系外行星$τ$牧夫座b的空间环境和磁层坡印亭通量
Space environment and magnetospheric Poynting fluxes of the exoplanet $τ$ Boötis b
摘要：
内容：Turner等人（2021年）最近报告了热木星型系外行星$τ$牧夫座b上的磁场的首次尝试性探测。磁场是根据LOFAR望远镜观测到的圆偏振射电辐射推断出来的。无线电辐射可能是周围恒星风与行星磁场相互作用的结果。研究方法：我们进行磁流体动力学模拟周围的空间环境$τ$牧夫座b和它的相互作用与恒星风使用PLUTO代码。我们研究了磁层能量通量和不同磁场方向的影响，以了解在Turner等人（2021）提出的磁场强度下，导致观测到的无线电发射的能量通量的物理过程。在太阳系外星风预报不确定的情况下，我们进一步研究了星风密度和压力对磁层能量通量的影响。结果如下：我们在模拟中发现，这种相互作用很可能是超阿尔夫维尼克的，恒星风-行星相互作用产生的能量通量与观测到的无线电功率一致。磁层坡印廷通量的数量级为1-8 $\times10^{18}$ W的开放，半开放和封闭的磁层。当行星和恒星的磁场对齐时，坡印廷通量与Turner等人（2021）的磁层坡印廷通量-射电效率$> 10^{-3}$的射电功率在能量上一致。在效率因子较低的情况下，磁层发射情景根据本研究中模拟的参数空间不够强大。如果主星的磁极性反转，朝向反对齐的场配置，磁层发射情景中的预期无线电功率将低于可观测的阈值。
英文摘要：
Context: The first tentative detection of a magnetic field on the Hot Jupiter type exoplanet $τ$ Boötis b was recently reported by Turner et al. (2021). The magnetic field was inferred from observations of circularly-polarized radio emission obtained with the LOFAR telescopes. The radio emission is possibly a consequence of the interaction of the surrounding stellar wind with the planet's magnetic field. Methods: We perform magnetohydrodynamic simulations of the space environment around $τ$ Boötis b and its interaction with the stellar wind using the PLUTO code. We study the magnetospheric energy fluxes and effects of different magnetic field orientations in order to understand the physical processes which cause energy fluxes leading to the observed radio emission given the proposed magnetic field strength in Turner et al. (2021). Furthermore we study the effect of stellar wind density and pressure on magnetospheric energy fluxes given the uncertainty of extrasolar stellar wind predictions. Results: We find in our simulations that the interaction is most likely super--Alfvénic and energy fluxes generated by the stellar wind--planet interaction are consistent with the observed radio powers. Magnetospheric Poynting fluxes are of the order of 1-8 $\times10^{18}$ W for open, semi-open and closed magnetospheres. The Poynting fluxes are energetically consistent with the radio powers in Turner et al. (2021) for a magnetospheric Poynting flux-to-radio efficiency $> 10^{-3}$ when the magnetic fields of the planet and star are aligned. In case of lower efficiency factors the magnetospheric emission scenario is according to the parameter space modeled in this study not powerful enough. In case of a magnetic polarity reversal of the host star towards an anti-aligned field configuration, expected radio powers in the magnetospheric emission scenario fall below the observable threshold.
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