EWOCS-XI: Anisotropic expansion and shear in the cluster Westerlund 1. A 2D kinematic analysis with Gaia DR3
Abstract
Westerlund 1 (Wd1) is one of the Milky Way's most massive young clusters, in which heavy extinction and field contamination have historically hampered its kinematic characterization. We aim to assess Wd1's internal kinematics, dynamical state, and formation scenario. Using a robust 5D clustering approach, we identified 559 bona fide members. We modeled their proper motions via a 2D first-order Taylor expansion to derive the velocity gradient tensor, mapping expansion, rotation, and shear. Restricting the kinematic analysis to 524 stars with proper motion uncertainties <0.5 mas yr−1, we find at least 103 gravitationally unbound members, including 14 walkaway stars. The 2D analysis reveals a statistically significant divergence (>4σ) highlighting preferred expansion along the Galactic latitude (position angle 164∘±10.1∘). A detected cross-gradient (∂ vb / ∂ l = −0.41 ± 0.08 km s−1 pc−1) provides a clear signature of shear on the plane of the sky. Wd1 does not behave as a single cohesive population, but comprises two decoupled subsystems: a stable, bound core in dynamical equilibrium and an escaping, unbound envelope. This prominent anisotropic expansion perpendicular to the Galactic plane has a kinematic timescale (Δtkin = 2.08 ± 0.44 Myr) matching the independent orbital mid-plane crossing time (Δtorbit = 2.10 ± 0.05 Myr). This temporal alignment indicates the expansion onset coincided with Wd1's Galactic disk passage, triggered either by internal dynamics (e.g., early supernova feedback) or external tidal perturbations. Finally, the robust unbound population demonstrates that active cluster dissolution is underway, driven by discrete dynamical ejection events rather than long-term stellar evaporation.
Related
- EWOCS-IX: JWST/NIRCam observations of Westerlund 2 - Identification of candidate substellar members
- EWOCS-IX: JWST/NIRCam observations of Westerlund 2
- Extended gamma-ray emission in the vicinity of the Westerlund 1 massive star cluster and Kes 41 supernova remnant seen by the Fermi Large Area Telescope
- A Hectospec Spectroscopic Census of Kinematic Members in the Orion Nebula Cluster
- A Hectospec Spectroscopic Census of Kinematic Members in the Orion Nebula Cluster
BibTeX
@misc{prisinzano2026ewocsxi,
title = {{EWOCS-XI: Anisotropic expansion and shear in the cluster Westerlund 1. A 2D kinematic analysis with Gaia DR3}},
author = {L. Prisinzano and F. Damiani and C. Ordenes-Huanca and M. Tarantino and S. Sciortino and N. J. Wright and V. Almendros-Abad and M. Andersen and A. Bayo and R. Bonito and V. Cusimano and M. Haberle and K. Mužic and I. Negueruela and E. Sabbi and A. Winter and M. G. Guarcello},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.12117},
eprint = {2610.12117},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.12117},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - EWOCS-XI: Anisotropic expansion and shear in the cluster Westerlund 1. A 2D kinematic analysis with Gaia DR3
AU - L. Prisinzano
AU - F. Damiani
AU - C. Ordenes-Huanca
AU - M. Tarantino
AU - S. Sciortino
AU - N. J. Wright
AU - V. Almendros-Abad
AU - M. Andersen
AU - A. Bayo
AU - R. Bonito
AU - V. Cusimano
AU - M. Haberle
AU - K. Mužic
AU - I. Negueruela
AU - E. Sabbi
AU - A. Winter
AU - M. G. Guarcello
PY - 2026
DA - 2026/10/08
PB - arXiv
DO - 10.48550/arxiv.2610.12117
UR - https://arxiv.org/abs/2610.12117
AB - Westerlund 1 (Wd1) is one of the Milky Way's most massive young clusters, in which heavy extinction and field contamination have historically hampered its kinematic characterization. We aim to assess Wd1's internal kinematics, dynamical state, and formation scenario. Using a robust 5D clustering approach, we identified 559 bona fide members. We modeled their proper motions via a 2D first-order Taylor expansion to derive the velocity gradient tensor, mapping expansion, rotation, and shear. Restricting the kinematic analysis to 524 stars with proper motion uncertainties <0.5 mas yr^−1, we find at least 103 gravitationally unbound members, including 14 walkaway stars. The 2D analysis reveals a statistically significant divergence (>4σ) highlighting preferred expansion along the Galactic latitude (position angle 164^∘±10.1^∘). A detected cross-gradient (∂ v_b / ∂ l = −0.41 ± 0.08 km s^−1 pc^−1) provides a clear signature of shear on the plane of the sky. Wd1 does not behave as a single cohesive population, but comprises two decoupled subsystems: a stable, bound core in dynamical equilibrium and an escaping, unbound envelope. This prominent anisotropic expansion perpendicular to the Galactic plane has a kinematic timescale (Δt_kin = 2.08 ± 0.44 Myr) matching the independent orbital mid-plane crossing time (Δt_orbit = 2.10 ± 0.05 Myr). This temporal alignment indicates the expansion onset coincided with Wd1's Galactic disk passage, triggered either by internal dynamics (e.g., early supernova feedback) or external tidal perturbations. Finally, the robust unbound population demonstrates that active cluster dissolution is underway, driven by discrete dynamical ejection events rather than long-term stellar evaporation.
ER -