[论文解读] The CARMENES search for exoplanets around M dwarfs: Wing asymmetries of H$\alpha$, Na I D, and He I lines

B. Fuhrmeister, S. Czesla+14 more|arXiv (Cornell University)|Jan 31, 2018

Stellar, planetary, and galactic studies参考文献 69被引用 26

一句话总结

本研究利用CARMENES数据对28颗活跃M型矮星的高分辨率光谱进行分析，探究Hα、Na I D和He I线轮廓的非对称性与展宽现象。研究提出，红移非对称性源于日冕色球层的凝聚或冕雨，而蓝移非对称性则与耀斑爆发初期的色球层蒸发有关。主要发现为：Hα非对称性普遍存在——约15%的光谱中出现该现象——且与耀斑关联性微弱，表明其可能反映超越瞬时耀斑的持续性大气动力学。

ABSTRACT

Stellar activity is ubiquitously encountered in M dwarfs and often characterised by the H$\alpha$ line. In the most active M dwarfs, H$\alpha$ is found in emission, sometimes with a complex line profile. Previous studies have reported extended wings and asymmetries in the H$\alpha$ line during flares. We used a total of 473 high-resolution spectra of 28 active M dwarfs obtained by the CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs) spectrograph to study the occurrence of broadened and asymmetric H$\alpha$ line profiles and their association with flares, and examine possible physical explanations. We detected a total of 41 flares and 67 broad, potentially asymmetric, wings in H$\alpha$. The broadened H$\alpha$ lines display a variety of profiles with symmetric cases and both red and blue asymmetries. Although some of these line profiles are found during flares, the majority are at least not obviously associated with flaring. We propose a mechanism similar to coronal rain or chromospheric downward condensations as a cause for the observed red asymmetries; the symmetric cases may also be caused by Stark broadening. We suggest that blue asymmetries are associated with rising material, and our results are consistent with a prevalence of blue asymmetries during the flare onset. Besides the H$\alpha$ asymmetries, we find some cases of additional line asymmetries in \ion{He}{i} D$_{3}$, \ion{Na}{i}~D lines, and the \ion{He}{i} line at 10830\,\AA\, taken all simultaneously thanks to the large wavelength coverage of CARMENES. Our study shows that asymmetric H$\alpha$ lines are a rather common phenomenon in M~dwarfs and need to be studied in more detail to obtain a better understanding of the atmospheric dynamics in these objects.

研究动机与目标

• 调查活跃M型矮星中非对称且展宽的Hα线轮廓的普遍性及其物理成因。
• 确定此类线轮廓非对称性是否主要与恒星耀斑相关，或代表持续的色球层动力学。
• 检查其他色球层谱线（Na I D、He I D3、He I 10830 Å）中非对称性的出现情况，以评估不同谱线特征间的相干性。
• 探讨线轮廓非对称性与恒星活动水平（特别是旋转速度和Hα光度）之间的关联。
• 评估利用多线非对称性模式推断大气动力学（如质量运动和能量传输机制）的可行性。

提出的方法

• 获取CARMENES光谱仪覆盖0.5–2.5 μm波段的473组高分辨率光谱，信噪比高，光谱分辨率R ≈ 80,000。
• 基于Hα辐射增强及其时间演化特征，采用耀斑检测标准识别耀斑。
• 通过比较线轮廓红翼与蓝翼相对于轮廓峰值的通量比，定量测量线轮廓非对称性。
• 将Hα非对称性与Na I D、He I D3及He I 10833 Å线的非对称性进行比较，评估多线相干性。
• 采用熄灭态减去的通量轮廓，以分离活跃谱线成分并减少连续谱贡献。
• 将非对称性出现频率与恒星活动指标（如Hα光度中位数和投影旋转速度v sin i）进行相关性分析。

实验结果

研究问题

• RQ1 活跃M型矮星中非对称Hα线轮廓的普遍性如何？其与恒星活动水平之间存在何种关联？
• RQ2 Hα线非对称性是否主要与耀斑相关，还是在平静状态下亦可独立出现？
• RQ3 何种物理机制（如色球层蒸发或凝聚）可解释观测到的红移与蓝移非对称性？
• RQ4 Hα线非对称性在频率与相干性方面与其他色球层谱线（如Na I D、He I D3、He I 10833 Å）相比如何？
• RQ5 观测到的线轮廓非对称性是否可由Stark展宽解释，还是必须依赖动态质量运动？

主要发现

• 在473组光谱中检测到67组Hα线非对称性（占14.2%），相当于约15%的观测，表明其在活跃M型矮星中是一种普遍现象。
• 仅24%的Hα非对称性与研究定义的耀斑相关，表明非对称性并非完全由耀斑引起。
• 红移非对称性最一致的解释为色球层凝聚或冕雨，而蓝移非对称性主要在耀斑爆发初期被观测到，与色球层蒸发有关。
• Na I D与He I D3线中的非对称性极为罕见，仅在4%的Hα非对称性光谱中出现，表明这些谱线间相干性有限。
• 在34%的Hα非对称性案例中，He I 10833 Å线表现出展宽的翼部，且该线中观察到3例蓝移非对称性，全部与Hα的蓝移成分一致。
• 由于Na I D线也出现对称展宽（而其受Stark效应影响远小于Hα），因此排除Stark展宽作为OT Ser中对称Hα展宽的主要原因。