[论文解读] Na-O Anticorrelation and Horizontal Branches. V. The Na-O anticorrelation in NGC 6441 from Giraffe spectra

R. Gratton, S. Lucatello+17 more|HAL (Le Centre pour la Communication Scientifique Directe)|Jan 8, 2007

Stellar, planetary, and galactic studies参考文献 52被引用 58

一句话总结

本研究通过分析银河系球状星团NGC 6441中25颗红巨星的FLAMES-Giraffe光谱，探讨钠-氧反相关性。研究证实存在明确的钠-氧反相关性，约25%的恒星为富钠、缺氧，且发现红巨星分支上[O/Na]分布与水平分支恒星的颜色分布之间存在强烈相关性，支持富氦种群与蓝水平分支形态之间的关联。

ABSTRACT

We present an analysis of FLAMES-Giraffe spectra for several bright giants in NGC 6441, to investigate the presence and extent of the Na-O anticorrelation in this anomalous globular cluster. The field of NGC 6441 is very crowded, with severe contamination by foreground (mainly bulge) field stars. Appropriate membership criteria were devised to identify a group of 25 likely cluster members among the about 130 stars observed. Combined with the UVES data obtained with the same observations, high dispersion abundance analyses are now available for a total of 30 stars in NGC 6441, 29 of them having data for both O and Na. The spectra were analyzed by a standard line analysis procedure; care was taken to minimize the impact of the differential interstellar reddening throughout the cluster, and to extract reliable information from crowded, and moderately high S/N (30-70), moderately high resolution (R ~ 23,000) spectra. NGC 6441 is very metal-rich ([Fe/H]=$-0.34\pm 0.02\pm0.04$ dex). There is no clear sign of star-to-star scatter in the Fe-peak elements. The alpha-elements Mg, Si, Ca, and Ti are overabundant by rather large factors, suggesting that the cluster formed from material enriched by massive core collapse SNe. The O-Na anticorrelation is well defined, with about 1/4 of the stars being Na-rich and O-poor. One of the stars is a Ba-rich and moderately C-rich star. The distribution of [Na/O] ratios among RGB stars in NGC 6441 appears similar to the distribution of colors of stars along the horizontal branch. The fraction of Na-poor, O-rich stars found in NGC 6441 agrees well with that of stars on the red horizontal branch of this cluster (in both cases about 80%), with a sloping distribution toward lower values of [O/Na] (among RGB stars and bluer colors (among HB stars).

研究动机与目标

• 调查金属丰度较高的球状星团NGC 6441中钠-氧反相关性的存在及其范围，该星团具有异常的水平分支形态。
• 确定观测到的水平分支形态是否与红巨星分支恒星中钠和氧丰度的分布相关。
• 识别并表征星族中的稀有恒星，如钡富集或碳富集恒星。
• 评估差异性消光和场星污染对密集场中丰度测量的影响。
• 结合高分辨率UVES与中等分辨率Giraffe数据，对30颗恒星实现稳健的丰度测定，其中29颗同时具有氧和钠的测量值。

提出的方法

• 利用VLT-UT2上的FLAMES-Giraffe光谱仪，对NGC 6441密集场中的130颗恒星获取中等分辨率（R ~ 23,000）光谱。
• 通过径向速度和光度测量标准（使用2MASS的K星等和(V-K)色指数）识别出25颗可能的星团成员，以最小化银晕场星的污染。
• 对Giraffe光谱进行逐线丰度分析，重点研究[O I] 6300.3、6363.8 Å和Na I 6154–6160 Å谱线，以推导氧和钠的丰度。
• 将Giraffe数据与已有的UVES高分辨率数据结合，对30颗恒星实现一致且高精度的丰度测量，其中29颗同时具有氧和钠的测量值。
• 利用K波段星等与(V-K)色指数关系校正差异性星际消光，以确保丰度测定的准确性。
• 使用由光度测量获得的大气参数，以最小化系统误差，特别是在信噪比中等（S/N ~ 30–70）的情况下。

实验结果

研究问题

• RQ1 NGC 6441中是否存在钠-氧反相关性？其范围如何，特别是富钠、缺氧恒星的比例是多少？
• RQ2 红巨星分支恒星中[O/Na]比值的分布是否与NGC 6441中水平分支恒星的颜色分布相关？
• RQ3 NGC 6441的金属丰度是多少？铁峰元素或α元素是否存在星等间分散？
• RQ4 NGC 6441中是否存在稀有恒星（如钡富集或碳富集恒星）？其丰度模式对它们的起源有何启示？
• RQ5 NGC 6441中观测到的丰度模式能否由大质量核心塌缩超新星的增丰作用解释？

主要发现

• NGC 6441具有较高的金属丰度[Fe/H] = -0.34 ± 0.02 ± 0.04 dex，是目前已知最金属丰度较高的球状星团之一。
• 铁峰元素未观测到显著的星等间分散，表明这些元素具有化学同质性。
• α元素（Mg、Si、Ca、Ti）的过量丰度与大质量核心塌缩超新星增丰的预期因子一致。
• 钠-氧反相关性清晰可辨，约25%的29颗恒星表现出富钠、缺氧特征。
• 红巨星分支恒星中[O/Na]比值的分布以富含氧、钠贫乏的恒星为主（约70%），并向低氧、高钠方向延伸出尾部。
• 一颗恒星（6007741）被识别为钡富集且中等程度碳富集的恒星，属于球状星团中罕见的类型，其丰度模式与来自低质量AGB恒星的质量转移一致。