Constraints on the magnetic field strength of HAT-P-7 b and other hot giant exoplanets
Constraints on the magnetic field strength of HAT-P-7 b and other hot giant exoplanets
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HAT-P-7 b 和其他热巨系外行星磁场强度的约束
DOI：
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发表时间：
2017
期刊：
影响因子：
14.1
通讯作者：
Tamara M. Rogers
中科院分区：
文献类型：
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作者：
Tamara M. Rogers;Tamara M. Rogers
对热巨系外行星的红外和光学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，该数值模拟结果产生了快速的、向东的风，这些风将亚恒星点3，4以东的大气中最热点平流输送。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着时间的推移而变化很大，最亮的点有时会向西移动。这些亮度偏移的变化需要风的变化，有或没有云的存在。虽然这种风的变化在热巨型系外行星大气的流体动力学模拟中没有看到，但在磁流体动力学模拟中已经看到了。在这里，我表明，磁流体动力学模拟的HAT-P-7 B确实显示可变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，我将其最小磁场强度限制为6 G。对热的巨型系外行星上风的变化进行类似的观测，或者缺乏这种观测，可能有助于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，这种观测约束可能会被证明是非常有用的。磁流体动力学模型能够解释热巨系外行星HAT-P-7 B大气中峰值亮度点偏移随风变率的变化。最小场强为6 G，以重现观测结果。
Observations of the infrared and optical light curves of hot giant exoplanets have demonstrated that the peak brightness is generally offset eastwards from the substellar point1,2. This observation is consistent with hydrodynamic numerical simulations producing fast, eastwards directed winds that advect the hottest point in the atmosphere eastwards of the substellar point3,4. However, recent continuous Kepler measurements of HAT-P-7 b show that its peak brightness offset varies considerably over time, with excursions such that the brightest point is sometimes westwards of the substellar point5. 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 giant exoplanet atmospheres, it has been seen in magnetohydrodynamic simulations6. Here I show that magnetohydrodynamic simulations of HAT-P-7 b indeed display variable winds and a corresponding variability in the position of the hottest point in the atmosphere. Assuming that the observed variability in HAT-P-7 b is due to magnetism, I constrain its minimum magnetic field strength to be 6 G. Similar observations of wind variability on hot giant exoplanets, or the lack thereof, could help constrain their magnetic field strengths. As dynamo simulations of these planets do not exist and theoretical scaling relations7 may not apply, such observational constraints could prove immensely useful. A magnetohydrodynamic model is able to explain the peak brightness point offset variations in the atmosphere of the hot giant exoplanet HAT-P-7 b with wind variability. A minimum field strength of 6 G is required to reproduce the observations.