Very low metallicity massive star models : Pre – SN evolution and primary nitrogen production
Very low metallicity massive star models : Pre – SN evolution and primary nitrogen production
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极低金属丰度大质量恒星模型：超新星演化前和初级氮的产生
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发表时间：
2021
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通讯作者：
R. Hirschi
中科院分区：
文献类型：
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作者：
R. Hirschi
上下文近年来，极低金属丰度恒星表面丰度的精确测量为恒星演化模型提供了新的约束条件。目标。计算恒星演化模型，以解释观测到的表面丰度，特别是氮的丰度。方法.计算了两个系列的模型。第一个系列包括20个M模型，它们具有不同的初始金属丰度（Z = 0.02到Z = 10）和旋转（M ini = 0−600 kms）。第二个模型的初始金属丰度为Z = 10，质量在9到85 M之间，初始自转速度快（Mm = 600 - 800 kms）。结果最有趣的模型是Z = 10（[Fe/H]<$-6.6）的模型。在氦燃烧的过程中，碳和氧混合到氢燃烧的外壳中。这提高了壳层的重要性，并导致CO核心质量的减少。在演化后期，氢壳层加深并产生大量的原生氮。对于质量最大的模型（M & 60 M），在红超巨星阶段会发生显著的质量损失。这种质量损失是由于通过旋转和对流混合在CNO元素的表面富集。85 M的超新星模型最终成为WO型的E-Rayet星星。因此，该模型预测的SNe类型Ic和可能的长和软的伽玛暴在非常低的金属丰度。旋转的20 M的模型可以最好地再现所观察到的CNO丰度极贫金属（EMP）的恒星表面和金属量的趋势时，它们的角动量内容是在太阳的金属量相同（因此有一个增加的表面速度与金属量减少）。大质量星星模型的恒星风也可以重现迄今为止已知的最贫金属富碳星星HE 1327 -2326的CNO丰度。
Context. Precise measurements of surface abundances of extremely low metallicity stars have recently been obtained and provide new constraints for the stellar evolution models. Aims. Compute stellar evolution models in order to explain the surface abundances observed, in particular of nitrogen. Methods. Two series of models were computed. The first series consists of 20 M⊙ models with varying initial metallicity (Z = 0.02 down to Z = 10) and rotation (υini = 0−600 kms ). The second one consists of models with an initial metallicity of Z = 10, masses between 9 and 85 M⊙ and fast initial rotation velocities (υini = 600− 800 kms ). Results. The most interesting models are the models with Z = 10 ([Fe/H]∼ −6.6). In the course of helium burning, carbon and oxygen are mixed into the hydrogen burning shell. This boosts the importance of the shell and causes a reduction of the CO core mass. Later in the evolution, the hydrogen shell deepens and produces large amount of primary nitrogen. For the most massive models (M & 60M⊙), significant mass loss occurs during the red supergiant stage. This mass loss is due to the surface enrichment in CNO elements via rotational and convective mixing. The 85 M⊙ model ends up as a WO type wolf-Rayet star. Therefore the models predict SNe of type Ic and possibly long and soft GRBs at very low metallicities. The rotating 20 M⊙ models can best reproduce the observed CNO abundances at the surface of extremely metal poor (EMP) stars and the metallicity trends when their angular momentum content is the same as at solar metallicity (and therefore have an increasing surface velocity with decreasing metallicity). The wind of the massive star models can also reproduce the CNO abundances of the most metal-poor carbon–rich star known to date, HE1327-2326.