four wide and six medium-band filters, using DOLPHOT. The resulting catalog was used to identify BD candidates in Wd2 through spectral energy distribution (SED) fitting with atmospheric models. The methodology was validated using synthetic JWST photometry of spectroscopically confirmed young BDs together with simulated populations of typical contaminants, allowing us to define an empirical selection criteria. Results . The 50% detection limit of our NIRCam catalog is ∼0.015–0.02 M _⊙ , at the distance, age, and extinction of Wd2, providing the deepest view of a supermassive star cluster to date. The validation demonstrates that the adopted SED-fitting methodology reliably recovers young brown dwarfs while remaining robust against contamination from reddened evolved stars. Applying this procedure to Wd2, we identify 353 substellar candidates. Most candidates (301) lie above the 10 Myr isochrone in the Hertzsprung-Russell diagram consistent with cluster membership and are defined as strong candidates. Comparison with a control field indicates a contamination level of ≲5% for the strong candidate sample down to masses of ∼0.01–0.015 M _⊙ . We identify 73 candidates exhibiting an infrared excess indicative of circumstellar disks. These objects occupy the expected infrared-excess locus in de-reddened color diagrams and represent 27.7 ^+3.4 _−3.2 % of the candidates detected in F410M. Conclusions . These results demonstrate the capability of JWST/NIRCam medium-band photometry to identify robust samples of candidate brown dwarfs in distant, embedded, and crowded massive star clusters using imaging observations alone. The resulting catalog provides a well-characterized sample for future spectroscopic confirmation and subsequent studies of the substellar population of Wd2.

Context. Westerlund 1 (Wd1) is the most massive young star cluster known in the Milky Way and a key laboratory for studying the early dynamical evolution of massive clusters. Owing to its high extinction, the internal kinematics of its intermediate-mass stellar population remain largely unexplored. Aims. We aim to characterise the internal kinematic properties of Wd1 using a homogeneous census of near-infrared-selected cluster members and their proper motions (PMs). We considered PMs from the VIRAC2 catalogue, based on multi-epoch VVV/VVVX observations. Methods. For 1286 candidate members previously identified through near-IR photometry and astrometry (Paper I), we computed their PMs relative to the cluster mean motion and analysed them as a function of position and radius. We also investigated radial trends, tested the robustness of the results against the assumed cluster centre, and searched for preferred directions of motion. Results. We detected a statistically significant signature of expansion in the outer regions of Wd1, with the radial component of the relative PMs increasing with distance from the cluster centre. The expansion appears asymmetric, with the strongest gradient detected along a position angle PA= 84° ± 8° in the plane of the sky and with 5.9σ significance. We also found hints of inward radial motions in the central region, consistent with ongoing mass segregation, but only at a ≈2σ significance. Taken together, these results are consistent with a nearly monolithic formation scenario.

We present a spectroscopic survey of young stellar objects (YSOs) in Sh2-284, a distant (∼4.5 kpc), low-metallicity ( Z ∼ 1/3 Z _⊙ ) star-forming region (SFR) toward the Galactic anticenter. Candidate YSOs were selected using mid-infrared Spitzer/IRAC data with optical and near-infrared photometry. Follow-up spectroscopy was conducted with the Multi-Object Double Spectrograph (MODS) at the Large Binocular Telescope. We characterize the stellar and accretion properties of the disk-bearing population in a metal-poor environment, probing stellar masses from ∼2.95 to ∼0.35 M _⊙ . This work constitutes the first wide-field ( ∼ 4 5 ′ × 4 5 ′ ) spectroscopic investigation of YSOs in Sh2-284, providing a comprehensive view of star formation in this Galactic environment. We provide tentative first estimates of iron abundance for three low-mass targets and lithium abundance for a limited number of objects, offering an initial characterization of the chemical properties of these stars. We adopt a multidiagnostic approach based on nine tracers, exploiting the spectral coverage of the instrument, using H α , H β , H γ , Ca ii infrared triplet, Pa η , Pa ζ , and Pa ϵ emission lines. We derive key accretion properties and compare them with those measured in nearby, solar-metallicity SFRs to explore potential metallicity-driven differences in accretion behavior and disk evolution. We tentatively find indications of a flattening in the flux–flux relations of the metal-poor YSOs compared to solar-metallicity samples, a behavior that is recovered across diagnostics. Our observations indicate that the selected disk-bearing YSOs in this metal-poor environment exhibit resilient accretion activity, spanning from ∼2.3 × 10 ⁻¹⁰ to ∼1.0 × 10 ⁻⁶ M _⊙ yr ⁻¹ and having a median rate of ∼ 2.2 × 10 ⁻⁸ M _⊙ yr ⁻¹ . Overall, this demonstrates that gas accretion can be efficiently sustained in subsolar-metallicity environments.

