[论文解读] Binary-induced magnetic activity? Time-series echelle spectroscopy and photometry of HD123351 = CZ CVn

K. G. Strassmeier, T. A. Carroll+5 more|arXiv (Cornell University)|Aug 23, 2011

Stellar, planetary, and galactic studies参考文献 71被引用 11

一句话总结

本研究首次为活跃的K0IV-III型星HD 123351提供了高精度轨道解，揭示了一个具有147.89天轨道周期和58.32天自转周期的高偏心率双星系统。星周冕发射与磁通量的反相声调调制——在近日点时最低——表明存在一种双星间的磁场，导致磁通量稀释和自转不同步，为演化恒星中双星诱导的磁活动提供了证据。

ABSTRACT

We present a first and detailed study of the bright and active K0IV-III star HD 123351. The star is found to be a single-lined spectroscopic binary with a period of 147.8919+-0.0003 days and a large eccentricity of e=0.8086+-0.0001. The rms of the orbital solution is just 47 m/s, making it the most precise orbit ever obtained for an active binary system. The rotation period is constrained from long-term photometry to be 58.32+-0.01 days. It shows that HD 123351 is a very asynchronous rotator, rotating five times slower than the expected pseudo-synchronous value. Two spotted regions persisted throughout the 12 years of our observations. Four years of Halpha, CaII H&K and HeI D3 monitoring identifies the same main periodicity as the photometry but dynamic spectra also indicate that there is an intermittent dependence on the orbital period, in particular for Ca ii H&K in 2008. Line-profile inversions of a pair of Zeeman sensitive/insensitive iron lines yield an average surface magnetic-flux density of 542+-72 G. The time series for 2008 is modulated by the stellar rotation as well as the orbital motion, such that the magnetic flux is generally weaker during times of periastron and that the chromospheric emissions vary in anti-phase with the magnetic flux. We also identify a broad and asymmetric lithium line profile and measure an abundance of log n(Li) = 1.70+-0.05. The star's position in the H-R diagram indicates a mass of 1.2+-0.1 Msun and an age of 6-7 Gyr. We interpret the anti-phase relation of the magnetic flux with the chromospheric emissions as evidence that there are two magnetic fields present at the same time, a localized surface magnetic field associated with spots and a global field that is oriented towards the (low-mass) secondary component.

研究动机与目标

• 利用长期多波段观测，研究演化中的活跃K0IV-III型星HD 123351的磁活动本质。
• 确定在高偏心率双星系统中轨道运动是否诱导或调制磁活动。
• 解析单线光谱双星中自转不同步与持久黑子区的成因。
• 检验双星间磁场影响表面磁通量与星周冕发射的假设。

提出的方法

• 2006–2010年间获取955组高分辨率STELLA echelle光谱，测量径向速度变化并推导轨道解。
• 利用波茨坦自动光电望远镜（APT）对12年$VI_C$光变曲线进行监测，以确定恒星自转周期。
• 对H$α$、Ca ii H&K及He i D3线进行时间序列分析，检测星周冕活动的调制。
• 应用线轮廓反演技术，测量磁敏感与非敏感铁线的表面磁通量密度。
• 将Küker-Stix差速旋转代码应用于MESA恒星模型，估算对流周转时间与罗斯比数。
• 结合Hipparcos视差与演化轨迹估算质量与年龄，并分析锂线不对称性以评估同位素贡献。

实验结果

研究问题

• RQ1 HD 123351的高偏心率双星轨道是否在主星中诱导或调制磁活动？
• RQ2 为何主星自转不同步，周期为58.32天，显著慢于预期的伪同步值？
• RQ3 为何在近日点附近，星周冕发射与磁通量之间出现反相声调调制？
• RQ4 是否存在双星间磁场影响观测到的磁活动与周期的证据？
• RQ5 具有150°经向分离的持久、长寿命黑子区的成因是什么？

主要发现

• HD 123351为单线光谱双星，轨道周期为147.8919 ± 0.0003天，偏心率为0.8086 ± 0.0001，是迄今对活跃双星系统最精确的轨道解（均方根误差=47 m s⁻¹）。
• 恒星自转周期为58.32 ± 0.01天，表明其自转速度比预期的伪同步值慢五倍。
• 在12年观测中持续观测到两个黑子区，经向间隔为150° ± 17°，黑子温度比有效温度低约1160 K，覆盖表面面积的5–10%。
• 星周冕发射线（Ca ii H&K、H$α$、Ca ii红外三重线）的通量约为10⁶ erg cm⁻² s⁻¹，He i D3的探测证实存在非热加热过程。
• 塞曼展宽分析得出平均表面磁通量密度为542 ± 72 G，其随轨道周期的微弱调制表明近日点时磁通量较低。
• 磁通量与星周冕发射的反相声调关系——近日点时最低——表明存在双星间磁场，可能负责磁通量稀释与自转不同步。