Structure and Dynamics of the Young Massive Star Cluster Westerlund 1

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 ${\Lambda }_{\mathrm{MSR}}=1.11\pm 0.11$, 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 $10M_{\odot }$, 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.

25 pages, 24 figures, 7 tables

References in corpus (64)

• Astropy: A Community Python Package for Astronomy
• emcee: The MCMC Hammer
• The Astropy Project: Building an inclusive, open-science project and status of the v2.0 core package
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• MultiNest: an efficient and robust Bayesian inference tool for cosmology and particle physics
• MESA Isochrones and Stellar Tracks (MIST). I: Solar-Scaled Models
• A new extensive library of PHOENIX stellar atmospheres and synthetic spectra
• Grids of stellar models with rotation - I. Models from 0.8 to 120 Msun at solar metallicity (Z = 0.014)
• X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue
• New evolutionary models for pre-main sequence and main sequence low-mass stars down to the hydrogen-burning limit
• Young massive star clusters
• Models of Stars, Brown Dwarfs and Exoplanets
• Stellar Orbits Around the Galactic Center Black Hole
• Resolved Massive Star Clusters in the Milky Way and its Satellites: Brightness Profiles and a Catalogue of Fundamental Parameters
• An ACS Survey of Globular Clusters V: Star Catalog for Each Cluster
• On the fraction of star formation occurring in bound stellar clusters
• A Disk of Young Stars at the Galactic Center as Determined by Individual Stellar Orbits
• Stellar Populations in the Central 0.5 pc of the Galaxy II: The Initial Mass Function
• Surface Brightness Profiles and Structural Parameters for 53 Rich Stellar Clusters in the Large Magellanic Cloud
• On the massive stellar population of the Super Star Cluster Westerlund 1
• The Pisa pre-main sequence tracks and isochrones. A database covering a wide range of Z, Y, mass, and age values
• Using the minimum spanning tree to trace mass segregation
• A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy
• PAndAS' progeny: extending the M31 dwarf galaxy cabal
• Properties of the Remnant Clockwise Disk of Young Stars in the Galactic Center
• The Effect of Gas Loss on the Formation of Bound Stellar Clusters
• Gravitational fragmentation and the formation of brown dwarfs in stellar clusters
• The State-of-the-Art HST Astro-Photometric Analysis of the core of $\omega$ Centauri. I. The Catalog
• The impact of mass loss on star cluster formation. I. Analytic results
• Intermediate to low-mass stellar content of Westerlund 1
• Atmospheres from very low-mass stars to extrasolar planets
• The Physics of Star Cluster Formation and Evolution
• Surface Brightness Profiles and Structural Parameters for 10 Rich Stellar Clusters in the Small Magellanic Cloud
• Stellar Populations in the Central 0.5 pc of the Galaxy I: A New Method for Constructing Luminosity Functions and Surface-Density Profiles
• The HST large programme on $\omega$ Centauri - II. internal kinematics
• The population of OB supergiants in the starburst cluster Westerlund 1
• The Unusual Initial Mass Function of the Arches Cluster
• Proper motions of the Arches cluster with Keck-LGS Adaptive Optics: the first kinematic mass measurement of the Arches
• Mass segregation and elongation of the starburst cluster Westerlund 1
• Disruption of giant molecular clouds and formation of bound star clusters under the influence of momentum stellar feedback
• Diffuse, Non-Thermal X-ray Emission from the Galactic Star Cluster Westerlund 1
• Objective priors for the bivariate normal model
• On the velocity dispersion of young star clusters: super-virial or binaries?
• The Starburst Cluster Westerlund 1: The Initial Mass Function and Mass Segregation
• The effect of binaries on the dynamical mass determination of star clusters
• Is the massive young cluster Westerlund I bound?
• Comparisons between different techniques for measuring mass segregation
• Extinction law in the range 0.4-4.8 $\mu$m and the 8620 $\mathring{A}$ DIB towards the stellar cluster Westerlund 1
• The Formation and Early Evolution of Young Massive Clusters
• Grids of stellar models with rotation VII: Models from 0.8 to 300 M${}_{\odot }$ at super-solar metallicity (Z = 0.020)
• The very low-mass stellar content of the young supermassive Galactic star cluster Westerlund 1
• SPISEA: A Python-Based Simple Stellar Population Synthesis Code for Star Clusters
• The Age of Westerlund 1 Revisited
• The Optical/Near-Infrared Extinction Law In Highly Reddened Regions
• Tracing the stellar component of low surface brightness Milky Way Dwarf Galaxies to their outskirts I: Sextans
• Stellar models and isochrones from low-mass to massive stars including pre-main sequence phase with accretion
• Westerlund 1 under the light of Gaia EDR3: Distance, isolation, extent, and a hidden population
• The Quintuplet Cluster: Extended Structure and Tidal Radius
• Inferring the parallax of Westerlund 1 from Gaia DR2
• Westerlund 1: bound or unbound?
• Hierarchical formation of Westerlund 1: a collapsing cluster with no primordial mass segregation?
• Distance and age of the massive stellar cluster Westerlund 1. I. Parallax method using Gaia-EDR3
• Distance and age of the massive stellar cluster Westerlund 1. II. The eclipsing binary W36
• The 3D Kinematics of the Orion Nebula Cluster II: Mass-dependent Kinematics of the Inner Cluster