[论文解读] The origin of nitrogen: Implications of recent measurements of N/O in Galactic metal-poor stars

C. Chiappini, F. Matteuccí+1 more|ArXiv.org|Mar 23, 2005

Educational Leadership and Practices参考文献 1被引用 42

一句话总结

本文提出，由快速自转驱动的极低金属量大质量恒星中氮的增强生成，可解释银河系晕星中观测到的出人意料的高N/O比。通过将Z < 10⁻⁵时60 M⊙恒星的¹⁴N产量提高达200倍，该模型成功再现了贫金属恒星中的高N/O比以及太阳邻域的观测N/O比，同时预测银河系盘内存在负的N/O丰度梯度。

ABSTRACT

Recent new high-precision abundance data for Galactic halo stars suggest important primary nitrogen production in very metal-poor massive stars. Here, we compute a new model for the chemical evolution of the Milky Way aimed at explaining these new abundance data. The new data can be explained by adopting: a) the stellar yields obtained from stellar models that take into account rotation and b) an extra production of nitrogen in the very metal-poor massive stars. In particular, we suggest an increase of nearly a factor of 200 in 14N for a star of 60 Msun and 40 for a star of 9Msun, for metallicities below Z=10$^{-5}$, with respect to the yields given in the literature for Z=10$^{-5}$ and rotational velocity of 300 km/s. We show that once we adopt the above prescriptions, our model is able to predict high N/O abundance ratios at low metallicities and still explains the nitrogen abundances observed in thin disk stars in the solar vicinity. The physical motivation for a larger nitrogen production in massive stars in very metal-poor environments could be the fact that some stellar models as well as observational data suggest that at low metallicities stars rotate faster. If this is the case, such large nitrogen production seen in the pristine phases of the halo formation would not necessarily happen in Damped Lyman-alpha systems which have metallicities always above [Fe/H]$\simeq -$2.5, and could have been pre-enriched. We also compute the abundance gradient of N/O along the Galactic disk and show that a negative gradient is predicted once we adopt stellar yields where rotation is taken into account. The latter result implies that intermediate mass stars contribute less to the primary nitrogen than previously thought.

研究动机与目标

• 将近期对贫金属晕星中N/O的高精度测量结果与银河系的化学演化模型相协调。
• 研究在低金属度下大质量恒星中增强的原生氮生成是否可解释观测到的丰度模式。
• 评估恒星自转与质量损失对氮产量的影响及其对银河系化学演化的影响。
• 探讨对衰减莱曼-阿尔法系统（DLAs）的启示，特别是其N/O丰度二分性。
• 确定在修订的产量下，中等质量恒星是否对原生氮生成有显著贡献。

提出的方法

• 采用Meynet & Maeder (2002)的恒星产量，其中包含自转与质量损失，并对低金属度下的¹⁴N生成进行修改。
• 构建了一个银河系的启发式化学演化模型，调整Z = 10⁻⁵以下大质量恒星的氮产量。
• 将模型预测与晕星和薄盘星中观测到的N/O、C/O和C/Fe丰度进行比较。
• 利用自转影响的产量模型，计算银河系盘内N/O丰度梯度。
• 评估热底燃烧（HBB）与第三次包层混合在中等质量恒星中的作用，尽管这些过程未包含在基础模型中。
• 使用Spite et al. (2005) 和 Israelian et al. (2004) 的观测数据来约束模型参数。

实验结果

研究问题

• RQ1 为何贫金属晕星表现出高于标准恒星产量预测的N/O比？
• RQ2 在低金属度下大质量恒星中增强的氮生成是否可解释银河系晕中观测到的丰度模式？
• RQ3 恒星自转在提高贫金属大质量恒星中原生氮产量中起什么作用？
• RQ4 包含自转影响的产量如何改变银河系盘内预测的N/O丰度梯度？
• RQ5 衰减莱曼-阿尔法系统中观测到的N/O二分性是否可由恒星形成历史差异而非年龄差异来解释？

主要发现

• 为使60 M⊙恒星在Z < 10⁻⁵时与贫金属晕星中观测到的N/O比一致，¹⁴N产量需提高200倍。
• 在相同金属度下，9 M⊙恒星的¹⁴N产量需提高约40倍才能再现数据。
• 该模型成功再现了贫金属晕星中的高N/O比以及太阳邻域的观测N/O比。
• 包含自转影响的产量预测了银河系盘内存在负的N/O丰度梯度。
• 当考虑自转时，中等质量恒星对原生氮生成的贡献比以往认为的要小。
• 该模型表明，金属度高于[Fe/H] ≈ -2.5的DLA系统可能未经历与晕星相同的快速旋转大质量恒星阶段，从而解释其较低的N/O比。