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arXiv 1105.0346astro-ph.SR
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NGC 2808巨星氦丰度差的测量

Measuring helium abundance difference in giants of NGC 2808

L. Pasquini, P. Mauas, H. U. Kaufl, C. Cacciari

备注：
Accepted for publication in A&A
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摘要：

在几个球状星团（GC）中发现了多个种群，这些星团没有显示出金属丰度的分布。它们的CMD的不寻常特征可以解释为属于子种群的恒星的氦含量的差异。He丰度的差异从未被直接观察到。我们试图测量这些差异在两个巨大的NGC 2808恒星非常相似的参数，但不同的Na和O丰度，因此，可能属于不同的子群体，通过直接比较他们的He I 10830线。He 10830谱线形成于色球上层。我们的详细模型推导出色球结构使用Ca II和H $α$，并模拟相应的He I 10830线轮廓。我们表明，在一个给定的值，他丰富，他我10830等效宽度不能显着改变，而相应的更大的变化，在钙II线。我们用VLT-CRIRES获得了10830 nm区域的高分辨率光谱，用VLT-UVES获得了目标星的Ca II和H $α$线的光谱。这两颗靶星有非常相似的Ca II和H $α$线，但在He区的外观不同。一条线，蓝移17公里秒$^{-1}$相对于他10830休息波长，检测到的光谱中的钠丰富的恒星，而钠贫星光谱是一致的非检测。光谱的差异是一致的，最接近的解释是两颗恒星的He丰度差异为$ΔY \ge$ 0.17。我们提供了直接的证据，证明NGC 2808中不同亚族的巨星存在显著的He线强度差异，这是以前用其他光度和光谱方法探测到的。

英文摘要：

Multiple populations have been detected in several globular clusters (GC) that do not display a spread in metallicity. Unusual features of their CMD can be interpreted in terms of differences in the Helium content of the stars belonging to the sub-populations. Differences in He abundance have never been directly observed. We attempt to measure these differences in two giant stars of NGC 2808 with very similar parameters but different Na and O abundances, hence that presumably belong to different sub-populations, by directly comparing their He I 10830 Å lines. The He 10830 Å line forms in the upper chromosphere. Our detailed models derive the chromospheric structure using the Ca II and H$α$, and simulate the corresponding He I 10830 line profiles. We show that, at a given value of He abundance, the He I 10830 equivalent width cannot significantly change without a corresponding much larger change in the Ca II lines. We have used the VLT-CRIRES to obtain high-resolution spectra in the 10830 Å region, and the VLT-UVES to obtain spectra of the Ca II and H$α$ lines of our target stars. The two target stars have very similar Ca II and H$α$ lines, but different appearances in the He region. One line, blueshifted by 17 km s$^{-1}$ with respect to the He 10830 rest wavelength, is detected in the spectrum of the Na-rich star, whereas the Na-poor star spectrum is consistent with a non-detection. The difference in the spectra is consistent and most closely explained by an He abundance difference between the two stars of $ΔY \ge$ 0.17.We provide direct evidence of a significant He line strength difference in giant stars of NGC 2808 belonging to different sub-populations, which had been previously detected by other photometric and spectroscopic means.

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