[论文解读] Extended stellar systems in the solar neighborhood - II. Discovery of a nearby 120° stellar stream in Gaia DR2

Stefan Meingast, J. Alves+1 more|arXiv (Cornell University)|Jan 18, 2019

Stellar, planetary, and galactic studies参考文献 27被引用 35

一句话总结

该论文报道在约100 pc 处发现并表征一个巨大的、动态冷却的、同代恒星流，在 Gaia DR2 数据中被追踪，具有约120° 的天空跨度和约400 pc 的长度。

ABSTRACT

We report the discovery of a large, dynamically cold, coeval stellar stream that is currently traversing the immediate solar neighborhood at a distance of only 100 pc. The structure was identified in a wavelet decomposition of the 3D velocity space of all stars within 300 pc to the Sun. Its members form a highly elongated structure with a length of at least 400 pc, while its vertical extent measures only about 50 pc. Stars in the stream are not isotropically distributed but instead form two parallel lanes with individual local overdensities, that may correspond to a remnant core of a tidally disrupted cluster or OB association. Its members follow a very well-defined main sequence in the observational Hertzsprung-Russel diagram and also show a remarkably low 3D velocity dispersion of only 1.3 km s$^{-1}$. These findings strongly suggest a common origin as a single coeval stellar population. An extrapolation of the present-day mass function indicates a total mass of at least 2000 M$_\odot$, making it larger than most currently known clusters or associations in the solar neighborhood. We estimated the stream's age to be around 1 Gyr based on a comparison with a set of isochrones and giant stars in our member selection and find a mean metallicity of $\left[ \mathrm{Fe/H} ight] = -0.04$. This structure may very well represent the Galactic disk counterpart to the prominent stellar streams observed in the Milky Way halo. As such, it constitutes a new valuable probe to constrain the Galaxy's mass distribution.

研究动机与目标

• 在太阳邻近区域，识别超出已知群体的动态冷恒星结构。
• 表征新发现的恒星流的形态、运动学和同代特征。
• 估计年龄、金属丰度和总质量，以评估起源及作为银河盘质量探针的潜在价值。

提出的方法

• 为距太阳 300 pc 的恒星构建三维速度空间直方图（v_r, v_phi, v_z）。
• 应用小波分解以在 ~1.5 km/s 的尺度上识别过度密度。
• 提取一个速度峰并在 (r, phi, z, v_r, v_phi, v_z) 上使用 DBSCAN 进行六维聚类。
• 施加空间密度过滤以得出最终流成员名单，并与 HR 图和等时线进行交叉检验。
• 通过跨银河平面的镜像相空间采样来估计污染。
• 将流成员与 LAMOST 进行交叉匹配以估计金属丰度。

实验结果

研究问题

• RQ1 太阳邻近区域在已知群体之外，是否存在并延伸的动态冷恒星流？
• RQ2 新识别恒星流的运动学、结构和恒星种群特征（年龄、金属丰度、质量）为何？
• RQ3 该盘系相关流如何帮助理解银河的质量分布？

主要发现

• 识别出一个约 400 pc 长、垂直尺度约 50 pc、轴比为 8.4:2.7:1.0 的大而拉长的流。
• 在结构中测得异常低的三维速度色散为 1.3 km/s。
• 发现该流位于银河平面下方约 75–100 pc，并向远离平面的方向移动但尚未逃逸到星 halo。
• 在观测的 HR 图中清晰地观察到主序带，支持单一同代族群，年龄约为 1 Gyr。
• 估计的现在质量函数表明总质量至少 ≳2000 Msun，比大多数近邻簇/关联更大。
• 来自 LAMOST 交叉匹配的平均金属丰度： [Fe/H] ≈ -0.04 ± 0.15。