[论文解读] Stellar evolution with rotation X: Wolf-Rayet star populations at solar metallicity

G. Meynet, A. Maeder|ArXiv.org|Apr 3, 2003

Stellar, planetary, and galactic studies参考文献 24被引用 300

一句话总结

本文利用更新的损失率和旋转效应，研究了在太阳金属丰度下旋转对Wolf-Rayet (WR) 星星种群的影响。结果表明，旋转显著延长了WR星的寿命，特别是富含氢的WNL阶段；将WR星形成的质量阈值降低至约22 M⊙；使60 M⊙以下的WN/WC相变阶段成为可能；并改善了与观测到的WR/O、WN/WC及过渡星比例的一致性，解决了非旋转模型长期存在的不一致问题。

ABSTRACT

We examine the properties of Wolf--Rayet (WR) stars predicted by models of rotating stars taking account of the new mass loss rates for O--type stars and WR stars (Vink et al. \cite{Vink00}, \cite{Vink01}; Nugis & Lamers \cite{NuLa00}) and of the wind anisotropies induced by rotation. We find that the rotation velocities $v$ of WR stars are modest, i.e. about 50 km s$^{-1}$, not very dependant on the initial $v$ and masses. For the most massive stars, the evolution of $v$ is very strongly influenced by the values of the mass loss rates; below $\sim$12 M$_\odot$ the evolution of rotation during the MS phase and later phases is dominated by the internal coupling. Massive stars with extreme rotation may skip the LBV phase. Models having a typical $v$ for the O--type stars have WR lifetimes on the average two times longer than for non--rotating models. The increase of the WR lifetimes is mainly due to that of the H--rich eWNL phase. Rotation allows a transition WN/WC phase to be present for initial masses lower than 60 M$_\odot$. The durations of the other WR subphases are less affected by rotation. The mass threshold for forming WR stars is lowered from 37 to 22 M$_\odot$ for typical rotation. The comparisons of the predicted number ratios WR/O, WN/WC and of the number of transition WN/WC stars show very good agreement with models with rotation, while this is not the case for models with the present--day mass loss rates and no rotation. As to the chemical abundances in WR stars, rotation brings only very small changes for WN stars, since they have equilibrium CNO values. However, WC stars with rotation have on average lower C/He and O/He ratios. The luminosity distribution of WC stars is also influenced by rotation.

研究动机与目标

• 解决非旋转恒星模型与太阳金属丰度下Wolf-Rayet (WR) 星星种群观测之间的差异。
• 研究更新的损失率（Vink et al. 2000, 2001；Nugis & Lamers 2000）和旋转效应对WR星演化的影响。
• 检验旋转是否能解释观测到的WR/O、WN/WC比例以及过渡WN/WC星的频率。
• 评估旋转对WR星化学丰度和光度分布的影响。
• 评估旋转是否能解决标准模型中WC星过量预测和WR星预测不足的问题。

提出的方法

• 采用包含风团块化和双稳态极限的旋转恒星演化模型，更新了O型星和WR星的损失率。
• 引入旋转混合和内部耦合，以模拟表面丰度和角动量演化。
• 通过应用校正因子至经验数据，对非旋转恒星采用降低的损失率，以隔离非旋转行为。
• 在赫罗图中追踪演化序列，重点关注WR阶段和表面成分的变化。
• 将模型预测与恒星形成速率恒定区域中观测到的WR/O、WN/WC及过渡WN/WC星比例进行比较。
• 分析表面丰度演化（C/N、C/He、O/He）以及光度-成分图（例如，(C+O)/He 与 L/L⊙ 的关系），以评估WR亚型分布。

实验结果

研究问题

• RQ1 旋转如何影响Wolf-Rayet星的寿命，特别是WNL阶段和WN/WC阶段？
• RQ2 与非旋转模型相比，旋转对观测到的WR星数量比（WR/O、WN/WC）有何影响？
• RQ3 旋转能否解释非旋转模型无法重现的过渡WN/WC星（约4.4%）的观测频率？
• RQ4 旋转如何影响WR星中的化学丰度，特别是WN和WC亚型？
• RQ5 旋转对WR星形成的质量阈值以及WC星光度分布有何影响？

主要发现

• 与非旋转模型相比，旋转使WR星的平均寿命延长了两倍，主要归因于富含氢的WNL阶段延长。
• 当引入旋转时，WR星形成的质量阈值从37 M⊙降低至22 M⊙。
• 旋转使初始质量低于60 M⊙的恒星能够经历WN/WC相变阶段，与观测到的此类恒星约4.4%的频率一致。
• 旋转导致WC星的C/He和O/He比值低于非旋转模型，改变了其化学成分。
• 旋转模型使WC星在(C+O)/He与光度图中的演化轨迹向较低光度移动，这是由于WC阶段开始时质量减少所致。
• 观测到的WR/O和WN/WC数量比在旋转模型中得到良好再现，而使用当前损失率的非旋转模型则无法匹配观测结果。