[论文解读] The Gaia-ESO Survey. Mg-Al anti-correlation in iDR4 globular clusters

E. Pancino, D. Romano|arXiv (Cornell University)|Feb 20, 2017

Stellar, planetary, and galactic studies参考文献 67被引用 52

一句话总结

本研究利用盖亚-ESO巡天iDR4数据，分析了11个球状星团中的Mg-Al反相关关系，确认其同时依赖于金属丰度和现今质量，而与年龄无关。研究首次对金属丰度较高的星团NGC 5927（[Fe/H] = –0.49 dex）进行了详细的丰度测量，表明正常的Mg-Al反相关关系无法解释某些星系外球状星团中观测到的低[Mg/α]，必须引入更极端的化学组成，如NGC 2419中的组成。

ABSTRACT

We use Gaia-ESO Survey iDR4 data to explore the Mg-Al anti-correlation in globular clusters, that were observed as calibrators, as a demonstration of the quality of Gaia-ESO Survey data and analysis. The results compare well with the available literature, within 0.1 dex or less, after a small (compared to the internal spreads) offset between the UVES and the GIRAFFE data of 0.10-0.15 dex was taken into account. In particular, we present for the first time data for NGC 5927, one of the most metal-rich globular clusters studied in the literature so far with [Fe/H]=-0.49 dex, that was included to connect with the open cluster regime in the Gaia-ESO Survey internal calibration. The extent and shape of the Mg-Al anti-correlation provide strong constraints on the multiple population phenomenon in globular clusters. In particular, we studied the dependency of the Mg-Al anti-correlation extension with metallicity, present-day mass, and age of the clusters, using GES data in combination with a large set of homogenized literature measurements. We find a dependency with both metallicity and mass, that is evident when fitting for the two parameters simultaneously, but no significant dependency with age. We confirm that the Mg-Al anti-correlation is not seen in all clusters, but disappears for the less massive or most metal-rich ones. We also use our dataset to see whether a normal anti-correlation would explain the low [Mg/$α$] observed in some extragalactic globular clusters, but find that none of the clusters in our sample can reproduce it, and more extreme chemical compositions (like the one of NGC 2419) would be required. We conclude that GES iDR4 data already meet the requirements set by the main survey goals, and can be used to study in detail globular clusters even if the analysis procedures were not specifically designed for them.

研究动机与目标

• 评估盖亚-ESO巡天iDR4数据在研究球状星团中Mg-Al反相关关系方面的质量和可靠性。
• 研究Mg-Al反相关关系延伸程度对星团金属丰度、现今质量及年龄的依赖性。
• 确定标准Mg-Al反相关关系是否能解释某些星系外球状星团中观测到的低[Mg/α]比值。
• 将NGC 5927（最富含金属的球状星团之一）纳入分析，以弥合球状星团与疏散星团化学之间的差距。

提出的方法

• 利用盖亚-ESO巡天iDR4数据中UVES（高分辨率）和GIRAFFE（中等分辨率）光谱仪对11个球状星团的数据。
• 采用最先进的技术，由多个研究团队协作进行迭代丰度分析，并对UVES与GIRAFFE数据进行交叉校准。
• 整合来自GES iDR4、FLAMES GC巡天和APOGEE巡天的28个球状星团中约1,300颗恒星的数据库，实现数据同质化。
• 应用线性拟合模型，将Mg-Al反相关关系的延伸程度作为金属丰度和质量的函数进行建模，年龄作为次要参数。
• 通过将单颗恒星的平均丰度与观测到的星系外星团趋势进行比较，评估个体恒星丰度与积分光谱测量结果的一致性。
• 使用统计建模评估标准Mg-Al反相关关系是否能再现星系外星团中观测到的低[Mg/α]值。

实验结果

研究问题

• RQ1 球状星团中Mg-Al反相关关系的延伸程度是否依赖于金属丰度和现今质量？这种依赖关系是否具有统计显著性？
• RQ2 Mg-Al反相关关系是否表现出对星团年龄的可测量依赖性，或是否独立于演化阶段？
• RQ3 如同银河系典型球状星团中所见的标准Mg-Al反相关关系，能否解释某些星系外球状星团中观测到的低[Mg/α]比值？
• RQ4 盖亚-ESO巡天iDR4数据在Mg和Al丰度方面是否能准确再现文献值，特别是与UVES和GIRAFFE仪器的比较结果如何？
• RQ5 将NGC 5927（一个富含金属的球状星团）纳入分析，是否能改善球状星团与疏散星团化学体系之间的校准？

主要发现

• 当同时拟合金属丰度和现今质量时，Mg-Al反相关关系的延伸程度显著依赖于两者，而与年龄无显著依赖关系。
• UVES与GIRAFFE数据之间观测到的偏移较小（0.10–0.15 dex），校正后结果与文献值一致，差异在0.1 dex以内。
• NGC 5927（[Fe/H] = –0.49 dex）是本研究中研究的最富含金属的球状星团，其纳入显著改善了与疏散星团的校准。
• 标准Mg-Al反相关关系无法再现某些星系外球状星团中观测到的低[Mg/α]值，必须引入更极端的化学组成，如NGC 2419中的组成。
• 盖亚-ESO巡天iDR4数据满足详细球状星团研究的质量要求，尽管分析流程并非专为此类研究设计。
• 对28个星团中约1,300颗恒星的同质化数据库，支持了丰度变化的稳健统计分析，为未来多元素研究提供了基础。