发表机构
Krishnaguru Adhyatmik Visvavidyalaya; Bhattadev University（克里希纳古鲁阿迪亚特米克维什瓦迪维亚拉亚; 巴塔德夫大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
利用MESA程序同时拟合八项观测性质与化学丰度比，首次确定极端贫金属红巨星BD-18°5550的质量、年龄、半径和光度，年龄精度提高7倍，并揭示氮元素在红巨星分支中段开始富集，将其确立为约束恒星演化与早期银河系化学演化的基准恒星。
AI 中文摘要
我们对银河系晕中的极端贫金属红巨星BD$-18^{\circ}$5550进行了详细的恒星演化研究。这类恒星是已知最古老的天体之一，保存了银河系最早阶段化学特征的印记。BD$-18^{\circ}$5550表现出$\alpha$元素增强，$\rm[\alpha/Fe]=+0.4$，这是第二星族晕星的典型特征，其氧、镁及其他$\alpha$元素相对于铁的含量有所增强。利用\texttt{MESA}恒星演化程序，我们通过同时拟合八项观测恒星性质和多个化学丰度比，首次直接确定了其质量、年龄、半径和光度。我们的分析得出其当前质量为$\rm M=0.76,M_{\odot}$，半径为$R=38.4,R_{\odot}$，光度为$\log L=2.8,L_{\odot}$，年龄为$t=11.87$ Gyr，年龄精度提高了7倍。该模型再现了所有观测约束，支持了我们对恒星物理、核反应和内部混合的处理。我们进一步研究了预测的直至铁的表面化学成分，并与观测进行了比较。在演化过程中，直至铁的元素表面丰度基本保持不变，而氮则逐渐富集，表明在红巨星分支中段混合过程开始启动。其他表面丰度的保持为早期银河系环境的化学条件提供了宝贵的窗口。我们的结果将BD$-18^{\circ}$5550确立为约束恒星演化和早期银河系化学演化的基准恒星。其推导出的年龄使其跻身银河系最古老恒星之列，为早期恒星形成时代以及银河系的最初组装和演化提供了重要约束。
英文摘要
We present a detailed stellar evolution study of BD$-18^{\circ}$5550, an extremely metal-poor red giant star in the Galactic halo. Such stars are among the oldest known objects and preserve chemical signatures of the earliest stages of Galactic history. BD$-18^{\circ}$5550 exhibits $α$-enhancement, $\rm[α/Fe]=+0.4$, typical of Population II halo stars, with enhanced abundances of oxygen, magnesium, and other $α$-elements relative to iron. Using the \texttt{MESA} stellar evolution code, we perform the first direct determination of its mass, age, radius, and luminosity by simultaneously fitting eight observed stellar properties and multiple chemical abundance ratios. Our analysis yields a current mass of $\rm M=0.76,M_{\odot}$, radius of $R=38.4,R_{\odot}$, luminosity of $\log L=2.8,L_{\odot}$, and an age of $t=11.87$ Gyr, with a 7-fold improvement in age precision. The model reproduces all observed constraints, supporting our treatment of stellar physics, nuclear reactions, and internal mixing. We further investigate the predicted surface chemical composition up to iron and compare it with observations. The surface abundances of elements up to iron remain essentially unchanged throughout the evolution, while nitrogen shows gradual enrichment, indicating the onset of mixing processes at intermediate RGB stages. The preservation of the other surface abundances provides a valuable window into the chemical conditions of the early Galactic environment. Our results establish BD$-18^{\circ}$5550 as a benchmark star for constraining stellar evolution and early Galactic chemical evolution. Its derived age places it among the oldest stars in the Milky Way, providing important constraints on the epoch of early star formation and the initial assembly and evolution of our Galaxy.
CommentsSubmitted to The Astrophysical Journal. 24 pages, 7 figures, 11 tables. Part I of a two-part series on the stellar parameters and nucleosynthesis of BD$-18^{\circ}$5550; Part II on heavy element abundances is in preparation
