[论文解读] Populations of rotating stars. - I. Models from 1.7 to 15 Msun at Z = 0.014, 0.006, and 0.002 with {\Omega}/{\Omega}crit between 0 and 1

C. Georgy, Sylvia Ekström+5 more|arXiv (Cornell University)|Mar 10, 2013

Stellar, planetary, and galactic studies参考文献 28被引用 124

一句话总结

本论文针对1.7–15 M⊙恒星在金属丰度Z = 0.014、0.006和0.002下，提供了270组完整的旋转恒星演化模型，初始旋转速率最高达临界速度的95%。采用改进版本的日内瓦恒星演化代码，能够追踪临界旋转期间的角动量损失和赤道质量损失。研究发现，快速旋转的低质量恒星（1.7 M⊙）在核心氢燃烧阶段形成较小的对流核心；金属贫乏恒星（Z = 0.002）在主序中期表现出强烈的氮元素富集（最高达0.4 dex），且在高旋转速率下，碳和氧的抛射量显著增加，这是由于有效的旋转混合所致。

ABSTRACT

B-type stars are known to rotate at various velocities, including very fast rotators near the critical velocity as the Be stars. In this paper, we provide stellar models covering the mass range between 1.7 to 15 Msun, which includes the typical mass of known Be stars, at Z = 0.014, 0.006, and 0.002 and for an extended range of initial velocities on the zero-age main sequence. We used the Geneva stellar-evolution code, including the effects of shellular rotation, with a numerical treatment that has been improved so the code can precisely track the variation in the angular momentum content of the star as it changes under the influence of radiative winds and/or mechanical mass loss. We discuss the impact of the initial rotation rate on the tracks in the Hertzsprung-Russell diagram, the main-sequence (MS) lifetimes, the evolution of the surface rotation and abundances, as well as on the ejected masses of various isotopes. Among the new results obtained from the present grid we find that 1) fast-rotating stars with initial masses around 1.7 Msun present at the beginning of the core hydrogen-burning phase quite small convective cores with respect to their slowly rotating counterparts. This fact may be interesting to keep in mind in the framework of the asteroseismic studies of such stars. 2) The contrast between the core and surface angular velocity is higher in slower rotating stars. The values presently obtained are in agreement with the very few values obtained for B-type stars from asteroseismology. 3) At Z = 0.002, the stars in the mass range of 1.7 to 3 Msun with a mean velocity on the MS of the order of 150 km/s show N/H enhancement superior to 0.2 dex at mid-MS, and superior to 0.4 dex at the end of the MS phase. At solar metallicity the corresponding values are below 0.2 dex at any time in the MS.

研究动机与目标

• 提供覆盖B型星和Be星质量范围的详细、高分辨率旋转恒星模型网格。
• 改进在接近临界速度旋转时角动量损失与赤道质量损失的追踪能力。
• 研究初始旋转速率与金属丰度对恒星演化、表面元素丰度及质量抛射的影响。
• 支持基于真实初始旋转速率与金属丰度分布的合成星族合成模拟。

提出的方法

• 使用改进数值处理的日内瓦恒星演化代码，结合层状旋转模型，追踪角动量演化。
• 引入辐射与机械质量损失的各向异性，尤其在临界旋转阶段。
• 在整个主序阶段追踪表面速度、有效温度及化学元素丰度（N、C、O）的演化。
• 基于改进的风模型，计算处于或接近临界旋转的恒星的赤道质量损失率。
• 在三个金属丰度（Z = 0.014、0.006、0.002）下，模拟10个质量点与9种初始旋转速率（Ω/Ωcrit 从0到0.95）。
• 计算氢、氦、碳、氮、氧及CO核心质量的抛射量，并对比非旋转与快速旋转模型。

实验结果

研究问题

• RQ1 初始旋转速率如何影响低质量（1.7 M⊙）恒星在核心氢燃烧阶段对流核心的大小？
• RQ2 金属丰度对旋转恒星表面氮元素富集的影响如何，特别是在主序中期与末期？
• RQ3 旋转混合如何影响不同金属丰度与旋转速率下恒星的碳与氧抛射量？
• RQ4 临界旋转在多大程度上导致增强的赤道质量损失与非球面对称质量抛射？
• RQ5 后主序演化中的蓝 loop 现象如何依赖于旋转速率与金属丰度？

主要发现

• 在核心氢燃烧开始时，快速旋转的1.7 M⊙恒星形成的对流核心显著小于慢速旋转的对应恒星，该结果对星震学建模具有重要意义。
• 慢速旋转恒星的核与表面角速度差异更大，计算值与B型恒星的星震学测量结果一致。
• 在Z = 0.002时，主序中期表面平均速度约为150 km·s⁻¹的恒星，其氮氢比（N/H）在主序末期的富集程度超过0.4 dex；而在太阳金属丰度下，此类富集程度仍低于0.2 dex。
• 旋转混合使SMC金属丰度模型（Z = 0.002）中碳的抛射质量增加最多达2.15倍，氧的抛射质量增加最多达3.36倍，初始旋转速率达临界速度95%时。
• 当初始旋转速率达临界速度的95%时，SMC金属丰度模型（Z = 0.002）的CO核心质量增加43%，LMC与银河系模型分别增加27%与25%。
• 在Z = 0.002时，所有初始旋转速率ωini > 0.60的模型在主序中期即表现出可探测的表面氮富集；而在太阳金属丰度下，仅在最快速旋转的恒星（ωini > 0.8）中，1.7 M⊙模型才可探测到此类富集。