Stellar Alpha-Element and CNO Abundances in the Galactic Center

银河系中心的恒星阿尔法元素和 CNO 丰度

• 批准号: 0206331
• 负责人: Kristen Sellgren
• 金额: --
• 依托单位: Ohio State University Research Foundation -DO NOT USE
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-08-01 至 2006-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0206331&HistoricalAwards=false
• 关键词: Stellar; Alpha; Element; CNO; Abundances

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. ***

AST 0206331Sellgren 恒星丰度对于理解银河系晕、盘和核球的形成和化学演化至关重要。随着大型望远镜红外成像和光谱技术的出现，现在可以详细探索银河系中心 100 % 范围内的恒星群。银河系中心包含许多年轻、明亮的大质量恒星，它们集中在银河系中央 60 秒差距内的三个独立星团中。它们是中央星团、拱门星团和五重星团，最年轻恒星的年龄分别为 3-9 Myr、1-4.5 Myr 和 3-4 Myr。中央星团位于银河系的动力中心，与巴德窗中的核球星群相比，其年轻、明亮的恒星数量惊人地多。这些年轻恒星与年龄高达数百密尔或以上的较老恒星共存于中央星团。银河系中心最亮恒星的相对年轻年龄（1 - 100 Myr）支持这样的观点，即银河系中心棒将盘状气体驱动到银河系中心，为整个银河系历史中的恒星形成提供燃料，无论是连续的还是以爆发式的恒星形成。俄亥俄州立大学的 Kristen Sellgren 博士之前对中央星团和五重星团内 [Fe/H] 的恒星丰度差异测量发现，银河系中央 60 秒差距内恒星 [Fe/H] 的分布和平均值与它们对太阳附近温度和光度相似的恒星得出的值没有区别。这一结果令人惊讶，因为 H II 区域的气相丰度测量显示，太阳邻域和银河系中心之间的 α 元素 O 和 S 显着增加。对此的一种解释是，与 Fe 相比，α 捕获元素 [α/Fe] 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恒星形成条件非常不同，预计会导致初始质量函数（IMF）偏向大质量恒星。此外，以大质量恒星为主的环境中的超新星富集模型预测[α/Fe]的值很高。目前由 Sellgren 博士指导的项目将通过获取光谱来确定银河系中心恒星中的 [α/H] 来测试这一假设，其中 [Fe/H] 已经从之前的工作中推导出来。这项研究的一个关键部分是对银河系中心恒星与有效温度和表面重力范围相似的太阳附近恒星进行差异分析。为了减少系统误差的影响，对太阳附近恒星的观察和分析对于银河系中心丰度研究至关重要。丰度分析本身涉及使用光谱合成程序 MOOG 和晚期巨星和超巨星的模型大气来计算每颗恒星的合成光谱。该项目期望获得更多太阳邻域M超巨星和更多银河中心M超巨星的光谱并推导CNO丰度，以确定源IRS 7中的异常CNO丰度是否是由于银河系中心与太阳邻域相比独特的恒星形成环境、中央星团中黑洞的存在，或者太阳邻域和银心恒星形成的初始CNO丰度所致。对银河系中心恒星样本中α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性质的基本问题。 ***