Westerlund 1 under the light of Gaia EDR3: Distance, isolation, extent, and a hidden population

Abstract

Context. The young massive cluster Westerlund 1 offers the promise of a grand laboratory for the study of high-mass star evolution, but its basic parameters are still poorly known. Aims. In this paper, we aim at a better characterisation of the cluster by determining some basic kinematic properties and analysing the area surrounding the cluster and the population in its foreground. Methods. We have used Gaia early data release 3 (EDR3) data, together with spectra of a large sample of luminous stars in the field surrounding Westerlund 1, to explore the extent of the cluster. We carried out a non-parametric analysis of proper motions and membership determination. We investigated the reddening and proper motions of several dozen OB stars and red supergiants less than one degree away from Westerlund 1. Results. We identify a population of kinematic members of Westerlund 1 that largely includes the known spectroscopic members. From their EDR3 parallaxes, we derive a distance to the cluster of 4.23 +0.23 −0.21 kpc. We analyse the extinction in this direction, finding that it increases by a large amount around 2.8 kpc, which in all likelihood is due to dark clouds associated with the Scutum-Crux arm. As a consequence, we hardly see any stars at distances comparable (or higher) than that of the cluster. The proper motions of Westerlund 1, however, are very similar to those of stars in the field surrounding it which are -almost without exception -less distant, but distinct. We find a second, astrometrically well-defined population in the foreground (d ≈ 2 kpc), centred ∼ 8 ′ away, which is likely connected to the possible open cluster BH 197. Westerlund 1 is very elongated, an effect that seems real and not driven by the very heavy extinction to the east and south. We find a low-density halo extending to distances up to 10 ′ from the cluster centre, mainly in the north-west quadrant. A few OB stars at larger distances from the cluster, most notably the luminous blue variable (LBV) MN48, share its proper motions, suggesting that Westerlund 1 has little or no peculiar motion with respect to the field population of the Norma arm. Despite this, we are unable to find any red supergiant that could belong to an extended population related to the cluster, although we observe several dozen such objects in the foreground, demonstrating the richness of the field population along this sightline. We find a substantial population of luminous OB members obscured by several more magnitudes of extinction than most known members. These objects, mostly located in the central region of the cluster, increase the population of OB supergiants by about 25%.

A deep spectromorphological study of the γ-ray emission surrounding the young massive stellar cluster Westerlund 1

“…Recently, data from the Gaia spacecraft were used to obtain new distance estimates. While most of them are compatible with the old estimates (Davies & Beasor 2019;Rate et al 2020;Beasor et al 2021;Negueruela et al 2022), closer distances of ∼2.7 kpc have also been obtained (Aghakhanloo et al 2020(Aghakhanloo et al , 2021. Clark et al (2019) have questioned the reliability of Gaia (DR2) data in the Westerlund 1 field altogether, rendering the new estimates somewhat uncertain.…”

Understanding the TeV γ-ray emission surrounding the young massive star cluster Westerlund 1

“…and reported in Aharonian et al (2022) (henceforth HC22). Westerlund 1 is a young (4-5 Myr, Beasor et al 2021), massive (3-5 × 10 4 M , Brandner et al 2008, see also Portegies Zwart et al 2010 andLim et al 2013), compact (half-mass radius ∼1 pc) star cluster, located at a distance of ∼4 kpc from Earth and ∼4.6 kpc from the Galactic Center (GC) (Kothes & Dougherty 2007;Davies & Beasor 2019;Negueruela et al 2022). The cluster contains a large collection of young massive stars, including 24 Wolf-Rayet (WR) stars 1 , one Luminous Blue Variable (LBV), 10 Yellow Hypergiants (YHGs) and Red Supergiants (RSGs), and several bright OB supergiants (Clark et al 2020).…”

Gaia-ESO survey: Massive stars in the Carina Nebula

“…As some examples of such studies, Figer (2005) determined the upper mass limit of the initial mass function (IMF) in the Arches Cluster, a result that was later challenged by studies in R136 (Crowther et al 2010). Other examples are the determination of the chemical composition of stars in Orion (Simón-Díaz 2010); the membership, chemical and stellar parameter determination studies in Cygnus OB2 (Berlanas et al 2018a(Berlanas et al ,b, 2020; the characterization of very massive obscured clusters in the Milky Way such as Westerlund 1 (Clark et al 2005;Negueruela et al 2010Negueruela et al , 2022; and the analysis of the multiplicity of massive stars in clusters (De Becker et al 2004Becker et al , 2006Mahy et al 2009Mahy et al , 2013Sana & Evans 2011;Banyard et al 2023) and in the whole northern hemisphere (Maíz Apellániz et al 2019b;Trigueros Páez et al 2021;Mahy et al 2022). Outside the Milky Way, the most thorough spectroscopic analysis is that of the many papers 1 published by the VLT-FLAMES Tarantula Survey collaboration (VFTS, Evans et al 2011).…”