[论文解读] omega Centauri - a former nucleus of a dissolved dwarf galaxy? New evidence from Stroemgren photometry

M. Hilker, T. Richtler|ArXiv.org|Aug 30, 2000

Stellar, planetary, and galactic studies被引用 6

一句话总结

本文提出ω中心星团起源于一个已瓦解的矮星系核心，利用斯特伦格伦测光揭示了其复杂的恒星形成历史，包含多个富金属种群。数据表明，富金属星（最高达−0.7 dex）比最古老种群年轻5–8 Gyr，表现出空间分布不对称性，并与C+N丰度强烈相关，表明存在持续的气体流入和原位富集，支持其为星系核心起源，而非其他星团形成情景。

ABSTRACT

CCD vby Stroemgren photometry of a statistically complete sample of red giants and stars in the main sequence turn-off region in omega Centauri has been used to analyse the apparently complex star formation history of this cluster. From the location of stars in the (b-y),m_1 diagram metallicities have been determined. These have been used to estimate ages of different sub-populations in the color-magnitude diagram. The metal-poor population around -1.7 dex is the oldest population found. More metal-rich stars between [Fe/H]=-1.5 and -1.0 dex tend to be 1-3 Gyr younger. These stars are more concentrated towards the center than the metal-poor ones. The most-metal rich stars (-0.7 dex) might be up to 6 Gyr younger than the oldest population. They are asymmetrically distributed around the center with an excess of stars towards the South. We argue that the Stroemgren metallicity in terms of element abundances has another meaning than in other globular clusters. From a comparison with spectroscopic element abundances, we find the best correlation with the sum C+N. The high Stroemgren metallicities, if interpreted by strong CN-bands, result from progressively higher N and perhaps C abundances in comparison to iron. We see an enrichment already among the metal-poor population, which is difficult to explain by self-enrichment alone. An attractive speculation is that omega Cen was the nucleus of a dwarf galaxy. We propose a scenario in which omega Cen experienced mass inflow over a long period of time, until the gas content of its host galaxy was so low that star formation in omega Cen stopped, or alternatively the gas was stripped off during its infall in the Milky Way potential. A mass inflow occuringcn a clumpy and discontinuous manner could explain several of our findings.

研究动机与目标

• 利用高精度斯特伦格伦测光研究ω中心星团的恒星形成历史与金属量分布。
• 基于测光金属量，确定ω中心星团中不同恒星种群的年龄与空间分布。
• 检验观测到的丰度模式与空间不对称性是否可由原位富集或外部吸积解释。
• 评估ω中心星团是已瓦解矮星系核心残余的假设的有效性。
• 澄清斯特伦格伦金属量在ω中心星团中的物理意义，与其它球状星团相比，特别是其与C+N丰度的相关性。

提出的方法

• 在ω中心星团多个区域获取红巨星与转折星的CCD vby斯特伦格伦测光，确保统计完整性。
• 利用(b−y), m₁图导出测光金属量，随后与光谱[Fe/H]值校准，以估算年龄与金属量。
• 构建颜色-星等图（CMD），并与等年龄线比较，估算不同恒星种群的年龄。
• 分析贫金属与富金属星的径向分布，评估其空间集中度与不对称性，特别是向南方向的特征。
• 将斯特伦格伦金属量与光谱[C+N/H]及[Fe/H]相关联，以确定驱动测光金属量的主要丰度因素。
• 将恒星形成历史建模为持续或成团的气体流入，持续3–6 Gyr，与观测到的年龄弥散与丰度模式一致。

实验结果

研究问题

• RQ1 斯特伦格伦测光与等年龄线拟合揭示的ω中心星团的恒星形成历史是什么？
• RQ2 为何ω中心星团中最富金属的恒星表现出强烈的空间不对称性，特别是向南方向存在过量？
• RQ3 ω中心星团中的斯特伦格伦测光金属量如何与实际铁与C+N丰度相关？为何其与其它球状星团不同？
• RQ4 观测到的丰度模式与年龄弥散是否可由原位自富集解释，还是需要外部起源（如来自矮星系的吸积）？
• RQ5 何种物理机制可产生观测到的高斯特伦格伦金属量与C+N丰度之间的相关性，而非铁？

主要发现

• 围绕[Fe/H] = −1.7 dex的主导贫金属种群最为古老，年龄约为17–18 Gyr。
• [Fe/H]在−1.5至−1.0 dex之间的恒星比最古老种群年轻1–3 Gyr，且表现出更高的中心集中度。
• 围绕[Fe/H] = −0.7 dex的最富金属种群比最古老恒星年轻最多6 Gyr，且表现出强烈的空间不对称性，向南方向存在过量。
• ω中心星团中的斯特伦格伦金属量与[C+N/H]相关性最佳，而非[Fe/H]，表明高测光金属量主要由氮与碳富集驱动，而非铁。
• 贫金属种群已显示出CNO循环处理的迹象，表明富集发生在恒星形成初期或同时发生，挑战了简单的自富集模型。
• 观测到的年龄弥散、丰度模式与空间不对称性，最合理的解释是持续3–6 Gyr的气体流入，支持ω中心星团是已瓦解矮星系核心残余的假设。