Context. Westerlund 1 (Wd 1) is the closest supermassive star cluster to the Sun, containing more than 100 000 stars of all spectral types and masses down to brown dwarfs. This population considerably heats the surrounding interstellar medium (ISM), making Wd 1 a relevant place to investigate stellar feedback on intracluster gas. Aims. We present the most detailed X-ray study to date of the diffuse emission in this region, aimed at providing the most favourable observational configuration for distinguishing and quantifying the point-source contribution to the true diffuse emission. Methods. We analysed 36 Chandra ACIS-I observations of Wd 1 within an 8 × 8 arcmin window. After removing 4922 point sources with ACIS Extract, using energy-dependent point spread function (PSF) models and adaptive 99% enclosed-energy masks, we subtracted PSF-wing contamination from overlapping neighbours. It is critical in this crowded field to correct for background. We applied adaptive smoothing to reveal hot gas between stars in the soft (0.5–1.2 keV), medium (1.2–1.9 keV), and hard (1.9–7.0 keV) bands. Results. The spectral fitting suggests a shocked thermal plasma, with soft emission extending beyond the stellar core and diffuse X-rays correlating spatially with the massive stars. The cluster core (region #2) is best described by an APEC+PSHOCK model, capturing the continuous range of post-shock ionisation timescales produced by the many coexisting stellar wind shocks; it is characterised by collisionally excited plasma temperatures of 1.81 ± 0.24 and 4.11 ± 1.98 keV and the 6.7 keV FeK _α emission line. The outer region (Region #1) exhibits charge exchange emission (CXE) and is best described by a CXE+NEI model, with softer temperatures of 0.27 ± 0.04 and 1.09 ± 0.05 keV. Absorption column densities ( N _H ) range between 1.84 and 2.24 ×10 ²² cm ⁻² and the total (0.5–8.0 keV) diffuse luminosity of both regions is ≃9.6 (±0.3)×10 ³³ erg s ⁻¹ . Conclusions. Near the core, the hard emission likely originates from thermalised Wolf-Rayet (WR) stellar winds and intense wind–wind collisions in regions of high massive-star density, while the adiabatic expansion of the hot plasma and turbulent mixing with the denser, cooler ISM ends up softening it. The soft diffuse component extends farther out and may be associated with CXE processes, spatially correlated with regions of lower extinction. No hint of a non-thermal contribution is revealed. The analytical cluster wind model, using the observed WR population, predicts L _X ^CWM ≃ 2.1 (±0.10 × 10 ³⁴ erg s ⁻¹ , a factor of ∼2.2 above the observed luminosity, consistent with the low-density, partially evacuated intracluster medium (ICM) inferred from the X-ray and infrared morphology. This low-density morphology at the cluster centre suggests that ultraviolet (UV) radiation can clear the ambient medium, producing an apparent outward displacement that likely reduces the L _w -to- L _X conversion efficiency, measured as η = L _X / L _w ≃ 4.3 × 10 ⁻⁶ , approximately two orders of magnitude below that of the Cygnus OB2 association and systematically lower than every other massive star-forming region compiled here, resulting in comparatively subluminous diffuse X-ray emission.

