氮增丰贫金属场星的证认与起源研究

One of the characteristics of multiple populations in globular clusters (GCs) is the existence of a group of nitrogen (N) and sodium (Na) enhanced but carbon (C) and oxygen (O) depleted stars. N-enhanced metal-poor field stars with low C abundances in the field have been recently identified using large spectroscopic survey data. These stars share similar C and N abundances with some GC stars, but if they are ejected from GCs is still uncertain. In this work, we identify a group of N-enhanced metal-poor field stars using LAMOST low resolution spectra, which greatly increases the current sample size, and later data releases will be used to further expand the sample. We will perform chemical (C, N, and Mg) and dynamical statistics on our newly defined sample to understand the origin of these stars. At the same time, this project will use mid to high resolution spectra to obtain more information on their chemistry (e.g., Na, O, Al, and other neutron capture elements), in order to investigate the nucleosynthesis they have gone through; the velocity changes of these stars from spectra of different epochs will be used to investigate its binary fraction. Understanding the origins of these rare N-enhanced metal-poor field stars will add an important piece in our Milky Way formation and evolution theory.

球状星团多重星族的一个重要表现是拥有一类富氮、钠但贫碳、氧的恒星。近年来，大型的光谱巡天项目在场星里发现了一类贫碳的氮增丰贫金属场星，与部分球状星团成员星的化学特性有相似之处。但它们是否从球状星团中抛出还有待进一步的确认。本项目利用LAMOST低分辨率光谱找到了一群贫碳的氮增丰贫金属场星，极大地增大了现有样本的大小，并计划利用后续数据释放继续扩大样本。本项目将会对已有样本进行化学（碳、氮、镁）和动力学统计，以了解这些恒星的起源。同时也会利用中高谱分辨率光谱得到更多的化学元素信息（如钠、氧、铝、中子俘获元素等），以了解它们经历过的核反应过程；不同时刻的光谱可以测量这一类恒星的速度变化，以确定它们的双星起源比例。了解这类稀有的氮增丰贫金属场星的起源，将会为银河系形成与演化理论再添新机遇。

球状星团是星系早期形成的恒星系统，并且存活至今，是研究星系演化的最佳天体之一。球状星团的逃逸恒星被认为是银晕的重要组成部分，但因其与晕星具有相似的金属丰度和动力学参数，搜寻较为困难。近年来发现的一类氮增丰贫金属场星为寻找球状星团逃逸恒星带来了新的契机：它们与球状星团的增丰星族有着相似的氮增丰性质，与一般场星明显不同。但它们的样本数量较少，而且许多化学和动力学的性质尚不明确。本项目利用LAMOST低分辨率光谱数据找到了一大批氮增丰贫金属场星，将当时已有样本扩大了一倍。结合高分辨率光谱的化学元素丰度和动力学信息，本项目证明了这一类恒星与球状星团性质相符。多时刻的视向速度观测表明这一类恒星的双星比例较低，说明双星通道并不是主要产生机制。上述的研究表明氮增丰贫金属场星是球状星团逃逸恒星的最佳候选体，可以用作进一步研究星系与球状星团共同演化的探针。