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arXiv 1704.06271astro-ph.EP
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对HAT-P-7 b和其他热巨系外行星磁场强度的约束

Constraints on the Magnetic Field Strength of HAT-P-7 b and other Hot Giant Exoplanets

T. M. Rogers

摘要:

对热彗星的红外光变曲线和光变曲线的观测表明,峰值亮度通常从亚恒星点向东偏移[1,2]。这一观察结果与流体动力学数值模拟结果一致,后者产生快速的、向东的风,这些风平流输送亚恒星点以东大气中最热点[3,4]。然而,最近开普勒对HAT-P-7 b的连续测量表明,它的峰值亮度偏移随时间变化很大,最亮的点有时会在亚恒星点的西边[5]。这些亮度偏移的变化需要风的变化,有或没有云的存在。虽然在热木星大气的流体动力学模拟中没有看到这种风的变化,但在磁流体动力学(MHD)模拟中已经看到了[6]。在这里,我们表明,HAT-P-7 b的MHD模拟确实显示了变化的风和相应的变化在大气中最热点的位置。假设HAT-P-7 b中观察到的变化是由于磁性,我们将其最小磁场强度限制为6 μ g。对热的巨型系外行星上风的变化进行类似的观测,或者缺乏类似的观测,可以帮助限制它们的磁场强度。由于这些行星的发电机模拟并不存在,理论上的比例关系可能不适用,这样的观测约束可能会被证明是非常有用的。

英文摘要:

Observations of infrared and optical light curves of hot Jupiters have demonstrated that the peak brightness is generally offset eastward from the substellar point [1,2]. This observation is consistent with hydrodynamic numerical simulations that produce fast, eastward directed winds which advect the hottest point in the atmosphere eastward of the substellar point [3,4]. However, recent continuous Kepler measurements of HAT-P-7 b show that its peak brightness offset varies significantly in time, with excursions such that the brightest point is sometimes westward of the substellar point [5]. These variations in brightness offset require wind variability, with or without the presence of clouds. While such wind variability has not been seen in hydrodynamic simulations of hot Jupiter atmospheres, it has been seen in magnetohydrodynamic (MHD) simulations [6]. Here we show that MHD simulations of HAT-P-7 b indeed display variable winds and corresponding variability in the position of the hottest point in the atmosphere. Assuming the observed variability in HAT-P-7 b is due to magnetism we constrain its minimum magnetic field strength to be 6\,G. Similar observations of wind variability on hot giant exoplanets, or lack thereof, could help constrain their magnetic field strengths. Since dynamo simulations of these planets do not exist and theoretical scaling relations [7] may not apply, such observational constraints could prove immensely useful.

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