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

B. Fuhrmeister, S. Czesla+14 more|DIGITAL.CSIC (Spanish National Research Council (CSIC))|Jan 31, 2018

Stellar, planetary, and galactic studies参考文献 73被引用 24

一句话总结

本研究利用CARMENES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线的不对称性，发现红不对称性可能源于日冕冷凝或日冕雨，而蓝不对称性则与耀斑爆发初期的色球层蒸发有关。约15%的光谱显示Hα不对称性，其中仅24%与耀斑相关，表明不对称性可能持续至耀斑峰值之后。

ABSTRACT

Stellar activity is ubiquitously encountered in M dwarfs and often characterised by the H$α$ line. In the most active M dwarfs, H$α$ is found in emission, sometimes with a complex line profile. Previous studies have reported extended wings and asymmetries in the H$α$ 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$α$ 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$α$. The broadened H$α$ 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$α$ 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\,Å\, taken all simultaneously thanks to the large wavelength coverage of CARMENES. Our study shows that asymmetric H$α$ 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

• 研究活跃M型矮星中Hα线轮廓不对称性的发生情况及其物理成因。
• 确定线不对称性是否与恒星耀斑或其他活动现象相关。
• 研究Na I D和He I线中不对称性的普遍性及其特征，与Hα线进行对比。
• 探讨色球层动力学（如冷凝与蒸发）在塑造线轮廓中的作用。
• 评估当前光谱时间采样率在解析耀斑期间不对称性形成时机方面的局限性。

提出的方法

• 使用CARMENES光谱仪获取28颗活跃M型矮星的473组高分辨率光谱，波段覆盖0.5–2.4 μm。
• 基于Hα线中通量增强的定义准则识别耀斑。
• 通过测量相对于线心的红翼或蓝翼过剩发射来分析线轮廓不对称性。
• 将不对称性分类为红移型（红翼过剩）或蓝移型（蓝翼过剩），相对于静止波长。
• 将不对称性发生情况与耀斑活动、恒星自转及Hα中值通量（活动水平指标）进行关联。
• 利用日冕雨和色球层蒸发的物理模型解释观测到的不对称性，并与太阳类比进行比较。

实验结果

研究问题

• RQ1 活跃M型矮星中Hα线不对称性的普遍性如何？其与恒星耀斑的相关性如何？
• RQ2 Hα线中的红不对称性与蓝不对称性是否分别与色球层冷凝或蒸发等不同物理机制相关？
• RQ3 Na I D和He I D3线中不对称性的发生频率如何？是否与Hα不对称性相关？
• RQ4 线轮廓不对称性在耀斑峰值或衰减阶段之后持续存在的程度如何？
• RQ5 Stark展宽或其他展宽机制能否解释对称的Hα轮廓？与动态过程相比有何差异？

主要发现

• 在分析的473组光谱中，15%检测到Hα线不对称性，共涉及28颗恒星中的67个不对称轮廓。
• 仅24%的Hα不对称性与耀斑相关，表明不对称性可能持续至衰减阶段或独立于耀斑发生。
• 红不对称性被解释为色球层冷凝或日冕雨的证据，与太阳观测结果一致。
• 蓝不对称性最可能由耀斑早期猝发相期间的色球层蒸发引起。
• Na I D和He I D3线中的不对称性极为罕见，仅在4%的Hα不对称光谱中出现，表明相关性有限。
• 在34%的Hα不对称情况中，He I 10830 Å线表现出展宽的翼部，表明更高阶巴耳末线/He线中存在相似的动力学过程。