窥探精度下限之下——II. 探究潜在球状星团姊妹天体NGC 288与NGC 362的化学动力学历史
Peeking beneath the precision floor -- II. Probing the chemo-dynamical histories of the potential globular cluster siblings, NGC 288 and NGC 362
AI总结：
本研究通过高精度元素丰度对比，探究球状星团NGC 288与NGC 362是否为随GSE并合事件吸积的姊妹天体，发现二者化学相似但与GSE恒星特征存在差异，推测二者或非姊妹天体，或GSE化学不均一。
摘要：
银河系（MW）的并合历史是一个发展迅速的研究领域，已有研究提出银河系历史中发生过多次小质量吸积事件及至少一次主并合事件。球状星团（GCs）随这些矮星系一同被吸积，是这些过往事件空间上连贯的遗迹。利用我们近期发表研究中的高精度微分丰度测量结果，我们探究了银河系星团NGC 362与NGC 288作为星系姊妹天体、随盖亚-香肠-恩克拉多斯（GSE）并合事件被吸积的可能性。为此，我们首次在0.01 dex的精度水平上对比了这两个球状星团的20余种元素丰度。研究发现二者高度相似，其化学相似性程度与大麦哲伦云（LMC）的三个球状星团NGC 1786、NGC 2210和NGC 2257之间的相似性相当。然而，将球状星团的丰度直接与GSE恒星对比时，观测到了显著差异。NGC 362的Eu与Mg、Si的比值与GSE恒星吻合良好，且呈现出明显的r过程相对于s过程的主导地位，而NGC 288仅表现出轻微的r过程主导特征。用类GSE星系化学演化模型拟合两个球状星团的丰度时，在相同金属丰度下（考虑Si、Ni、Ba和Eu元素），NGC 362的丰度与模型预测及GSE丰度比均吻合，NGC 288则不然。我们提出，二者要么并非星系姊妹天体，要么GSE的化学不均匀性足够高，足以形成两个相似但不完全相同、且与其组成恒星化学特征存在差异的星团。
英文摘要：
The assembly history of the Milky Way (MW) is a rapidly evolving subject, with numerous small accretion events and at least one major merger proposed in the MW's history. Accreted alongside these dwarf galaxies are globular clusters (GCs), which act as spatially coherent remnants of these past events. Using high precision differential abundance measurements from our recently published study, we investigate the likelihood that the MW clusters NGC 362 and NGC 288 are galactic siblings, accreted as part of the Gaia-Sausage-Enceladus (GSE) merger. To do this, we compare the two GCs at the 0.01 dex level for 20+ elements for the first time. Strong similarities are found, with the two showing chemical similarity on the same order as those seen between the three LMC GCs, NGC 1786, NGC 2210 and NGC 2257. However, when comparing GC abundances directly to GSE stars, marked differences are observed. NGC 362 shows good agreement with GSE stars in the ratio of Eu to Mg and Si, as well as a clear dominance in the r- compared to the s-process, while NGC 288 exhibits only a slight r-process dominance. When fitting the two GC abundances with a GSE-like galactic chemical evolution model, NGC 362 shows agreement with both the model predictions and GSE abundance ratios (considering Si, Ni, Ba and Eu) at the same metallicity. This is not the case for NGC 288. We propose that the two are either not galactic siblings, or GSE was chemically inhomogeneous enough to birth two similar, but not identical clusters with distinct chemistry relative to constituent stars.
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