[论文解读] The Galactic WN stars: Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation

W.‐R. Hamann, G. Gräfener+1 more|ArXiv.org|Aug 3, 2006

Stellar, planetary, and galactic studies参考文献 50被引用 190

一句话总结

本研究利用先进的线致遮蔽模型大气（PoWR）对银河系WN星进行了全面的光谱分析，以精确测定其恒星与风参数。尽管模型有所改进并已在演化轨道中引入了自转效应，观测参数与理论预测之间仍存在显著的定量偏差，且未显示出对自转模型的明确偏好，表明尽管理论研究取得进展，大质量恒星演化机制仍未能得到充分理解。

ABSTRACT

CONTEXT: Very massive stars pass through the Wolf-Rayet (WR) stage before they finally explode. Details of their evolution have not yet been safely established, and their physics are not well understood. Their spectral analysis requires adequate model atmospheres, which have been developed step by step during the past decades and account in their recent version for line blanketing by the millions of lines from iron and iron-group elements. However, only very few WN stars have been re-analyzed by means of line-blanketed models yet. AIMS: The quantitative spectral analysis of a large sample of Galactic WN stars with the most advanced generation of model atmospheres should provide an empirical basis for various studies about the origin, evolution, and physics of the Wolf-Rayet stars and their powerful winds. METHODS: We analyze a large sample of Galactic WN stars by means of the Potsdam Wolf-Rayet (PoWR) model atmospheres, which account for iron line blanketing and clumping. The results are compared with a synthetic population, generated from the Geneva tracks for massive star evolution. RESULTS: We obtain a homogeneous set of stellar and atmospheric parameters for the Galactic WN stars, partly revising earlier results. CONCLUSIONS: Comparing the results of our spectral analyses of the Galactic WN stars with the predictions of the Geneva evolutionary calculations, we conclude that there is rough qualitative agreement. However, the quantitative discrepancies are still severe, and there is no preference for the tracks that account for the effects of rotation. It seems that the evolution of massive stars is still not satisfactorily understood.

研究动机与目标

• 利用最先进的模型大气，对银河系WN星进行统一的、基于实测的恒星与大气参数测定。
• 检验现代恒星演化模型（包括自转）是否能够再现WN星的观测特性。
• 评估铁线遮蔽与风致凝聚对光谱拟合及参数推导的影响。
• 评估WN星在赫罗图中的观测分布与日内瓦演化轨道生成的合成星族之间的一致性。
• 确定自转模型是否能更好地解释观测到的WN与WC星比例及光度函数。

提出的方法

• 应用波茨坦沃尔夫-拉叶（PoWR）模型大气代码，包含铁及铁族元素的完整线致遮蔽效应。
• 使用预先计算的模型网格，覆盖关键参数：有效温度、光度、质量损失率及凝聚因子。
• 采用非局部热动平衡、球对称、定态风模型对观测到的WN星谱进行拟合，速度律采用β-律且β = 1。
• 通过体积填充因子$f_{\rm V}$引入风致凝聚，假设$D = f_{\rm V}^{-1} = 4$。
• 将实测参数与包含自转与不包含自转的日内瓦恒星演化轨道生成的合成星族进行比较。
• 采用萨尔皮特初始质量函数及更陡的初始质量函数（$\beta = 1.35, 2.0$）进行星族合成，以检验模型预测与观测赫罗图分布的一致性。

实验结果

研究问题

• RQ1 线致遮蔽模型大气是否相比以往模型能更优地拟合观测到的WN星谱？
• RQ2 当前包含自转的日内瓦恒星演化轨道在多大程度上能再现WN星在赫罗图中的观测分布？
• RQ3 与实测数据相比，是否存在对自转演化模型的偏好？
• RQ4 合成星族中WC与WN星的预测比例是多少？与观测结果相比如何？
• RQ5 当前演化模型能否再现WNL星的观测光度函数？

主要发现

• 包含铁线遮蔽的PoWR模型显著改善了光谱拟合效果，从而对银河系WN星的恒星参数进行了重新标定。
• 尽管模型有所改进，观测到的WNL星光度仍显著高于自转与非自转演化轨道的预测值，且无任何合成WNL星达到$\log L/L_\odot > 6.2$。
• 观测到的WNE星的光球半径大于模型预测值，表明可能存在由“热铁峰”引起的缓慢膨胀、扩展的包层。
• 包含自转的合成星族中WNE与WNL星的比例为30:29，与观测值30:29高度一致，但无法再现WNE星的温度弥散特征。
• 自转模型中WC/WN比例预测值（17）低于非自转模型（45与37），但观测到的比例约0.9更接近非自转情况，尽管尚未能明确确认。
• 未发现对自转模型的明确偏好；自转与非自转轨道均与观测存在严重定量偏差，表明大质量恒星演化机制仍未得到令人满意的理解。