Stellar Alpha-Element and CNO Abundances in the Galactic Center

恒星丰度对于理解银河系的晕、盘和凸起的形成和化学演化是至关重要的。随着大型望远镜的红外成像和光谱学的出现，现在有可能详细地探索银河系中心100pc以内的恒星群。银河系中心包含许多年轻的、发光的、大质量的恒星，它们集中在银河系中心60秒内的三个独立星团中。它们是中央星系团、拱星团和五胞胎星团，最年轻的恒星年龄分别为3-9 Myr、1-4.5 Myr和3-4 Myr。中央星系团位于银河系的动力中心，与巴德窗口中膨胀的恒星数量相比，年轻明亮的恒星数量惊人地过剩。这些年轻的恒星与年龄高达100英里或更长的年长恒星共存于中央星系团。银河系中心最亮的恒星的年龄相对较年轻(1-100英里)，支持这样的观点，即银河系的中央条将盘状气体驱动到银河系中心，在整个银河系历史上不断地或以恒星形成的爆发为恒星形成提供燃料。俄亥俄州立大学的Kristen Sellgren博士之前在中央星系团和五重星系团内对恒星[Fe/H]的差分恒星丰度测量工作发现，银河系中心60秒内恒星[Fe/H]的分布和平均值与他们得出的太阳附近类似温度和光度的恒星的值没有什么区别。这一结果令人惊讶，因为H II区域的气相丰度测量显示，太阳附近和银河系中心之间的阿尔法元素O和S显著增加。对此的一种解释是，与铁相比，阿尔法俘获元素[阿尔法/铁]在银河系中心得到增强。银河系中心的恒星形成条件与太阳附近的非常不同，预计会导致初始质量函数(IMF)加权到大质量恒星。此外，在一个由大质量恒星主导的环境中，超新星丰富的模型预测了[α/Fe]的高值。Sellgren博士指导的当前项目将通过获取光谱来测试这一假设，以确定银河系中心恒星的[Fe/H]，这些恒星的[Fe/H]已经从以前的工作中得到。这项研究的一个关键部分将是对有效温度和表面重力范围相似的银河系中心恒星和邻近太阳的恒星进行差异分析。为了减少系统误差的影响，对太阳附近比较星的观测和分析在银心丰度研究中是至关重要的。丰度分析本身涉及使用光谱合成程序MOOG和晚型巨星和超巨星的大气模型计算每颗恒星的合成光谱。本项目期望获得更多太阳近邻M超巨星和更多银河中心M超巨星的光谱和CNO丰度，以确定源IRS7中的异常CNO丰度是由于银河中心相对于太阳近邻独特的恒星形成环境，中央星团中黑洞的存在，还是形成太阳近邻和银心恒星的初始CNO丰度。对银河系中心恒星样本中的阿尔法元素的研究将回答有关恒星形成、气体流入和流出以及银河系核心化学演化的基本问题。***

AST 0206331SellgrenStellar abundances have been critical to understanding the formation and chemical evolution of the halo, disk, and bulge of the Galaxy. With the advent of infrared imaging and spectroscopy on large telescopes, it is now possible to explore the stellar population within 100 pc of the Galactic center in detail. The Galactic center contains many young, luminous, massive stars, which are concentrated in three separate clusters within the central 60 parsecs of the Galaxy. These are the Central Cluster, the Arches Cluster, and the Quintuplet Cluster, with respective ages for the youngest stars of 3-9 Myr, 1-4.5 Myr, and 3-4 Myr. The Central Cluster, located at the dynamical center of the Galaxy, has a striking excess of young, luminous stars compared to the bulge stellar population in Baade's window. These young stars co-exist in the Central Cluster with older stars whose ages range up to a few 100 Myr or more. The relatively young ages (1 - 100 Myr) of the brightest stars in the Galactic center support the idea that the Galaxy's central bar has driven disk gas into the Galactic center to fuel star formation throughout the Galaxy's history, continuously or in bursts of star formation. Previous work by Dr. Kristen Sellgren, at Ohio State University, on differential stellar abundance measurements of [Fe/H] within the Central Cluster and the Quintuplet Cluster have found that the distribution and mean value of stellar [Fe/H] in the central 60 parsec of the Galaxy is indistinguishable from the values they derive for stars of similar temperature and luminosity in the solar neighborhood. This result is surprising, because gas-phase abundance measurements of H II regions show a significant increase in the alpha-elements O and S between the solar neighborhood and the Galactic center. One explanation for this is that the alpha-capture elements compared to Fe, [alpha/Fe], are enhanced in the Galactic center. The star formation conditions in the Galactic center are very different from those in the solar neighborhood, and are predicted to result in an initial mass function (IMF) weighted toward massive stars. Furthermore, models of supernova enrichment in an environment dominated by massive stars predict a high value of [alpha/Fe]. The current project directed by Dr. Sellgren will test this hypothesis by obtaining spectra to determine [alpha/H] in Galactic center stars for which [Fe/H] has been already derived from previous work. A key part of this study will be to perform a differential analysis of Galactic center stars compared to solar-neighborhood stars with a similar range in effective temperature and surface gravity. The observation and analysis of comparison stars in the solar neighborhood is critically important in Galactic center abundance studies in order to reduce the effects of systematic errors. The abundance analysis itself involves the computation of the synthetic spectrum of each star, using the spectral synthesis program MOOG and model atmospheres for late type giants and supergiants. This project expects to obtain spectra and derive CNO abundances of more solar neighborhood M supergiants and more Galactic Center M supergiants to determine whether the anomalous CNO abundances in the source IRS 7 are due to the unique star-forming environment in the Galactic Center compared to the solar neighborhood, the presence of the black hole in the Central Cluster, or the initial CNO abundances from which solar neighborhood and Galactic center stars formed. The study of alpha-elements in a sample of Galactic center stars will answer fundamental questions about the nature of star formation, gas infall and outflow, and chemical evolution in the nucleus of the Galaxy. ***