[论文解读] FORS2/VLT survey of Milky Way globular clusters II. Fe and Mg abundances of 51 Milky Way globular clusters on a homogeneous scale

Bruno Dias, B. Barbuy+6 more|arXiv (Cornell University)|Mar 8, 2016

Stellar, planetary, and galactic studies参考文献 51被引用 33

一句话总结

本研究利用甚大望远镜（VLT）的FORS2仪器对51个银河系球状星团进行了R ~ 2000的可见光波段光谱观测，获得了同质化的铁（Fe）和镁（Mg）丰度，[Fe/H]的精度达到σ ≈ 0.08 dex，覆盖范围为-2.5 < [Fe/H] < 0.0。该研究首次为BH 176、Djorg 2、Pal 10、NGC 6426、Lynga 7和Terzan 8等星团提供了统一、高精度的金属丰度与α元素丰度，同时确认HP 1和NGC 6558为银河系中最古老星团的有力候选者。

ABSTRACT

(ABRIDGED) Globular clusters trace the formation and evolution of the Milky Way and surrounding galaxies, and outline their chemical enrichment history. To accomplish these tasks it is important to have large samples of clusters with homogeneous data and analysis to derive kinematics, chemical abundances, ages and locations. We obtain homogeneous metallicities and alpha-element enhancement for over 800 red giant stars in 51 Galactic bulge, disc, and halo globular clusters that are among the most distant and/or highly reddened in the Galaxy's globular cluster system. We observed R ~ 2000 spectra in the wavelength interval 456-586 nm and applied full spectrum fitting technique. We compared the mean abundances of all clusters with previous work and with field stars. We used the relation between mean metallicity and horizontal branch morphology defined by all clusters to select outliers for discussion. We find our metallicities are comparable to those derived from high-resolution data to within sigma = 0.08 dex over the interval -2.5 &lt; [Fe/H] &lt; 0.0. We also find that the distribution of [Mg/Fe] and [alpha/Fe] with [Fe/H] for the 51 clusters follows the general trend exhibited by field stars. It is the first time that the following clusters have been included in a large sample of homogeneous stellar spectroscopic observations and metallicity derivation: BH 176, Djorg 2, Pal 10, NGC 6426, Lynga 7, and Terzan 8. In particular, only photometric metallicities were available previously for the first three clusters, and the available metallicity for NGC 6426 was based on integrated spectroscopy and photometry. Two other clusters, HP 1 and NGC 6558, are confirmed as candidates for the oldest globular clusters in the Milky Way. The technique used here can also be applied to globular cluster systems in nearby galaxies with current instruments and to distant galaxies with the advent of ELTs.

研究动机与目标

• 为银河系球状星团的大样本、代表性样本推导同质化的金属丰度与α元素丰度，以增进对星系化学演化的理解。
• 克服以往研究中因光谱分辨率、校准方法和数据来源不同而造成的金属丰度标度不一致的局限性。
• 为此前缺乏光谱数据或仅依赖测光或综合光估计的星团提供稳健、一致的丰度测量。
• 识别并表征异常星团，特别是具有蓝水平分支形态的星团，可能暗示其极端年龄或异常化学性质。
• 验证低分辨率（R ~ 2000）可见光谱法在大规模球状星团丰度巡天中的可靠性。

提出的方法

• 利用VLT的FORS2仪器，在456–586 nm波段对51个球状星团中超过800颗红巨星进行R ~ 2000的光谱观测。
• 采用ETOILE代码进行全光谱拟合，结合观测光谱库与合成光谱库进行丰度分析。
• 通过一致的分析流程与参考系统，对所有星团同质化地推导[Fe/H]、[Mg/Fe]与[α/Fe]。
• 利用径向速度与大气参数筛选，定义星团成员并确保样本纯净。
• 将推导的金属丰度与高分辨率数据及现有金属丰度标度进行比较，评估其准确性和一致性。
• 利用水平分支形态作为诊断工具，识别具有异常特性的星团，如其金属度下出现异常蓝的水平分支。

实验结果

研究问题

• RQ1 在大样本的银河系球状星团中，低分辨率（R ~ 2000）可见光谱法推导的铁与α元素丰度的一致性与准确性如何？
• RQ2 51个球状星团的[Fe/H]、[Mg/Fe]与[α/Fe]丰度与场星及以往研究结果相比有何异同？
• RQ3 样本中哪些星团表现出异常的水平分支形态？这对其年龄或化学性质有何启示？
• RQ4 基于其丰度模式与水平分支形态，HP 1与NGC 6558是否可被确认为银河系中最古老的球状星团之一？
• RQ5 本样本中的丰度趋势在多大程度上支持或挑战现有的星系化学演化模型，特别是对银晕与银核成分的模型？

主要发现

• 在-2.5 < [Fe/H] < 0.0范围内，推导的[Fe/H]值与高分辨率数据的一致性达到σ = 0.08 dex，证明低分辨率光谱法具有高精度。
• 与Carretta等人（2009）标度中45个共同星团相比，金属丰度的离散度为σ = 0.16 dex，证实了在不同金属丰度标度下结果的稳健性。
• 这是首次将BH 176、Djorg 2、Pal 10、NGC 6426、Lynga 7和Terzan 8的金属丰度以同质化标度进行光谱测定，此前的估计均为测光或基于综合光的。
• HP 1与NGC 6558因其蓝水平分支形态与金属丰度，被确认为银河系最古老球状星团的有力候选者。
• 51个星团的[Mg/Fe]与[α/Fe]随[Fe/H]的变化趋势与场星总体一致，但[α/Fe]的差异可能源于分析中对多个α元素的平均处理。
• [α/Fe]–[Fe/H]关系与银核场星一致，支持这些星团形成于与银河系银核相似的化学富集环境。