[论文解读] Direct detection of a magnetic field in the photosphere of the single M giant EK Boo: How common is magnetic activity among M giants?

R. Konstantinova‐Antova, M. Aurière+7 more|arXiv (Cornell University)|Sep 10, 2010

Stellar, planetary, and galactic studies参考文献 40被引用 23

一句话总结

本研究利用伯纳德·勒伊奥特望远镜上的NARVAL高分辨率光谱偏振仪，首次直接探测到单颗M5型巨星EK Boo光球层中变化的磁场。纵向磁场在-0.1至-8 G之间变化，2009年3月的复杂斯托克斯V轮廓表明可能存在发电机活动，提示M型巨星的磁活动可能比以往认为的更为普遍。

ABSTRACT

We study the fast rotating M5 giant EK Boo by means of spectropolarimetry to obtain direct and simultaneous measurements of both the magnetic field and activity indicators, in order to infer the origin of the activity in this fairly evolved giant. We used the new spectropolarimeter NARVAL at the Bernard Lyot Telescope (Observatoire du Pic du Midi, France) to obtain a series of Stokes I and Stokes V profiles for EK Boo. Using the Least Square Deconvolution technique we were able to detect the Zeeman signature of the magnetic field. We measured its longitudinal component by means of the averaged Stokes V and Stokes I profiles. The spectra also permitted us to monitor the CaII K&H chromospheric emission lines, which are well known as indicators of stellar magnetic activity. From ten observations obtained between April 2008 and March 2009, we deduce that EK Boo has a magnetic field, which varied in the range of -0.1 to -8 G. We also determined the initial mass and evolutionary stage of EK Boo, based on up-to-date stellar evolution tracks. The initial mass is in the range of 2.0-3.6 M_sun, and EK Boo is either on the asymptotic giant branch (AGB), at the onset of the thermal pulse phase, or at the tip of the first (or red) giant branch (RGB). The fast rotation and activity of EK Boo might be explained by angular momentum dredge-up from the interior, or by the merging of a binary. In addition, we observed eight other M giants, which are known as X-ray emitters, or to be rotating fast for their class. For one of these, beta And, presumably also an AGB star, we have a marginal detection of magnetic field, and a longitudinal component Bl of about 1G was measured. More observations like this will answer the question whether EK Boo is a special case, or whether magnetic activity is, rather, more common among M giants than expected.

研究动机与目标

• 利用光谱偏振法直接探测并表征单颗M型巨星EK Boo光球层中的磁场。
• 研究演化后的M型巨星中磁活动的起源，特别是其高X射线光度和色球层活动的成因。
• 确定磁活动在M型巨星中是否为普遍现象，还是仅限于如EK Boo这类罕见个例。
• 利用更新的恒星演化轨迹确定EK Boo的演化状态和初始质量。
• 将磁场搜索扩展至其他M型巨星，包括快速旋转星和X射线发射星，以评估磁活动的普遍性。

提出的方法

• 使用伯纳德·勒伊奥特望远镜2米望远镜上的NARVAL仪器，以65,000的光谱分辨率获取高分辨率光谱偏振数据。
• 测量斯托克斯I（强度）和斯托克斯V（圆偏振）轮廓，以探测谱线中的塞曼分裂。
• 应用最小二乘去卷积（LSD）技术，对约12,700条谱线进行平均，提取平均纵向磁场。
• 监测Ca ii K和Hα发射线作为色球层活动的指标，以与磁场变化进行关联。
• 利用恒星演化模型估算EK Boo的初始质量（2.0–3.6 M⊙）和演化阶段（AGB或红巨星分支顶端）。
• 将观测扩展至另外八颗M型巨星，包括X射线发射星和快速旋转星，以评估磁场的普遍性。

实验结果

研究问题

• RQ1 是否存在EK Boo单颗M型巨星光球层中磁场的直接证据？
• RQ2 EK Boo中磁场的强度及其随时间的变异性如何？
• RQ3 EK Boo中观测到的磁活动是否可由发电机过程解释？若可，是何种类型的发电机在起作用？
• RQ4 在M型巨星中，磁活动有多普遍，特别是考虑到其高X射线光度和色球层活动？
• RQ5 其他M型巨星，尤其是快速旋转或X射线发射星，是否也存在可探测的磁场？

主要发现

• 在为期12个月的观测中，EK Boo的磁场被直接探测到，其纵向分量在-0.1 G至-8 G之间变化。
• 2009年3月13日观测到的斯托克斯V轮廓具有复杂、多组分的结构，表明该星可能存在发电机活动。
• EK Boo很可能是一颗单星，初始质量在2.0至3.6 M⊙之间，位于渐近巨星支（AGB）或红巨星分支顶端（tip of RGB）。
• 磁场变化与色球层活动（Ca ii K&H）相关，支持活动的磁起源。
• 对β And的观测获得磁场的微弱检测，纵向分量为-0.95 G，提示其他M型巨星中可能存在磁活动。
• EK Boo中锂丰度未见增强，排除了行星吞噬作为角动量来源的可能性，更支持内部过程（如角动量上翻或双星合并）的作用。