[论文解读] Wind properties of variable B supergiants

M. Haucke, L. S. Cidale+5 more|arXiv (Cornell University)|Jun 20, 2018

Stellar, planetary, and galactic studies参考文献 106被引用 9

一句话总结

本研究利用非局部热动平衡（non-LTE）合成轮廓拟合方法，结合FAST-WIND代码，分析了19颗银河系B型超巨星的风变异性，结果表明：周期大于6天的径向脉动很可能是导致质量损失变化的原因。主要发现包括：首次建立质量损失振幅与光变/光谱变异性（10–100天 timescales）之间的经验关联；位于双稳态跃迁冷侧的恒星表现出V∞/Vesc降低，尽管其质量损失率与热侧相似或更低，暗示风团块化或电离效应增强。

ABSTRACT

Context. Variable B supergiants (BSGs) constitute a heterogeneous group of stars with complex photometric and spectroscopic behaviours. They exhibit mass-loss variations and experience different types of oscillation modes, and there is growing evidence that variable stellar winds and photospheric pulsations are closely related. Aims. To discuss the wind properties and variability of evolved B-type stars, we derive new stellar and wind parameters for a sample of 19 Galactic BSGs by fitting theoretical line profiles of H, He, and Si to the observed ones and compare them with previous determinations. Methods. The synthetic line profiles are computed with the non-local thermodynamic equilibrium (NLTE) atmosphere code FASTWIND, with a β-law for hydrodynamics. Results. The mass-loss rate of three stars has been obtained for the first time. The global properties of stellar winds of mid/late B supergiants are well represented by a β-law with β > 2. All stars follow the known empirical wind momentum–luminosity relationships, and the late BSGs show the trend of the mid BSGs. HD 75149 and HD 99953 display significant changes in the shape and intensity of the Hα line (from a pure absorption to a P Cygni profile, and vice versa). These stars have mass-loss variations of almost a factor of 2.8. A comparison among mass-loss rates from the literature reveals discrepancies of a factor of 1 to 7. This large variation is a consequence of the uncertainties in the determination of the stellar radius. Therefore, for a reliable comparison of these values we used the invariant parameter Qr. Based on this parameter, we find an empirical relationship that associates the amplitude of mass-loss variations with photometric/spectroscopic variability on timescales of tens of days. We find that stars located on the cool side of the bi-stability jump show a decrease in the ratio V∞∕Vesc, while their corresponding mass-loss rates are similar to or lower than the values found for stars on the hot side. Particularly, for those variable stars a decrease in V∞∕Vesc is accompanied by a decrease in Ṁ. Conclusions. Our results also suggest that radial pulsation modes with periods longer than 6 days might be responsible for the wind variability in the mid/late-type. These radial modes might be identified with strange modes, which are known to facilitate (enhanced) mass loss. On the other hand, we propose that the wind behaviour of stars on the cool side of the bi-stability jump could fit with predictions of the δ−slow hydrodynamics solution for radiation-driven winds with highly variable ionization.

研究动机与目标

• 利用合成轮廓拟合方法，为19颗银河系B型超巨星样本精确推导恒星与风参数。
• 研究演化B型星中光球脉动与可变星风之间的关联。
• 通过引入不变参数Qr，最小化与半径相关的不确定性，以解决质量损失率测定中的差异。
• 探讨双稳态跃迁对风特性（尤其是V∞/Vesc与质量损失率）的影响。
• 识别中/晚型B型超巨星中导致风变异性的脉动模式。

提出的方法

• 使用非局部热动平衡大气模型FAST-WIND，结合β律风速度结构，计算H、He和Si的合成轮廓。
• 通过将合成轮廓与实测光谱拟合，推导恒星与风参数（Teff, log g, β, Ṁ, V∞, 微湍流, 宏湍流, Vsin i, R⋆）。
• 采用不变参数Qr = Ṁ × V∞ × R⋆⁻¹对质量损失率进行归一化，以减少对不确定恒星半径估计的依赖。
• 在样本范围内检验风动量-光度关系（WLR），以验证风模型的一致性。
• 分析Hα轮廓的变异性（P Cygni型与吸收型之间的转换），以量化质量损失率随时间的变化。
• 将文献中已知的脉动周期与新拟合结果，与风变异性进行相关性分析，以识别驱动模式。

实验结果

研究问题

• RQ1 中/晚型B型超巨星中，径向脉动模式与风变异性之间存在何种关系？
• RQ2 不同观测示踪剂推导出的质量损失率如何比较？文献中存在巨大差异的原因是什么？
• RQ3 在双稳态跃迁区域，风特性（尤其是V∞/Vesc与Ṁ）如何变化？何种物理机制可解释这些观测趋势？
• RQ4 尽管存在半径不确定性，不变参数Qr是否可用于可靠比较不同研究中的质量损失率？
• RQ5 奇异模脉动在驱动演化B型超巨星中时间可变质量损失方面扮演何种角色？

主要发现

• 首次为样本中三颗B型超巨星推导出质量损失率，显著提升了该类恒星的参数覆盖范围。
• 所有中/晚型B型超巨星均符合β律且β > 2，表明风具有高度加速特性，且全部满足经验风动量-光度关系。
• HD 75149与HD 99953的Hα轮廓在纯吸收与P Cygni型之间循环变化，表明其质量损失率随时间变化约2.8倍。
• 通过不变参数Qr，发现质量损失变异性振幅与数十天时间尺度的光变/光谱变异性之间存在新的经验关系。
• 位于双稳态跃迁冷侧的恒星，其V∞/Vesc显著降低（从~2.6降至~1.3），而质量损失率与热侧恒星相当或更低。
• 冷侧V∞/Vesc的降低与Ṁ的减小相关，提示风团块化增强或电离效应（如δ−slow流体动力学）可能起作用，与非定常辐射驱动风模型的预测一致。