[论文解读] The Gaia-ESO Survey: Sodium and aluminium abundances in giants and dwarfs - Implications for stellar and Galactic chemical evolution

R. Smiljanić, D. Romano+41 more|Edinburgh Research Explorer (University of Edinburgh)|Feb 10, 2016

Stellar, planetary, and galactic studies参考文献 128被引用 34

一句话总结

本研究利用盖亚-ESO巡天的高分辨率UVES光谱，分析了六个疏散星团中超过600颗太阳邻域矮星和低至中等质量巨星的钠和铝丰度。结果表明，当恒星质量超过~2.0 M⊙时，钠过量丰度随质量增加而上升，与恒星演化模型一致；而铝丰度在~3.0 M⊙以下保持恒定；然而，观测到的[Na/Fe]和[Al/Fe]趋势与化学演化模型之间仍存在差异，提示模型中可能存在缺失的晚期钠生产源，且铝核合成过程的理解尚不完整。

ABSTRACT

Stellar evolution models predict that internal mixing should cause some sodium overabundance at the surface of red giants more massive than ~ 1.5--2.0 Msun. The surface aluminium abundance should not be affected. Nevertheless, observational results disagree about the presence and/or the degree of the Na and Al overabundances. In addition, Galactic chemical evolution models adopting different stellar yields lead to quite different predictions for the behavior of [Na/Fe] and [Al/Fe] versus [Fe/H]. Overall, the observed trends of these abundances with metallicity are not well reproduced. We readdress both issues, using new Na and Al abundances determined within the Gaia-ESO Survey, using two samples: i) more than 600 dwarfs of the solar neighborhood and of open clusters and ii) low- and intermediate-mass clump giants in six open clusters. Abundances of Na in giants with mass below ~2.0 Msun, and of Al in giants below ~3.0 Msun, seem to be unaffected by internal mixing processes. For more massive giants, the Na overabundance increases with stellar mass. This trend agrees well with predictions of stellar evolutionary models. Chemical evolution models that are able to fit well the observed [Na/Fe] vs. [Fe/H] trend in solar neighborhood dwarfs can not simultaneously explain the run of [Al/Fe] with [Fe/H], and viceversa. The comparison with stellar ages is hampered by severe uncertainties. Indeed, reliable age estimates are available for only a half of the stars of the sample. We conclude that Al is underproduced by the models, except for stellar ages younger than about 7 Gyr. In addition, some significant source of late Na production seems to be missing in the models. Either current Na and Al yields are affected by large uncertainties, and/or some important Galactic source(s) of these elements has not been taken into account up to now. [abridged]

研究动机与目标

• 通过观测不同质量和金属度恒星的钠和铝丰度，改进对恒星演化和银河系化学演化模型的约束。
• 解决观测到的[Na/Fe]和[Al/Fe]趋势与当前银河系化学演化模型预测之间的差异。
• 研究内部混合过程在红巨星和矮星表面钠和铝丰度变化中的作用。
• 评估恒星产物（尤其是大质量恒星）对银河系盘中钠和铝化学演化的影响。
• 查明环境因素或缺失核合成源是否能解释铝丰度异常，特别是在NGC 6705等星团中。

提出的方法

• 采用高分辨率UVES光谱测量盖亚-ESO巡天中恒星的钠和铝谱线深度。
• 对钠丰度应用非局部热动平衡（non-LTE）修正，对代表性样本中的铝丰度进行数量级估算的修正。
• 基于演化轨迹和星团成员关系推断恒星质量，以与理论模型进行比较。
• 将丰度随金属度和年龄的变化趋势与银河系盘的详细化学演化模型进行对比。
• 样本包括来自六个疏散星团的超过600颗太阳邻域矮星和巨星，支持质量与年龄相关的分析。
• 数据来自欧洲南方天文台档案，并通过CASU和FLAMES/UVES数据处理流程进行处理。

实验结果

研究问题

• RQ1 低至中等质量恒星内部混合是否在红巨星中产生可测量的钠过量丰度，且是否与恒星演化模型一致？
• RQ2 巨星中的铝丰度是否受内部混合影响，且这种影响是否在~3.0 M⊙以上恒星质量下表现不同？
• RQ3 为何当前银河系化学演化模型无法再现太阳邻域[Fe/H]与[Na/Fe]和[Al/Fe]之间的观测趋势？
• RQ4 NGC 6705中铝过量丰度是源于恒星内部过程，还是环境因素（如初始条件或星团形成环境）所致？
• RQ5 观测到的高金属度下[Na/Fe]增加是否可由现有恒星产物解释，还是需要引入新的晚期钠生产源？

主要发现

• 当恒星质量超过~2.0 M⊙时，钠过量丰度随质量增加而上升，与涉及内部混合的恒星演化模型预测高度一致。
• 在~3.0 M⊙以下的巨星中，铝丰度保持恒定，无确凿证据表明其丰度随质量变化而富集。
• 唯一表现出铝增强的星团NGC 6705（主序星质量>3.0 M⊙）表现出异常化学特征，可能源于环境因素而非恒星内部过程。
• 太阳邻域矮星中[Na/Fe]随[Fe/H]的变化趋势在整体上可被模型再现，但高金属度区域存在未解释的上升，暗示模型中可能存在缺失的晚期钠生产源。
• 化学演化模型在低金属度的晕星中系统性过量生产铝，而在[Fe/H] > -1.0的太阳邻域矮星中则系统性生产不足，除最年轻恒星外均如此。
• 由于[Na/Fe] > 0.00的恒星缺乏可靠的年龄测定，丰度-年龄图的实用性受限，使模型验证更加复杂。