关于盘星化学双峰的起源：合并和迁移的故事
On the origin of the chemical bimodality of disk stars: A tale of merger and migration
摘要：
银河系的恒星盘在[Fe/H]与[ $α$ /Fe]平面上呈现双峰性，显示出明显的高- $α$和低- $α$序列，其起源仍在争论中。我们研究了NIHAO-UHD项目中类银河星系的宇宙学流体动力学模拟中的[Fe/H]-[ $α$ /Fe]丰度平面，并表明双峰$α$序列是银河系演化过程中某个时刻富气合并的一般结果。高$α$序列在早期星系中首先演化，延伸到高金属丰度，而低$α$序列是在富含气体的合并之后形成的。合并带来了新鲜的贫金属气体，稀释了星际介质的金属丰度，同时保持[ $α$ /Fe]丰度几乎不变。双峰$α$序列的运动学，结构和空间特性在我们的模拟再现的观测。在所有模拟中，高$α$盘都很旧，径向集中在星系中心，并显示出大尺度的高度。相比之下，低$α$椎间盘更年轻，径向延伸更多，并集中在椎间盘中平面。我们的研究结果表明，丰度平面很好地描述了这两个人口已径向分布在整个磁盘的迁移：在今天的太阳附近，低- $α$星起源于内外盘，而大多数的高- $α$星已从内盘迁移。我们发现，年龄测定的恒星在[Fe/H]-[ $α$ /Fe]平面可以限制的时间低$α$序列形成合并，并得出结论，$α$双峰可能是一个并不罕见的功能盘星系。
英文摘要：
The Milky Way's stellar disk exhibits a bimodality in the [Fe/H] vs. [$α$/Fe] plane, showing a distinct high-$α$ and low-$α$ sequence whose origin is still under debate. We examine the [Fe/H]-[$α$/Fe] abundance plane in cosmological hydrodynamical simulations of Milky Way like galaxies from the NIHAO-UHD project and show that the bimodal $α$-sequence is a generic consequence of a gas-rich merger at some time in the Galaxy's evolution. The high-$α$ sequence evolves first in the early galaxies, extending to high metallicities, while it is the low-$α$ sequence that is formed after the gas-rich merger. The merger brings in fresh metal-poor gas diluting the interstellar medium's metallicity while keeping the [$α$/Fe] abundance almost unchanged. The kinematic, structural and spatial properties of the bimodal $α$-sequence in our simulations reproduces that of observations. In all simulations, the high-$α$ disk is old, radially concentrated towards the galaxy's center and shows large scale heights. In contrast, the low-$α$ disk is younger, more radially extended and concentrated to the disk mid-plane. Our results show that the abundance plane is well described by these two populations that have been distributed radially across the disk by migration: at present-day in the solar neighbourhood, low-$α$ stars originate from both the inner and outer disk while most of the high-$α$ stars have migrated from the inner disk. We show that age dating the stars in the [Fe/H]-[$α$/Fe] plane can constrain the time of the low-$α$ sequence forming merger and conclude that $α$-bimodality is likely a not uncommon feature of disk galaxies.
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