[论文解读] Escape from the Bermuda cluster: Orphanization by multiple stellar ejections

J. Maíz Apellániz, M. Pantaleoni González+2 more|arXiv (Cornell University)|May 31, 2022

Stellar, planetary, and galactic studies参考文献 71被引用 15

一句话总结

本研究利用盖亚 EDR3 天体测量数据和多波段数据，从百慕大星团（Villafranca O-014 NW）中识别出三次独立的恒星抛射事件——分别命名为巴哈马、多伦多和 HD 201 795。这三次事件分别发生在161万年、150万年和191万年前，抛射质量超过200 M⊙，包括星团中三颗最质量的恒星，证实百慕大星团为一个孤立星团，其当前质量函数显著改变，与Kroupa型初始质量函数一致。

ABSTRACT

[Context] Dynamical interactions in young stellar clusters can eject massive stars early in their lives and significantly alter their mass functions. If all of the most massive stars are lost, we are left with an orphan cluster. [Aims] We study the Bermuda cluster (Villafranca O-014 NW), the most significant young stellar group in the North America and Pelican nebulae, and the massive stars that may have been ejected from it to test if it has been orphane-d. [Methods] We use Gaia EDR3 parallaxes and proper motions to search for walkaway and runaway stars in the vicinity of the North America and Pelican nebulae. The candidates are analyzed with a combination of spectroscopy and photometry to assess their nature and their trajectories are traced back in time to determine at what time they left the Bermuda cluster. [Results] We detect three ejection events, dubbed the Bajamar, Toronto, and HD 201 795 events, which expelled (a minimum of) five, two, and two systems, respectively, or six, three, and three stars if we count the individual components in spectroscopic and eclipsing binaries. The events took place 1.611 ± 0.011 Ma, 1.496 ± 0.044 Ma, and 1.905 ± 0.037 Ma ago, respectively, but our analysis is marginally consistent with the first two being simultaneous. We detect bow shocks in WISE images associated with four of the ejected systems; their orientation agrees with that of their relative proper motions with respect to the cluster. Combining the three events, the Bermuda cluster has lost over 200 M, including its three most massive stars, so it can be rightfully considered an orphan cluster. One consequence is that the present-day mass function of the cluster has been radically altered from its top-heavy initial value to one compatible with a Kroupa-like function. Another is that the cluster is currently expanding with a dynamical timescale consistent with the cause being the ejection events. A scenario in which the Bermuda cluster was formed in a conveyor-belt fashion over several hundreds of ka or even 1 Ma is consistent with all the observables.

研究动机与目标

• 确定百慕大星团（Villafranca O-014 NW）是否因早期抛射其最质量恒星而成为孤立星团。
• 利用盖亚 EDR3 天体测量数据，识别并表征北美洲星云和野马星云区域内的逃逸星和慢行星。
• 追溯被抛射恒星的轨迹，以估算其从百慕大星团抛射的时间，并评估其对星团质量函数的动力学影响。
• 调查红外数据中是否存在弓形激波，作为高速抛射的证据。
• 评估星团当前的扩张与质量函数演化是否与多次抛射事件一致。

提出的方法

• 利用盖亚 EDR3 的视差和自行数据，识别北美洲星云和野马星云区域附近高速恒星。
• 结合光谱和测光分析，分类候选抛射恒星的性质，评估其径向速度和双星性质。
• 将被抛射恒星的轨迹向后追溯，以确定其从百慕大星团抛射的时期。
• 分析WISE红外图像，检测与被抛射恒星相关的弓形激波，并将其方向与自行矢量进行比较。
• 通过累加被抛射系统的质量（包括双星系统中的成员）计算星团损失的总质量。
• 评估抛射情景与星团当前扩张 timescale 和质量函数演化的自洽性。

实验结果

研究问题

• RQ1 百慕大星团是否因动力学抛射其最质量恒星而成为当前的孤立星团？
• RQ2 从星团中抛射的最质量恒星的抛射时间与运动学特性是什么？
• RQ3 是否存在可观测的特征（如弓形激波）与被抛射恒星相关？
• RQ4 由于多次抛射事件，星团损失了多少质量？对当前质量函数有何影响？
• RQ5 星团当前的扩张是否与抛射事件的时间和能量一致？

主要发现

• 三次独立的抛射事件——巴哈马、多伦多和 HD 201 795——分别发生在161.1±0.011 Ma、149.6±0.044 Ma 和190.5±0.037 Ma 之前，微小的一致性表明前两次事件可能近乎同时发生。
• 抛射事件导致至少5、2和2个系统被抛出（若计入双星系统，则为6、3和3颗恒星），总质量损失超过200 M⊙。
• 四个被抛射系统在 WISE 图像中显示出弓形激波，且激波方向与自行矢量一致，证实其为高速抛射。
• 星团当前的质量函数与 Kroupa 型初始质量函数一致，表明其原始的顶重态已发生根本性转变。
• 星团当前正以与推断的抛射事件相匹配的动力学 timescale 扩张，支持抛射情景。
• 一个持续数10万至100万年的“传送带”形成过程，与所有观测到的运动学和天体测量数据一致。