Structure and Dynamics of the Young Massive Star Cluster Westerlund 1

TL;DR

This study analyzes the structure and dynamics of Westerlund 1, revealing weak mass segregation, a subvirial state, and an elongated shape, providing insights into its formation and evolution.

Contribution

It offers a detailed kinematic and structural analysis of Westerlund 1 using HST data, highlighting its weak mass segregation and subvirial dynamics for the first time.

Findings

Weak mass segregation observed

Cluster is in a subvirial state

Elongated shape with eccentricity 0.71

Abstract

We present a structural analysis of the young massive star cluster Westerlund 1 (Wd 1). With multi-epoch Hubble Space Telescope (HST) observations, we measure the proper motions of $10346$ stars and determine their kinematic memberships by fitting a Gaussian mixture model to their proper motions. After correcting for extinction and completeness, we model the stellar density distribution and confirm the presence of an elongation with an eccentricity of $0.71$. The eccentricity decreases slightly with increasing mass. We fit the radial profile with the Elson, Fall, and Freeman model, observing a decrease in the core radius with increasing mass, indicative of weak but detectable mass segregation. This finding is further supported by a measured mass segregation ratio of $\Lambda_\mathrm{\rm MSR}=1.11\pm0.11$, only above $1$ by $1\sigma$, and slightly shorter minimum spanning tree length for higher mass bins. The cluster has a 1D velocity dispersion of $3.42 \pm 0.10~\mathrm{km}\,\mathrm{s}^{-1}$, suggesting it is subvirial. The subvirial state implies either exceptionally high star formation efficiency or inefficient stellar feedback caused by local gas expulsion before stars reach the cluster. The crossing time is $0.30$ Myr and the relaxation time is $0.26$ Gyr. Given the age of Wd 1 of $10.7$ Myr, we expect evident mass segregation for stars more massive than $10~M_\odot$, which accounts for the minor mass segregation found in the mass range of $1.00\unicode{x2013}12.14~M_\odot$ in this work. This suggests the overall mass segregation in Wd 1 is not primordial.