一种名为 Oosterhoff 二分法的“异类”？
An "alien" called Oosterhoff dichotomy?
AI总结：
本研究利用 Gaia DR3 中上万颗场天琴座 RR 型变星的轨道、运动积分与 Bailey 图分类，发现 Oosterhoff 二分法仅显著存在于吸积起源恒星中，表明其由古老星系并合引入银河系。
摘要：
在这篇快报中，我们结合近期有关外部星系与银河系（MW）发生的数次古老并合事件的发现，研究了 Oosterhoff 二分法的起源。我们尤其旨在阐明，银河系球状星团（GGC）与场天琴座 RR 型变星（RRL）在 Oosterhoff 类型上的划分，是否可以追溯到过去一个或多个与 MW 并合的古老星系。为此，我们首先依据不同文献研究，考察了 GGC 与过往并合事件之间的关联。随后，我们整理了来自 Gaia Data Release 3 的 10,138 颗场 RRL 变星的位置、自行和视向速度。为推断距离，我们采用 $M_G$–[Fe/H] 关系，其中 [Fe/H] 值通过涉及单个周期和 Fourier 参数的经验关系进行估计。然后，我们使用 Python 库 Galpy 计算了整个样本的轨道和运动积分（IoM）。通过比较场 RRL 与 GGC 在能量—角动量图中的位置，我们指定了它们可能的起源。最后，我们根据 RRL 恒星样本在 Bailey 图中的位置，从 Gaia G 波段光变曲线中判别其 Oosterhoff 类型。对具有 In-Situ（原位）与 Accreted（吸积）晕起源的银河系 RRL 恒星及 GGC 的 Bailey 图分析显示出显著差异。In-Situ 样本呈现广泛的金属丰度范围，分布连续，且没有 Oosterhoff 二分法的迹象。相反，Accreted RRL 清楚地表现出 Oosterhoff 二分法，并且金属丰度弥散显著更小。我们的结果表明，Oosterhoff 二分法是由塑造了银河系的并合事件带入 MW 的。
英文摘要：
In this letter we investigate the origin of the Oosterhoff dichotomy, considering recent discoveries related to several ancient merging events of external galaxies with the Milky Way (MW). In particular, we aim to clarify if the subdivision in Oosterhoff type of Galactic Globular Clusters (GGCs) and field RR Lyrae (RRLs) could be traced back to one or more ancient galaxies that merged with the MW in its past. To this purpose, we first explored the association of GGCs with the past merging events according to different literature studies. Subsequently we compiled positions, proper motions and radial velocity for 10,138 field RRLs variables from the $Gaia$ Data Release 3. To infer the distances, we adopted the $M_G$--[Fe/H] relation, with [Fe/H] values estimated through empirical relationships involving the individual periods and Fourier parameters. We then calculated the orbits and the integrals of motions (IoM) using the Python library Galpy for the whole sample. By comparing the location of the field RRLs in the energy-angular momentum diagram with that of the GGCs we assign their likely origin. Finally, we discriminate from the $Gaia$ G-band light curves the Oosterhoff type of our sample of RRL stars based on their location in the Bailey diagram. The analysis of the Bailey diagrams for Galactic RRLs stars and GGCs associated with \textit{In-Situ} vs \textit{Accreted} halo origin shows remarkable differences. The \textit{In-Situ} sample displays a wide range of metallicities with a continuous distribution and no sign of Oosterhoff dichotomy. Conversely, the \textit{Accreted} RRLs clearly shows the Oosterhoff dichotomy and a significantly smaller dispersion in metallicity. Our results suggest that the Oosterhoff dichotomy was imported into the MW by the merging events that shaped the Galaxy.
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