The young massive star cluster Westerlund 2 observed with MUSE. III. A cluster in motion -- the complex internal dynamics

TL;DR

This study investigates the internal dynamics of the young massive star cluster Westerlund 2 using MUSE and Gaia data, revealing complex stellar motions, gas expansion, and evidence that the cluster is not gravitationally bound.

Contribution

It provides a detailed analysis of the cluster's internal kinematics, linking stellar motions to initial cloud collapse and assessing its dynamical state with new mass estimates.

Findings

Gas radial velocity supports cloud-cloud collision origin.

Low-mass stars show five distinct velocity groups.

Cluster is likely unbound with a dynamical mass exceeding photometric estimates.

Abstract

Analyzing the dynamical state of nearby young massive star clusters is essential understanding star cluster formation and evolution during their earliest stages. In this work we analyze the stellar and gas kinematics of the young massive star cluster Westerlund 2 (Wd2) using data from the integral field unit MUSE and complement them with proper motions from the Gaia DR2. The mean gas radial velocity of $15.9\,{\rm km}\,{\rm s}^{-1}$ agrees with the assumption that Wd2 is the result of a cloud-cloud collision. The gas motions show the expansion of the HII region, driven by the radiation from the many OB stars in the cluster center. The velocity profile of the cluster member stars reveal an increasing velocity dispersion with decreasing stellar mass and that the low-mass stars show five distinct velocity groups. Based on their spatial correlation with the cluster's two clumps, we concluded that this is the imprint of the initial cloud collapse that formed Wd2. A thorough analysis of the dynamical state of Wd2, which determines a dynamical mass range of $M_{\rm dyn,Wd2} = (7.5 \pm 1.9)\cdot10^4 - (4.4 \pm 1.1)\cdot10^5\,{\rm M}_\odot$ and exceeds the photometric mass by at least a factor of two leads to the conclusion that Wd2 is not massive enough to remain gravitationally bound. Additionally we also identify 22 runaway candidates with peculiar velocities between 30 and $546\,{\rm km}\,{\rm s}^{-1}$.