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arXiv 1202.6176astro-ph.GAastro-ph.SR
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NGC 1851中MS和SGB星的C和N丰度

C and N abundances of MS and SGB stars in NGC 1851

C. Lardo, A. P. Milone, A. F. Marino, A. Mucciarelli, E. Pancino, M. Zoccali, M. Rejkuba, R. Carrera, O. Gonzalez

备注：
17 pages, 3 tables, 16 figures; typos corrected, added checks on temperature scale; A&A accepted (in press)
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摘要：

本文首次对球状星团NGC 1851的双子巨星支（SGB）进行了化学分析。我们获得了48颗次巨星和暗星的MagellanIMACS光谱，覆盖了3650- 6750 <$A的光谱区域，从4300 <$A（G波段）和~ 3883 <$A（CN波段）的光谱特征推导了C和N的丰度。我们在我们的样本中加入了以前在VLT用FORS 2观测到的~ 45颗未演化的恒星。对这两个数据集进行了均匀简化和分析。我们计算了64颗恒星的C和N丰度，发现在所有亮度下，[C/H]和[N/H]都有相当大的恒星间变化，一直延伸到红巨星分支（RGB）底部（V~18.9）。这些丰度似乎是强相关的，预计从CN-循环富集，但我们没有检测到任何双峰的C或N含量。我们使用HST和地基测光从视觉和Strömgren色星等图中选择了两组暗淡和明亮的SGB星。每个星系群的恒星之间碳和氮的丰度存在显著的差异，这表明每个SGB都有多个恒星的子代。亮星和暗星的C+N含量不同，暗星的C+N含量是亮星的2.5倍。耦合我们的研究结果与文献光度数据和丰度测定从高分辨率的研究，我们确定了较暗的SGB与红色RGB人口，这也应该是更丰富的平均钡和其他S-过程元素，以及在钠和N，当比较明亮的SGB和蓝色RGB人口。

英文摘要：

We present the first chemical analysis of stars on the double subgiant branch (SGB) of the globular cluster NGC 1851. We obtained 48 Magellan IMACS spectra of subgiants and fainter stars covering the spectral region between 3650-6750Å, to derive C and N abundances from the spectral features at 4300Å(G-band) and at ~ 3883Å(CN). We added to our sample ~ 45 unvevolved stars previously observed with FORS2 at the VLT. These two datasets were homogeneously reduced and analyzed. We derived abundances of C and N for a total of 64 stars and found considerable star-to-star variations in both [C/H] and [N/H] at all luminosities extending to the red giant branch (RGB) base (V~18.9). These abundances appear to be strongly anticorrelated, as would be expected from the CN-cycle enrichment, but we did not detect any bimodality in the C or N content. We used HST and ground-based photometry to select two groups of faint- and bright-SGB stars from the visual and Strömgren color-magnitude diagrams. Significant variations in the carbon and nitrogen abundances are present among stars of each group, which indicates that each SGB hosts multiple subgenerations of stars. Bright- and faint-SGB stars differ in the total C+N content, where the fainter SGB have about 2.5 times the C+N content of the brighter ones. Coupling our results with literature photometric data and abundance determinations from high-resolution studies, we identify the fainter SGB with the red-RGB population, which also should be richer on average in Ba and other s-process elements, as well as in Na and N, when compared to brighter SGB and the blue-RGB population.

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