Constraining the original composition of the gas forming first-generation stars in globular clusters
Constraining the original composition of the gas forming first-generation stars in globular clusters
复制标题
限制球状星团中形成第一代恒星的气体的原始成分
DOI：
10.1093/mnras/stac734
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发表时间：
2022
影响因子：
4.8
通讯作者：
Yong, D
中科院分区：
文献类型：
--
作者：
Legnardi, M V;Milone, A P;Armillotta, L;Marino, A F;Cordoni, G;Renzini, A;Vesperini, E;D’Antona, F;McKenzie, M;Yong, D
利用被称为染色体图（ChM）的伪双色图，可以沿着球状星团（GC）的红巨星分支（RGB）解开不同的恒星种群沿着。最有趣的发现之一是，所谓的第一代（1G）恒星，其特征是与其纳塔尔云相同的化学成分，在ChM中表现出扩展的序列。未解析的双星和氦或金属丰度的内部变化被认为可以解释这一现象。在这里，我们推导出GC NGC 6362和NGC 6838的高精度哈勃太空望远镜测光，并建立它们的ChM。我们发现，1G的RGB和主序（MS）星表现出更广泛的ChM序列比第二代（2G）。即使在未演化的1G MS恒星中也有这种特征的证据，表明化学不均匀性在原始气体中留下了印记。我们引入了一个伪二等图来区分氦和金属丰度，并证明恒星到星星的金属丰度变化是扩展的1G序列的原因。相反，二进制文件提供了一个小的贡献的现象。我们估计55个GC的1G恒星的金属丰度变化范围从小于[Fe/H]= 0.05到0.30，并与星团质量有一定的相关性。我们利用这些发现来约束多个种群的形成情景，表明它们在质量上与多代的发生是一致的。相比之下，2G恒星比1G恒星具有更均匀的铁含量，这一事实挑战了基于大质量1G恒星中处理的物质吸积到现有原恒星上的情况。
Disentangling distinct stellar populations along the red-giant branches (RGBs) of globular clusters (GCs) is possible by using the pseudo-two-colour diagram dubbed chromosome map (ChM). One of the most intriguing findings is that the so-called first-generation (1G) stars, characterized by the same chemical composition of their natal cloud, exhibit extended sequences in the ChM. Unresolved binaries and internal variations in helium or metallicity have been suggested to explain this phenomenon. Here, we derive high-precisionHubble Space Telescopephotometry of the GCs NGC 6362 and NGC 6838 and build their ChMs. We find that both 1G RGB and main-sequence (MS) stars exhibit wider ChM sequences than those of second-generation (2G). The evidence of this feature even among unevolved 1G MS stars indicates that chemical inhomogeneities are imprinted in the original gas. We introduce a pseudo-two-magnitude diagram to distinguish between helium and metallicity, and demonstrate that star-to-star metallicity variations are responsible for the extended 1G sequence. Conversely, binaries provide a minor contribution to the phenomenon. We estimate that the metallicity variations within 1G stars of 55 GCs range from less than [Fe/H]∼0.05 to ∼0.30 and mildly correlate with cluster mass. We exploit these findings to constrain the formation scenarios of multiple populations showing that they are qualitatively consistent with the occurrence of multiple generations. In contrast, the fact that 2G stars have more homogeneous iron content than the 1G challenges the scenarios based on accretion of material processed in massive 1G stars on to existing protostars.