arXiv 2104.097372021-04astro-ph.GA

银河系恒星星族的蓝图。III。局域晕星的空间分布和星族组成

A Blueprint for the Milky Way's Stellar Populations. III. Spatial Distributions and Population Fractions of Local Halo Stars

Deokkeun An, Timothy C. Beers

备注：: 32 pages, 14 figures, 3 tables, accepted for publication in the Astrophysical Journal after minor revision
DOI：: 10.3847/1538-4357/ac07a4

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摘要：

我们分析了观测到的空间，化学和动力学分布的本地贫金属恒星，光度计推导的金属丰度和距离估计以及适当的运动从盖亚任务的基础上。沿着银河本初子午线，我们确定恒星星族的金属丰度与旋转速度空间，包括盖亚香肠/土卫二（GSE），金属弱厚盘（MWTD），和飞溅（有时被称为“原位”晕）的不同属性。我们使用高斯混合物模型观测到的相空间分布，并完善其位置和分数的贡献作为从银河系平面（$|Z|$）和银河系中心（$R_{\rm GC}$）的距离的函数，提供了一个全球性的视角，主要的恒星星族在当地的晕。在样本体积（$|Z|<6$ kpc）内，与GSE相关的恒星在更大的$R_{\rm GC}$（$Δ\langle{\rm[Fe/H]}\rangle /ΔR_{\rm GC} =-0.05\pm0.02$ dex kpc $^{-1}$）下表现出更大比例的贫金属恒星。这一观测到的趋势，以及平均自转速度与金属丰度（$Δ\langle v_ϕ\rangle / Δ\langle{\rm[Fe/H]} \rangle \sim -10$ km s $^{-1}$ dex $^{-1}$）之间的温和的正相关关系，意味着在GSE前身星内部区域中更多的富含金属的恒星逐渐被剥离，而在合并时的反轨道在动力学摩擦的作用下变得辐射化。富含金属的GSE星与Splash结构有因果关系，Splash结构中的恒星大多数位于外圆轨道（$>94\%$）上，并且比GSE表现出更集中的分布。MWTD表现出类似的空间分布的飞溅，这表明早期的动力加热的恒星在银河系的原始磁盘，可能之前的GSE合并。

英文摘要：

We analyze the observed spatial, chemical and dynamical distributions of local metal-poor stars, based on photometrically derived metallicity and distance estimates along with proper motions from the Gaia mission. Along the Galactic prime meridian, we identify stellar populations with distinct properties in the metallicity versus rotational velocity space, including Gaia Sausage/Enceladus (GSE), the metal-weak thick disk (MWTD), and the Splash (sometimes referred to as the "in situ" halo). We model the observed phase-space distributions using Gaussian mixtures and refine their positions and fractional contributions as a function of distances from the Galactic plane ($|Z|$) and the Galactic center ($R_{\rm GC}$), providing a global perspective of the major stellar populations in the local halo. Within the sample volume ($|Z|<6$ kpc), stars associated with GSE exhibit a larger proportion of metal-poor stars at greater $R_{\rm GC}$ ($Δ\langle{\rm[Fe/H]}\rangle /ΔR_{\rm GC} =-0.05\pm0.02$ dex kpc$^{-1}$). This observed trend, along with a mild anticorrelation of the mean rotational velocity with metallicity ($Δ\langle v_ϕ\rangle / Δ\langle{\rm[Fe/H]} \rangle \sim -10$ km s$^{-1}$ dex$^{-1}$), implies that more metal-rich stars in the inner region of the GSE progenitor were gradually stripped away, while the prograde orbit of the merger at infall became radialized by dynamical friction. The metal-rich GSE stars are causally disconnected from the Splash structure, whose stars are mostly found on prograde orbits ($>94\%$) and exhibit a more centrally concentrated distribution than GSE. The MWTD exhibits a similar spatial distribution to the Splash, suggesting earlier dynamical heating of stars in the primordial disk of the Milky Way, possibly before the GSE merger.

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