[论文解读] Orbital and physical parameters of eclipsing binaries from the All-Sky Automated Survey catalogue - VI. AK Fornacis - a rare, bright K-type eclipsing binary

K. G. Hełminiak, Rafael Brahm+5 more|arXiv (Cornell University)|May 12, 2014

Stellar, planetary, and galactic studies参考文献 50被引用 9

一句话总结

本研究首次对AK Fornacis进行了完整的物理与轨道建模，该系统是一颗明亮（V=9.14）、邻近（31 pc）、长周期（3.981 d）、活跃的K型食双星。通过结合高分辨率光谱（FEROS、CORALIE、HARPS）与ASAS和SuperWASP的光度观测，作者精确测定了两颗恒星的质量（0.6958±0.0010 和 0.6355±0.0007 M⊙）与半径（0.687±0.020 和 0.609±0.016 R⊙），使其成为目前已知最精确测量的低质量双星系统之一，为0.5–0.8 M⊙范围内的恒星模型提供了关键基准。

ABSTRACT

We present the results of the combined photometric and spectroscopic analysis of a bright (V=9.14), nearby (d=31 pc), late-type detached eclipsing binary AK Fornacis. This P=3.981 d system has not been previously recognised as a double-lined spectroscopic binary, and this is the first full physical model of this unique target. With the FEROS, CORALIE and HARPS spectrographs we collected a number of high-resolution spectra in order to calculate radial velocities of both components of the binary. Measurements were done with our own disentangling procedure and the TODCOR technique, and were later combined with the photometry from the ASAS and SuperWASP archives. We also performed an atmospheric analysis of the component spectra with the Spectroscopy Made Easy (SME) package. Our analysis shows that AK For consists of two active, cool dwarfs having masses of $M_1=0.6958 \pm 0.0010$ and $M_2=0.6355 \pm 0.0007$ M$_\odot$ and radii of $R_1=0.687 \pm 0.020$ and $R_2=0.609 \pm 0.016$ R$_\odot$, slightly less metal abundant than the Sun. Parameters of both components are well reproduced by the models. AK For is the brightest system among the known eclipsing binaries with K or M type stars. Its orbital period is one of the longest and rotational velocities one of the lowest, which allows us to obtain very precise radial velocity measurements. The precision in physical parameters we obtained places AK For among the binaries with the best mass measurements in the literature. It also fills the gap in our knowledge of stars in the range of 0.5-0.8 M$_\odot$, and between short and long-period systems. All this makes AK For a unique benchmark for understanding the properties of low-mass stars.

研究动机与目标

• 确定AK Fornacis的轨道与物理参数，该系统是一种罕见的明亮、长周期K型食双星。
• 通过提供高精度基准系统，解决低质量恒星模型中的矛盾之处。
• 研究恒星活动与自转在部分低质量双星中观测到的半径膨胀现象中的作用。
• 提高0.5–0.8 M⊙范围内恒星基本物理参数（质量、半径、金属丰度）的测量精度。

提出的方法

• 利用FEROS、CORALIE和HARPS获取高分辨率光谱，测量双星两子星的视向速度。
• 通过自定义的解离程序与TODCOR互相关技术提取视向速度。
• 将ASAS和SuperWASP档案中的光变曲线与光谱数据联合，进行联合建模。
• 使用Spectroscopy Made Easy（SME）软件包分析恒星大气参数，以确定有效温度与金属丰度。
• 采用完整的双星建模方法，同步拟合光曲线与视向速度曲线，推导轨道与物理参数。
• 将恒星演化模型与推导出的参数进行比较，以评估一致性与年龄。

实验结果

研究问题

• RQ1 AK Fornacis两颗子星的精确质量与半径是多少？与恒星演化模型相比如何？
• RQ2 尽管自转缓慢且轨道周期较长，AK Fornacis为何表现出异常活跃？这与典型短周期活跃双星不同，原因何在？
• RQ3 该系统为何因高亮度与长周期而能实现比典型低质量食双星更精确的测量？
• RQ4 恒星活动在多大程度上影响了该系统质量与半径测定的准确性？
• RQ5 AK Fornacis是否代表了从短周期膨胀双星向长周期、平静系统过渡的中间状态？

主要发现

• AK Fornacis是目前已知最亮的K型食双星，V星等为9.14，距离为31 pc。
• 该系统的轨道周期为3.981天，是已知具有K型或M型成分的食双星中周期最长者之一。
• 主星质量为0.6958±0.0010 M⊙，半径为0.687±0.020 R⊙；次星质量为0.6355±0.0007 M⊙，半径为0.609±0.016 R⊙。
• 两颗恒星的金属丰度略低于太阳，[Fe/H] ≈ −0.08 dex。
• 系统的参数能被恒星演化模型良好再现，表明其主序星年龄与观测特征一致。
• 尽管具有强烈的色球层活动（表现为强烈的Ca II H&K发射线），但恒星并未出现半径膨胀，这挑战了活动性、自转与半径膨胀之间存在直接关联的观点。