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Published October 20, 2025 | Version Published
Journal Article Open

Structure and Dynamics of the Young Massive Star Cluster Westerlund 1

Creators

  • 1. ROR icon University of California, San Diego
  • 2. ROR icon University of California, Los Angeles
  • 3. ROR icon University of California, Berkeley
  • 4. ROR icon University of Maryland, College Park
  • 5. Cinamon Corp, ., 484 Gangnam-daero, Gangnam-gu, Seoul 06120, Republic of Korea
  • 6. ROR icon California Institute of Technology
  • 7. ROR icon W.M. Keck Observatory
  • 8. Verkada Inc., 406 East 3rd Avenue, San Mateo, CA 94401, USA
  • 9. ROR icon Space Telescope Science Institute

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 ΛMSR = 1.11 ± 0.11, only above unity by 1σ. 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.

Copyright and License

© 2025. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

The HST data presented in this article were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute. The specific observations analyzed can be accessed via DOI:10.17909/r5kb-7h31. We acknowledge the anonymous referee for providing constructive feedback. L.W. acknowledges Prof. Smadar Naoz for providing valuable advice on timescale analysis. This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5–26555. These observations are associated with programs GO-13044 and GO-13809. J.R.L., P.C.B., D.K., and M.S. acknowledge support from the National Science Foundation Astronomy and Astrophysics Grant AST-1764218. N.Z.R. acknowledges support from the National Science Foundation Graduate Research Fellowship under grant No. DGE–1745301. Portions of this work were conducted at the University of California, San Diego, which was built on the unceded territory of the Kumeyaay Nation, whose people continue to maintain their political sovereignty and cultural traditions as vital members of the San Diego community.

Facilities

HST WFC3 - .

Software References

astropy (Astropy Collaboration et al. 2013, 2018, 2022), emcee (D. Foreman-Mackey et al. 2013), Matplotlib (J. D. Hunter 2007).

Files

Wei_2025_ApJ_992_213.pdf

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2501.16755 (arXiv)
Is supplemented by
Dataset: 10.17909/r5kb-7h31 (DOI)

Funding

National Aeronautics and Space Administration
NAS 5-26555
National Science Foundation
AST-1764218
National Science Foundation
DGE-1745301

Dates

Accepted
2025-08-11
Available
2025-10-17
Published online

Caltech Custom Metadata

Caltech groups
TAPIR, Division of Physics, Mathematics and Astronomy (PMA)
Publication Status
Published