Structure and Dynamics of the Young Massive Star Cluster Westerlund 1

Abstract We present a structural and dynamical analysis of the young massive star cluster Westerlund 1 (Wd1). Using multiepoch Hubble Space Telescope observations, we measure the proper motions of 10,346 stars and present an unprecedented determination of cluster membership probability. We then determine the color membership after correcting for extinction. The stellar density map weighted by the membership probability and completeness correction displays a spatial elongation aligned with the Galactic plane with a high eccentricity of 0.71. The radial stellar density profile shows a decreasing core radius with increasing mass, indicative of weak but detectable mass segregation. Quantitatively, the measured mass segregation ratio is Λ MSR = 1.11 ± 0.11, only above unity by 1 σ . Building on the structural modeling along with a suite of stellar evolutionary and atmospheric models, we fit the cluster age and distance in two color–magnitude diagrams, concluding that Wd1 is approximately 7.45 ± 0.53 Myr old and at a distance of 3.7 ± 0.1 kpc, with a degeneracy in the posterior which is compatible with the literature values. We measure a 1D velocity dispersion of 4.13 ± 0.13 km s −1 , indicating a subvirialized state. The crossing time is 0.30 Myr and the relaxation time is 0.26 Gyr. Given the age of Wd1, we expect dynamical mass segregation for stars more massive than 12 M ⊙ , which accounts for the minor mass segregation observed in the mass range of 1.12–15.21 M ⊙ in this work. This suggests that the mass segregation in Wd1 is more likely a dynamical effect.