EWOCS-XI：Westerlund 1星团中的各向同性膨胀与剪切——基于Gaia DR3的二维运动学分析
EWOCS-XI: Anisotropic expansion and shear in the cluster Westerlund 1. A 2D kinematic analysis with Gaia DR3
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中文总结 AI 辅助
本研究通过Gaia DR3数据，采用5D聚类与二维泰勒展开方法分析Westerlund 1星团，发现其含两个解耦子系统，膨胀与银道面穿越时间匹配，星团正通过离散动力学抛射活跃瓦解。
中文摘要 AI 辅助
Westerlund 1（Wd1）是银河系中质量最大的年轻星团之一，长期以来，严重的消光和场星污染阻碍了对其运动学特征的表征。本研究旨在评估Wd1的内部运动学、动力学状态及形成场景。采用稳健的5D聚类方法，我们识别出559颗真实成员星；通过二维一阶泰勒展开对其自行进行建模，推导速度梯度张量，以绘制膨胀、旋转和剪切特征图。将运动学分析限制在524颗自行不确定度<0.5 mas yr⁻¹的恒星中，我们发现至少103颗引力未束缚成员，其中包括14颗逃逸恒星。二维分析显示存在统计显著的发散（>4σ），凸显出沿银纬的优先膨胀（位置角为164°±10.1°）；检测到的交叉梯度（∂v_b/∂l = -0.41±0.08 km s⁻¹ pc⁻¹）提供了天球平面上剪切的明确信号。Wd1并非表现为单一凝聚天体，而是包含两个解耦子系统：处于动力学平衡的稳定束缚核心，以及正在逃逸的未束缚包层。这种垂直于银道面的显著各向异性膨胀，其运动学时标（Δt_kin = 2.08±0.44 Myr）与独立的轨道中平面穿越时标（Δt_orbit = 2.10±0.05 Myr）匹配。这种时间一致性表明，膨胀的起始与Wd1穿过银道面的时间重合，触发因素可能是内部动力学（如早期超新星反馈）或外部潮汐扰动。最后，稳健的未束缚星群表明，星团正在经历活跃的瓦解，由离散的动力学抛射事件而非长期的恒星蒸发驱动。
英文摘要
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 $<0.5~\mathrm{mas\,yr^{-1}}$, we find at least 103 gravitationally unbound members, including 14 walkaway stars. The 2D analysis reveals a statistically significant divergence (>4$σ$) highlighting preferred expansion along the Galactic latitude (position angle $164^\circ\pm10.1^\circ$). A detected cross-gradient ($\partial v_b / \partial l = -0.41 \pm 0.08$ 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 ($Δt_{\mathrm{kin}} = 2.08 \pm 0.44$ Myr) matching the independent orbital mid-plane crossing time ($Δt_{\mathrm{orbit}} = 2.10 \pm 0.05$ 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.
发表机构
- INAF-Osservatorio Astronomico di Palermo（意大利国家天体物理研究所巴勒莫天文台）
- Instituto de Astrofísica, Pontificia Universidad Católica de Chile（智利天主教大学天体物理研究所）
- Departamento de Astronomía, Universidad de Concepción（康塞普西翁大学天文学系）
- Dipartimento di Scienze Economiche, Aziendali e Statistiche, Università degli Studi di Palermo（巴勒莫大学经济、企业统计科学系）
- Astrophysics Group, Keele University（基尔大学天体物理学组）
- European Southern Observatory（欧洲南方天文台）
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