[论文解读] The subgiant branch of omega Cen seen through high-resolution spectroscopy. II. The most metal-rich population

E. Pancino, A. Mucciarelli+3 more|arXiv (Cornell University)|Sep 7, 2011

Stellar, planetary, and galactic studies参考文献 73被引用 8

一句话总结

本研究分析了ω中心星系金属丰度较高的子巨星分支星（SGB-a）的高分辨率光谱，发现铝元素丰度均匀（[Al/Fe] = +0.32 ± 0.14 dex），且对氦增强模型大气的敏感性可忽略不计。这些结果支持该星族可能代表一个被破坏矮星系的场星族，锂的上限值进一步表明其氦含量增强。

ABSTRACT

We analyze spectra of 18 stars belonging to the faintest subgiant branch in omega Centauri (the SGB-a), obtained with GIRAFFE@VLT at a resolution o R~17000 and a S/N ratio between 25 and 50. We measure abundances of Al, Ba, Ca, Fe, Ni, Si, and Ti and we find that these stars have =-0.73 +/- 0.14 dex, similarly to the corresponding red giant branch population (the RGB-a). We also measure =+0.40 +/- 0.16 dex, and =+0.87 +/- 0.23 dex, in general agreement with past studies. It is very interesting to note that we found a uniform Al abundance, =+0.32 +/- 0.14 dex, for all the 18 SGB-a stars analysed here, thus supporting past evidence that the usual (anti-)correlations are not present in this population, and suggesting a non globular cluster-like origin of this particular population. In the dwarf galaxy hypothesis for the formation of omega Cen, this population might be the best candidate for the field population of its putative parent galaxy, although some of its properties appear contradictory. It has also been suggested that the most metal-rich population in omega Cen is significantly enriched in helium. If this is true, the traditional abundance analysis techniques, based on model atmospheres with normal helium content, might lead to errors. We have computed helium enhanced atmospheres for three stars in our sample and verified that the abundance errors due to the use of non-enhanced atmospheres are negligible. Additional, indirect support to the enhanced helium content of the SGB-a population comes from our Li upper limits.

研究动机与目标

• 利用高分辨率光谱学测定ω中心星系中最金属丰度较高的子巨星分支星族（SGB-a）的化学成分。
• 检验铝及其他轻元素中不存在（反）相关性是否支持该星族具有非球状星团起源的假设。
• 通过对比标准与氦增强模型大气，评估氦增强对丰度分析的影响。
• 评估锂丰度作为该SGB-a星族中氦增强的间接证据。

提出的方法

• 使用VLT的GIRAFFE在R ≈ 17,000的分辨率下进行高分辨率光谱观测，18颗SGB-a星的信噪比在25至50之间。
• 利用ATLAS12模型大气进行合成光谱拟合，测量铁、铝、钡、钙、镍、硅和钛的丰度。
• 针对三颗恒星计算了氦丰度增强的模型大气（Y = 0.35–0.40），以检验其对推导丰度的影响。
• 通过光谱中未检测到6708 Å处的Li I线，推导出锂的上限值。
• 通过铁线的电离平衡与激发能平衡，确定大气参数（Teff, log g, vt）。
• 使用在线数据库（NIST, VALD, Kurucz）提供光谱合成所需的原子谱线数据。

实验结果

研究问题

• RQ1 ω中心星系的SGB-a星是否表现出类似典型球状星团星族的碳、氮、氧、钠和铝等轻元素的（反）相关性？
• RQ2 在模型大气中假设标准氦丰度（Y ≈ 0.25）是否显著影响SGB-a星族的丰度分析？
• RQ3 SGB-a星中锂的上限值是否支持该星族存在氦增强的假设？
• RQ4 SGB-a星族的化学成分是否符合被破坏矮星系场星族的预期？

主要发现

• SGB-a星族的平均金属丰度为<[Fe/H]> = –0.73 ± 0.14 dex，与红巨星分支星族（RGB-a）一致。
• α-元素丰度比<[α/Fe]> = +0.40 ± 0.16 dex偏高，表明恒星形成 timescale 较短或早期由大质量超新星快速富集。
• 所有18颗SGB-a星的铝丰度均显著且均匀，<[Al/Fe]> = +0.32 ± 0.14 dex，其散布未超出测量误差范围，无明显变化。
• 通过标准与氦增强模型大气的对比，显示氦增强对丰度测定的影响可忽略不计。
• 未检测到6708 Å处的Li I线，其上限值与锂的耗尽一致，支持该星族存在氦增强的假设。
• 由于缺乏（反）相关性且金属丰度较高，SGB-a星族是ω中心星系假定母体矮星系场星族的最强候选者。