arXiv 2103.100342021-03-18astro-ph.GA

在麦哲伦星云中寻找年轻星团的化学丰度变化：NGC 411、NGC 1718和NGC 2213

Searching for chemical abundance variations in young star clusters in the Magellanic Clouds: NGC 411, NGC 1718 and NGC 2213

Shalmalee Kapse, Richard de Grijs, Daniel B. Zucker

备注：: 11 pages, 5 Figures
DOI：: 10.1093/mnras/stab813

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摘要：

对同时代、化学成分均匀、人口众多的星团--被称为“简单恒星星族”（SSP）--的传统看法是对过去的看法。光度学和光谱学研究表明，银河系和我们邻近星系中几乎所有古老的球状星团都表现出星与星之间的光元素丰度变化，通常被称为“多种群”（MP）。在这里，我们分析了三个年轻的（$<$ 2 Gyr-old）大，小麦哲伦星系团，NGC 411，NGC 1718和NGC 2213的测光$\it Hubble$ $\it Space$ $\it Telescope$观测。我们测量了它们的红巨星分支（RGB）的宽度。对于NGC 411，我们还使用了伪彩色幅度图（伪CMD）来评估其RGB的证据MP。我们比较了集群的RGBs与人工生成的SSP的形态。我们的结论是，他们的RGBs没有表现出显着的扩大超出内在的光度散射的证据，这表明在我们的样品集群中没有显着的化学丰度变化。具体来说，对于NGC 411、NGC 1718和NGC 2213，我们得出的最大氦丰度变化分别为δ_Y=0.003 $\pm$ 0.001（Y=0.300）、0.002 $\pm$ 0.001（Y=0.350）和0.004 $\pm$ 0.002（Y=0.300）。我们确定了NGC 411氮丰度变化的上限为$Δ$ [N/Fe] = 0.3 dex;我们其他星系团的可用数据不允许我们确定有用的上限。因此，从SSP到MP的转变似乎发生在约2 Gyr的年龄，这意味着年龄可能在这种转变中发挥重要作用。这就提出了一个问题，即这是否确实是组成议员的基本最低年龄限制。

英文摘要：

The conventional picture of coeval, chemically homogeneous, populous star clusters -- known as `simple stellar populations' (SSPs) -- is a view of the past. Photometric and spectroscopic studies reveal that almost all ancient globular clusters in the Milky Way and our neighbouring galaxies exhibit star-to-star light-element abundance variations, typically known as 'multiple populations' (MPs). Here, we analyse photometric $\it Hubble$ $\it Space$ $\it Telescope$ observations of three young ($<$2 Gyr-old) Large and Small Magellanic Cloud clusters, NGC 411, NGC 1718 and NGC 2213. We measure the widths of their red-giant branches (RGBs). For NGC 411, we also use a pseudo-colour--magnitude diagram (pseudo-CMD) to assess its RGB for evidence of MPs. We compare the morphologies of the clusters' RGBs with artificially generated SSPs. We conclude that their RGBs do not show evidence of significant broadening beyond intrinsic photometric scatter, suggesting an absence of significant chemical abundance variations in our sample clusters. Specifically, for NGC 411, NGC 1718 and NGC 2213 we derive maximum helium-abundance variations of delta_Y=0.003$\pm$0.001 Y=0.300), 0.002$\pm$0.001 (Y=0.350) and 0.004$\pm$0.002 (Y=0.300), respectively. We determined an upper limit to the NGC 411 nitrogen-abundance variation of $Δ$[N/Fe] = 0.3 dex; the available data for our other clusters do not allow us to determine useful upper limits. It thus appears that the transition from SSPs to MPs occurs at an age of ~2 Gyr, implying that age might play an important role in this transition. This raises the question as to whether this is indeed a fundamental minimum-age limit for the formation of MPs.

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