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arXiv 1902.02178astro-ph.SR
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大质量外晕球状星团NGC 2419中的四种恒星星族和极端氦变化

Four stellar populations and extreme helium variation in the massive outer-halo globular cluster NGC 2419

M. Zennaro, A. P. Milone, A. F. Marino, G. Cordoni, E. P. Lagioia, M. Tailo

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10 pages, 8 figures, submitted to MNRAS January 22nd
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摘要：

最近的研究表明，无论是球状星团（GC）内的氦变化，还是第一代和第二代恒星（1G，2G）的相对数量，都取决于主星团的质量。精确测定氦的内部变化和1G恒星的比例是多星族形成的关键约束条件。我们利用多波段哈勃空间望远镜测光研究NGC 2419中的MP，NGC 2419是银河系中最大和最遥远的GC之一，几乎与其潮汐影响无关。我们发现，1G主机的分析恒星的~37%，并确定了三个2G恒星，即2GA，2GB和2GC，其中包括~ 20%，~31%和~12%的恒星，分别。我们比较观察到的颜色，这四个群体的颜色来自适当的合成光谱，以推断相对氦丰度。我们发现2GA、2GB和2GC星的氦质量分数相对于具有原始氦的1G星（Y=0.246）分别增加了约0.01、0.06和0.19。2GC恒星的高氦丰度很难与目前大多数MP的情况相协调。此外，与其他大质量GC相比，1G恒星的比例相对较大（约37%）。利用文献的结果，我们发现，1G星的GC具有大的近星系距离的分数通常高于其他GC具有类似的质量。这表明NGC 2419与其他遥远的GC类似，失去了较低比例的1G恒星。

英文摘要：

Recent work revealed that both the helium variation within globular clusters (GCs) and the relative numbers of first and second-generation stars (1G, 2G) depend on the mass of the host cluster. Precise determination of the internal helium variations and of the fraction of 1G stars are crucial constraints to the formation scenarios of multiple populations (MPs). We exploit multi-band Hubble Space Telescope photometry to investigate MPs in NGC 2419, which is one of the most-massive and distant GCs of the Galaxy, almost isolated from its tidal influence. We find that the 1G hosts the ~37% of the analyzed stars, and identified three populations of 2G stars, namely 2GA, 2GB, and 2GC, which comprise the ~20%, ~31% and ~12% of stars, respectively. We compare the observed colors of these four populations with the colors derived from appropriate synthetic spectra to infer the relative helium abundances. We find that 2GA, 2GB, and 2GC stars are enhanced in helium mass fraction by deltaY ~0.01, 0.06, and 0.19 with respectto 1G stars that have primordial helium (Y=0.246). The high He enrichment of 2GC stars is hardly reconcilable with most of the current scenarios for MPs. Furthermore, the relatively larger fraction of 1G stars (~37%) compared to other massive GCs is noticeable. By exploiting literature results, we find that the fractions of 1G stars of GCs with large perigalactic distance are typically higher than in the other GCs with similar masses. This suggests that NGC 2419, similarly to other distant GCs, lost a lower fraction of 1G stars.

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