Magnetars are highly magnetised, isolated neutron stars with uncertain formation channels. They comprise a potentially significant fraction of the young neutron star population in the Milky Way and are implicated in the explosion mechanisms of some of the most powerful explosions in nature. We aim to identify magnetars in the near-infrared with Hubble Space Telescope (HST) and James Webb Space Telescope (JWST) imaging in order to measure their proper motions and search for their birth sites. Candidate infrared counterparts were selected based on variability, colours, and proper motions, which are outliers with respect to other sources in the field. Precise proper motions were obtained by tying HST/WCF3 and JWST/NIRcam images to the Gaia absolute astrometric reference frame. We identify new infrared counterpart candidates for PSRJ1622-4950, 1RXSJ,170849.0-400910, and CXOUJ164710.2-455216, representing a substantial increase in the population. The past trajectory of the 1RXSJ,170849.0-400910-associated source coincides with the supernova remnant G346.6-0.2. The transverse velocity distribution of magnetars is found to be marginally inconsistent with that of young pulsars, due primarily to a dearth of high velocity magnetars. A candidate birth site is identified inside the cone of possible past trajectories in nearly every case. We show, based on the inferred kinematic ages, that magnetar characteristic ages may frequently be lower than the true age but caution that this result depends on the reliability of the birth site associations. We conclude that magnetars are broadly similar to the wider Galactic neutron star population in terms of their kinematics and birth sites, consistent with magnetar formation being a common outcome of massive star core collapse. However, we find tentative evidence for a dearth of high-velocity magnetars. If real, this may arise from physical differences in the progenitor population giving rise to magnetars or from differences in their post-formation velocity evolution.

The evolutionary origin of Wolf-Rayet (WR) stars at solar metallicity is unclear. Single-star evolution from massive O stars, possibly via a luminous blue variable phase, is challenged by binary period distributions of different WR subtypes. Wd1-72 is a WN7b+O binary embedded in the collective wind of the Galactic young massive cluster Westerlund 1 (Wd 1). It is surrounded by highly structured nebulosity, with cometary tails pointing away from Wd 1 and quasi-spherical droplets towards it. In this Letter, we demonstrate that this morphology can be qualitatively reproduced by a hydrodynamic simulation of non-conservative Roche-lobe overflow (RLOF) mass loss into a cluster wind. Our model is based on a detailed binary evolution track consistent with key known properties of Wd1-72. Our work suggests Wd1-72 could be only ∼10 kyr post-RLOF, and the hydrogen-free nature of Wd1-72 favours this being a second or subsequent RLOF episode. Follow-up observations could make Wd1-72 a valuable benchmark for probing mass loss and mass transfer in forming gravitational-wave binary-progenitor systems.

Context . Being the most massive known young stellar cluster in the Milky Way, Westerlund 1 (Wd1) constitutes an ideal benchmark for understanding the evolution of massive stars. However, the cluster age remains highly controversial (∼4-10 Myr), hindering the use of Wd1 as a reference for massive star evolution. One of the main issues is a high foreground extinction, which has thus far prevented the detection of the main sequence. Aims . Using infrared (IR) spectroscopy, we seek to detect the cluster’s main sequence for the first time, to characterise the Hertzsprung-Russell diagram (HRD), and to use the cluster’s turn-off to obtain a robust age estimate. Methods . We obtained multi-epoch, near-IR VLT/KMOS spectroscopic observations of Wd1 to map its population of massive stars. The spectra of ∼110 members were analysed with CMFGEN models to derive the stellar parameters, populate the cluster HRD, and compare it with isochrones from evolutionary models. Results . Our observations returned 47 new spectroscopically identified cluster members, with spectral types O9-B1 III-V. The cluster turn-off indicates an age of 5.5 ± 1.0 Myr at a distance of 4.23 _−0.21 ^+0.23 kpc, displaying a moderate degree of coevality. We demonstrate that our estimate of the age of Wd1 is robust against reasonable changes in the distance and extinction law, as well as the adopted rotational velocity and metallicity of the stellar isochrones. We further find that ∼65% of the OB stars with multi-epoch coverage exhibit radial-velocity variability. Conclusions . The IR observations of the unevolved stellar population support a single episode of star formation with an age of ∼5.5 Myr, reinforcing its potential as a benchmark for massive star evolution and providing a reference sample for future binary population studies.

Recent years have seen a surge of interest in the community studying the effect of ultraviolet radiation environment, predominantly set by OB stars, on protoplanetary disc evolution and planet formation. This is important because a significant fraction of planetary systems, potentially including our own, formed in close proximity to OB stars. This is a rapidly developing field, with a broad range of observations across many regions recently obtained or recently scheduled. In this paper, stimulated by a series of workshops on the topic, we take stock of the current and upcoming observations. We discuss how the community can build on this recent success with future observations to make progress in answering the big questions of the field, with the broad goal of disentangling how external photoevaporation contributes to shaping the observed (exo)planet population. Both existing and future instruments offer numerous opportunities to make progress towards this goal.

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).