[论文解读] The HADES RV Programme with HARPS-N@TNG. III. Flux-flux and activity-rotation relationships of early-M dwarfs

J. Maldonado, G. Scandariato+24 more|arXiv (Cornell University)|Oct 19, 2016

Stellar, planetary, and galactic studies参考文献 93被引用 24

一句话总结

本研究利用TNG望远镜上的HARPS-N高分辨率光谱仪，对71颗早型M型矮星的色球层活动、自转及通量-通量关系进行了研究。结果表明，色球层发射在M0–M3星中大致恒定，活动与自转之间存在中等程度的相关性，且证实偏离通量-通量关系的场星极有可能为年轻恒星，支持早型M型矮星的活动-自转-年龄关系。

ABSTRACT

(Abridged) Understanding stellar activity in M dwarfs is crucial for the physics of stellar atmospheres as well as for ongoing radial velocity exoplanet programmes. Despite the increasing interest in M dwarfs, our knowledge of the chromospheres of these stars is far from being complete. We aim to test whether the relations between activity, rotation, and stellar parameters and flux-flux relationships also hold for early-M dwarfs on the main-sequence. We analyse in an homogeneous and coherent way a well defined sample of 71 late-K/early-M dwarfs that are currently being observed in the framework of the HArps-n red Dwarf Exoplanet Survey (HADES). Rotational velocities are derived using the cross-correlation technique while emission flux excesses in the Ca II H & K and Balmer lines from Halpha up to Hepsilon are obtained by using the spectral subtraction technique. The relationships between the emission excesses and the stellar parameters are studied. Relations between pairs of fluxes of different chromospheric lines are also studied. We find that the strength of the chromospheric emission in the Ca II H & K and Balmer lines is roughly constant for stars in the M0-M3 spectral range. Our data suggest that a moderate but significant correlation between activity and rotation might be present as well as a hint of kinematically selected young stars showing higher levels of emission. We find our sample of M dwarfs to be complementary in terms of chromospheric and X-ray fluxes with those of the literature, extending the analysis of the flux-flux relationships to the very low flux domain. Our results agree with previous works suggesting that the activity-rotation-age relationship known to hold for solar-type stars also applies to early-M dwarfs. We also confirm previous findings that the field stars which deviate from the bulk of the empirical flux-flux relationships show evidence of youth.

研究动机与目标

• 检验在FGK星和原主序星中观测到的活动-自转-年龄关系是否也适用于主序早型M型矮星。
• 在一大群同质的早型M型矮星样本中，研究色球层谱线（Ca ii H&K、巴尔默线）之间的通量-通量关系。
• 评估自转、有效温度和运动学在调制低质量恒星色球层发射中的作用。
• 确定偏离一般通量-通量关系的场星是否倾向于为年轻恒星，如其他恒星族中所建议的那样。
• 将色球层通量关系的实证分析拓展至低通量区域，补充现有文献。

提出的方法

• 利用高分辨率HARPS-N光谱的互相关函数（CCF）技术测量投影自转速度（v sin i）。
• 通过从合成模板中减去光谱，量化Ca ii H&K和巴尔默线（Hα至Hε）的色球层发射过量。
• 分析发射过量与恒星参数（v sin i、有效温度Teff、运动学和年龄估计值）之间的关系。
• 构建不同色球层线对之间的通量-通量关系（如F_Hα vs. F_Hβ、F_CaII vs. F_Hα），以识别经验趋势。
• 与文献样本比较，评估互补性，特别是在低通量区域。
• 使用BT-Settl大气模型进行光谱合成和减法过程中的通量定标。

实验结果

研究问题

• RQ1 在早型M型矮星（M0–M3）中，色球层活动与自转之间是否存在显著相关性？
• RQ2 早型M型矮星中不同色球层线之间的通量-通量关系是否与FGK星或原主序天体中观察到的趋势一致？
• RQ3 具有年轻运动学特征的恒星是否在Ca ii H&K和巴尔默线中表现出比老场星更强的色球层发射过量？
• RQ4 早型M型矮星中色球层发射强度在M0–M3光谱型范围内是否恒定，还是随Teff或光谱型系统性变化？
• RQ5 偏离色球层线一般通量-通量关系的场星是否表现出年轻恒星的证据？

主要发现

• Ca ii H&K和巴尔默线（Hα至Hε）的色球层发射在M0–M3光谱范围内大致恒定，表明该范围内的活动水平稳定。
• 观测到色球层发射与投影自转速度（v sin i）之间存在中等但统计显著的相关性，支持早型M型矮星中的活动-自转关系。
• 通过运动学选定的年轻恒星在Ca ii H&K和大多数巴尔默线中表现出更高的发射过量，表明其活动增强与年轻有关。
• 巴尔默递减比F_Hα/F_Hβ从Teff ~3900 K时的太阳耀斑状值下降至Teff ~3750 K时的太阳 plage 状值，随后在3600 K以下保持大致恒定。
• 本样本通过将通量-通量关系分析拓展至低色球层通量区域，补充了现有文献，证实偏离一般趋势的场星极有可能为年轻恒星。
• 日冕-色球层通量-通量关系显示色球层过量较低，证实本样本作为低活动M型矮星在系外行星和恒星活动研究中的基准样本具有重要价值。