[论文解读] The age structure of stellar populations in the solar vicinity. Clues of a two-phase formation history of the Milky Way disk

M. Haywood, P. Di Matteo+3 more|arXiv (Cornell University)|May 20, 2013

Stellar, planetary, and galactic studies参考文献 113被引用 260

一句话总结

本文基于太阳邻域恒星的高精度丰度与年龄数据，提出了银河系盘的两阶段形成历史。研究识别出厚盘在4–5 Gyr内形成，经历单调的化学增丰；而内盘薄盘则在厚盘结束时所设定的条件下较晚形成。关键结果表明，径向迁移在塑造本地盘结构中作用极小，且外盘薄盘与厚盘内区阶段同时形成，挑战了由内而外的盘形成范式。

ABSTRACT

We analyze high quality abundances data of solar neighborhood stars and show that there are two distinct regimes of [alpha/Fe] versus age which we identify as the epochs of the thick and thin disk formation. A tight correlation between metallicity and [alpha/Fe] versus age is clearly identifiable on thick disk stars, implying that this population formed from a well mixed ISM, over a time scale of 4-5 Gyr. Thick disk stars vertical velocity dispersion correlate with age, with the youngest objects having as small scale heights as those of thin disk stars. A natural consequence of these two results is that a vertical metallicity gradient is expected in this population. We suggest that the thick disk set the initial conditions for the formation of the inner thin disk. This provides also an explanation of the apparent coincidence between the step in metallicity at 7-10 kpc in the thin disk and the confinment of the thick disk at about R<10 kpc. We suggest that the outer thin disk developped outside the influence of the thick disk, but also that the high alpha-enrichment of the outer regions may originate from a primordial pollution by the gas expelled from the thick disk. Local metal-poor thin disk stars, whose properties are best explained by an origin in the outer disk, are shown to be as old as the youngest thick disk (9-10 Gyr), implying that the outer thin disk started to form while the thick disk formation was still on-going in the inner Galaxy. We point out that, given the tight age-abundance relations in the thick disk, an inside-out process would give rise to a radial gradient in abundances in this population which is not observed. Finally, we argue that the data discussed here leave little room for radial migration, either to have contaminated the solar vicinity, or, to have redistributed stars in significant proportion across the solar annulus.

研究动机与目标

• 利用太阳邻域高精度恒星年龄与丰度数据，确定银河系盘的形成历史。
• 解决长期以来关于厚盘与薄盘是否为独立族群或属于连续形成过程的争议。
• 通过检验年龄-金属丰度关系与年龄-[α/Fe]关系，验证由内而外盘形成模型的有效性。
• 评估径向迁移在太阳环带内重新分布恒星的作用。
• 基于形成条件，澄清厚盘与薄盘组分在命名与物理特性上的区别。

提出的方法

• 分析Adibekyan等（2012）调查中1,111颗FGK型恒星的样本，其具有高精度光谱丰度与年龄测定。
• 利用[α/Fe]与[Fe/H]关系图及年龄图，识别出对应于厚盘与薄盘族群的化学与动力学序列。
• 应用恒星演化模型与等距线拟合，从光度与金属丰度推导单颗恒星的年龄。
• 通过厚盘中垂直速度弥散度与年龄的相关性，推断结构演化与标高变化。
• 将观测到的年龄-金属丰度关系与年龄-[α/Fe]关系，与由内而外形成模型及径向迁移模型的预测进行比较。
• 利用运动学与化学数据，推断金属贫乏薄盘恒星的起源及其与外盘形成的关系。

实验结果

研究问题

• RQ1 太阳邻域观测到的年龄-金属丰度关系与年龄-[α/Fe]关系是否支持银河系盘的两阶段形成历史？
• RQ2 径向迁移（搅动）在多大程度上重新分布了太阳环带内的恒星，从而影响了观测到的局部运动学与化学特征？
• RQ3 太阳附近金属贫乏薄盘恒星的起源是什么？其年龄与厚盘形成时期有何关联？
• RQ4 能否通过同一形成机制解释薄盘中观测到的径向金属丰度梯度以及厚盘被限制在R < 10 kpc以内的现象？
• RQ5 当综合考虑形成时间、化学条件与运动学时，传统的‘薄’与‘厚’盘区分是否仍然有效？

主要发现

• 厚盘在4–5 Gyr内形成，经历单调化学增丰，[α/Fe]与年龄间存在紧密相关性，表明星际介质高度混合。
• 厚盘中垂直速度弥散度随时间从~50 km s⁻¹下降至~25 km s⁻¹，表明随着恒星形成演化，盘结构逐步变薄。
• 内盘薄盘约在8 Gyr前形成，继承了厚盘形成结束时留下的化学条件——具体为[Fe/H] ≈ -0.1至+0.1 dex，[α/Fe] ≈ 0.1 dex。
• 外盘薄盘与厚盘内区阶段同时形成，时间约为9–10 Gyr前，可能源自厚盘释放的富集气体与吸积的金属贫乏气体的混合。
• 厚盘中观测到的年龄-[α/Fe]关系与年龄-金属丰度关系排除了连续的由内而外形成过程，因为此类模型会产生未观测到的径向梯度。
• 径向迁移（搅动）在太阳附近或太阳环带内重新分布恒星的作用极小，因为观测到的运动学与化学模式与显著混合不一致。