[论文解读] Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. III. Detection of lithium in the metal-poor bulge dwarf MOA-2010-BLG-285S

T. Bensby, M. Asplund+9 more|Scientific Electronic Library Online (São Paulo Research Foundation, Latin American and Caribbean Center on Health Sciences Information, Conselho Nacional de Desenvolvimento Científico e Tecnológico)|Sep 29, 2010

Stellar, planetary, and galactic studies参考文献 43被引用 6

一句话总结

本研究利用引力透镜事件期间的高分辨率光谱观测，首次在银河系银心区域的金属贫乏矮星 MOA-2010-BLG-285S 中明确检测到锂元素。该恒星的非局部热动平衡（NLTE）修正后锂同位素丰度为 log ε(Li) = 2.16，与Spite平台一致，表明银心区域早期演化过程中未发生显著的锂增丰，支持了锂平台在银河系各组成部分中的普遍性。

ABSTRACT

Context: In order to study the evolution of Li in the Galaxy it is necessary to observe dwarf or subgiant stars. Abridged Although Li has been extensively studied in the Galactic disk and halo, to date there is only one uncertain detection of Li in an unevolved bulge star. AIMS: Our aim with this study is to provide the first clear detection of Li in the Galactic bulge, based on an analysis of a dwarf star that has largely retained its initial Li abundance. METHODS: We have performed a detailed elemental abundance analysis of the bulge dwarf star MOA-2010-BLG-285S using a high-resolution, and high signal-to-noise spectrum obtained with the UVES spectrograph at the VLT when the object was optically magnified during a gravitational microlensing event (visual magnification A~550 during observation). The lithium abundance was determined through synthetic line profile fitting of the 7Li resonance doublet line at 670.8 nm. The results have been corrected for departures from LTE. Results: MOA-2010-BLG-285S is, at [Fe/H]=-1.23, the most metal-poor dwarf star detected so far in the Galactic bulge. Its old age (12.5 Gyr) and enhanced [alpha/Fe] ratios agree well with stars in the thick disk at similar metallicity. This star represents the first unambiguous detection of Li in a metal-poor dwarf star in the Galactic bulge. We find an NLTE corrected Li abundance of log e(Li)=2.16, which is consistent with values derived for Galactic disk and halo dwarf stars at similar metallicities and temperatures. Conclusion: Our results show that there are no signs of Li enrichment or production in the Galactic bulge during its earliest phases. Observations of Li in other galaxies (omega Cen) and other components of the Galaxy suggest further that the Spite plateau is universal.

研究动机与目标

• 测定银河系银心区域金属贫乏矮星中的锂丰度，该区域此前因恒星过暗而难以实现此类测量。
• 利用一颗保留初始锂丰度的恒星，检验银河系银心在早期形成阶段是否经历了显著的锂生成或消耗。
• 通过将银心区域的锂丰度与球状星团、厚盘及其它星系中的结果进行比较，评估锂Spite平台的普适性。
• 通过分析具有高引力透镜放大倍数的恒星中α元素与铁峰元素的元素丰度，研究银心区域的化学演化历史。
• 确认引力透镜作为获取银心暗淡恒星高分辨率光谱观测数据的可靠性。

提出的方法

• 利用甚大望远镜（VLT）上的UVES仪器，在引力透镜事件期间获取了该恒星亮度放大约550倍的高分辨率、高信噪比光谱数据。
• 通过670.8 nm处 7Li共振双线的合成轮廓拟合方法测定锂丰度，并对非局部热动平衡（NLTE）效应进行了修正。
• 利用光谱分析技术及铁线的电离平衡法确定恒星参数，包括有效温度、表面重力和金属丰度。
• 基于Y2等时曲线模型估算恒星年龄，最佳拟合结果为12.5 ± 3 Gyr。
• 测量并比较了α元素（如O、Mg、Si、Ca、Ti）与铁峰元素（Cr、Ni）的丰度比，与厚盘和球状星团标准值进行对比。
• 将结果与理论模型（包括ν过程及AGB星贡献）进行比较，评估早期锂增丰的可能性。

实验结果

研究问题

• RQ1 银河系银心区域的锂丰度是否与球状星团和厚盘中观测到的Spite平台一致？
• RQ2 基于一颗金属贫乏矮星的观测，能否推断出银河系银心在早期形成阶段存在显著的锂生成或消耗？
• RQ3 能否通过引力透镜增强恒星亮度，从而可靠地测量金属贫乏银心矮星中的锂丰度？
• RQ4 在相似金属丰度下，银心区域的锂丰度与球状星团和厚盘等其他银河系组分相比如何？
• RQ5 锂Spite平台是否为包括银河系银心和外部系统（如ω Cen）在内的各类星系系统中的普遍特征？

主要发现

• MOA-2010-BLG-285S 是迄今在银河系银心中发现的最金属贫乏的矮星，其 [Fe/H] = -1.23。
• 该恒星年龄为12.5 ± 3 Gyr，[α/Fe] 比值为0.3至0.5 dex，与同金属丰度下厚盘恒星的特征一致。
• NLTE修正后的锂丰度为 log ε(Li) = 2.16，与同温度和金属丰度下金属贫乏球状星团及厚盘矮星中观测到的值高度一致。
• 该恒星位于锂Spite平台的金属丰度较高一端，表明其在银心早期形成阶段未发生显著的锂增丰。
• 银心、球状星团、厚盘及ω Cen中锂丰度的一致性表明，锂Spite平台是星系化学演化中的普遍特征。
• 结果支持理论模型预测：在银心区域，[Fe/H] > -1 之前，超新星ν过程、碳星或大质量AGB星对锂的生成可忽略不计。