Westerlund 1: bound or unbound?

TL;DR

This study measures the velocity dispersion and dynamical mass of Westerlund 1, a nearby young massive star cluster, to determine if it is gravitationally bound or dissolving, finding it likely remains bound.

Contribution

The paper provides improved velocity dispersion measurements and dynamical mass estimates for Westerlund 1, clarifying its bound status with new spectroscopic data.

Findings

Velocity dispersion measured at 9.2 km/s

Dynamical mass estimated at 1.5x10^5 Msun

Cluster appears gravitationally bound

Abstract

The Galactic open cluster Westerlund 1 (Wd 1) represents the ideal local template for extragalactic young massive star clusters, because it is currently the only nearby young cluster which reaches a mass of around 10^5 Msun. The proximity makes spatially resolved studies of its stellar population feasible, and additionally permits direct comparison of its properties with measurements of velocity dispersion and dynamical mass for spatially unresolved extragalactic clusters. Recently, we published the dynamical mass estimate based on spectra of four red supergiants. We have now identified six additional stars which allow a determination of radial velocity from the wavelength covered in our VLT/ISAAC near-infrared spectra (CO bandhead region near 2.29micron), improving statistics significantly. Using a combination of stepping and scanning the slit across the cluster centre, we covered an area which included the following suitable spectral types: four red supergiants, five yellow hypergiants, and one B-type emission line star. Our measured velocity dispersion is 9.2 km/s. Together with the cluster size of 0.86 pc, derived from archival near-infrared SOFI-NTT images, this yields a dynamical mass of 1.5x10^5 Msun. Comparing this to the mass derived via photometry, there is no indication that the cluster is currently undergoing dissolution.