Super-Helium-rich Populations and the Origin of Extreme Horizontal-Branch Stars in Globular Clusters
Super-Helium-rich Populations and the Origin of Extreme Horizontal-Branch Stars in Globular Clusters
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球状星团中超富氦星团和极端水平分支恒星的起源
DOI：
10.1086/428944
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发表时间：
2005
期刊：
影响因子：
--
通讯作者：
P. Demarque
中科院分区：
文献类型：
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作者：
Young;S. Joo;Sangil Han;C. Chung;C. Ree;Y. Sohn;Yong;Suk;S. Yi;P. Demarque
最近对大质量球状星团半人马座ω的色星等图（CMD）的观测表明，它有一个引人注目的双主序（MS），少数更蓝、更暗的MS星与大多数MS星很好地分离。在这里，我们通过最新的Y2等时线证实，只有假设该星系团中几个不同种群之间的原始氦丰度存在很大变化（ΔY = 0.15），才能重现这种特殊特征。我们进一步表明，MS上的这种特殊特征所需的相同氦增强本身就可以重现在ω Cen中观察到的极端水平分支（HB）星，它们比正常的HB星更热。类似地，其他球状星团（如NGC 2808）的HB上的复杂特征可以用氦丰度的巨大内部变化来解释。这些星团中的极端HB星的远紫外（FUV）观测也提供了支持富氦人口的证据，其中氦的增强可以自然地解释这些恒星观测到的较暗的FUV光度。在某些球状星团中存在的超氦丰富的人口表明，第三个参数，而不是金属丰度和年龄，也影响这些集群的CMD形态。
Recent observations for the color-magnitude diagrams (CMDs) of the massive globular cluster ω Centauri have shown that it has a striking double main sequence (MS), with a minority population of bluer and fainter MS stars well separated from a majority population of MS stars. Here we confirm, with the most up-to-date Y2 isochrones, that this special feature can only be reproduced by assuming a large variation (ΔY = 0.15) of primordial helium abundance among several distinct populations in this cluster. We further show that the same helium enhancement required for this special feature on the MS can by itself reproduce the extreme horizontal-branch (HB) stars observed in ω Cen, which are hotter than normal HB stars. Similarly, the complex features on the HBs of other globular clusters, such as NGC 2808, are explained by large internal variations of helium abundance. Supporting evidence for the helium-rich population is also provided by the far-UV (FUV) observations of extreme HB stars in these clusters, where the enhancement of helium can naturally explain the observed fainter FUV luminosity for these stars. The presence of super-helium-rich populations in some globular clusters suggests that a third parameter, other than metallicity and age, also influences the CMD morphology of these clusters.