[论文解读] Three-dimensional Atmospheric Circulation of Warm and Hot Jupiters: Effects of Orbital Distance, Rotation Period, and Non-Synchronous Rotation

Adam P. Showman, Nikole K. Lewis+1 more|arXiv (Cornell University)|Nov 18, 2014

Stellar, planetary, and galactic studies参考文献 1被引用 4

一句话总结

本研究针对宽范围轨道距离、自转速率和恒星辐照度下的温带与热木星，构建了三维大气环流模型，揭示了随着自转加快和辐照度降低，环流动力学从由昼夜加热驱动的超旋转赤道急流，过渡为由斜压不稳定性驱动的中纬度东风急流。关键贡献在于识别出红外相位曲线与光谱可区分这些环流型态，从而为未来观测提供自转速率与加热机制的推断依据。

ABSTRACT

Efforts to characterize extrasolar giant planet (EGP) atmospheres have so far emphasized planets within 0.05 AU of their stars. Despite this focus, known EGPs populate a continuum of orbital separations from canonical hot Jupiter values (0.03-0.05 AU) out to 1 AU and beyond. Unlike typical hot Jupiters, these more distant EGPs will not in general be synchronously rotating. In anticipation of observations of this population, we here present three-dimensional atmospheric circulation models exploring the dynamics that emerge over a broad range of rotation rates and incident stellar fluxes appropriate for warm and hot Jupiters. We find that the circulation resides in one of two basic regimes. On typical hot Jupiters, the strong day-night heating contrast leads to a broad, fast superrotating (eastward) equatorial jet and large day-night temperature differences. At faster rotation rates and lower incident fluxes, however, the day-night heating gradient becomes less important, and baroclinic instabilities emerge as a dominant player, leading to eastward jets in the midlatitudes, minimal temperature variations in longitude, and, in many cases, weak winds at the equator. Our most rapidly rotating and least irradiated models exhibit multiple eastward jets in each hemisphere--similar to the jets on Jupiter and Saturn--and illuminate the dynamical continuum between highly irradiated EGPs and the weakly irradiated giant planets of our own Solar System. We present infrared (IR) light curves and spectra of these models, which show that the amplitude and offset of the IR phase variation, as well as the shape of the spectra, depend significantly on incident flux and rotation rate. This provides a way to identify the regime transition in future observations and suggests that, in some cases, IR light curves can provide constraints on the rotation rate of non-synchronously rotating planets.

研究动机与目标

• 研究温带与热木星在经典同步热木星模式之外的大气环流动力学。
• 确定轨道距离、自转速率与非同步自转对大气环流型态的影响。
• 探索由昼夜加热对比主导与由斜压不稳定性主导的环流型态之间的过渡。
• 预测与不同环流型态相关的可观测特征——特别是红外相位曲线与光谱——以供未来观测对比。
• 评估相位曲线变化在非同步自转系外行星中约束行星自转速率的潜力。

提出的方法

• 利用三维一般环流模型（GCM）求解球形行星上的大气动力学原始方程。
• 系统性地改变恒星辐照度（从0.01至10倍地球接收到的太阳辐照度）与自转周期（从1.5至15天），以覆盖温带与热木星的条件。
• 通过调整自转速率相对于轨道周期，引入非同步自转，模拟未被潮汐锁定的行星。
• 模型包含辐射传输、热潮汐与非静力动力学，以捕捉波-平均流相互作用与斜压不稳定性。
• 合成多波段的合成红外相位曲线与光谱，以模拟可观测的发射特征。
• 通过分析纬向风廓线、温度结构与波活动，基于罗斯贝数与加热机制对环流型态进行分类。

实验结果

研究问题

• RQ1 随着自转速率增加与恒星辐照度降低，温带与热木星的大气环流如何变化？
• RQ2 在参数空间的不同区域，主导的动力机制是昼夜加热对比还是斜压不稳定性？
• RQ3 红外相位曲线与光谱能否区分由热对比驱动与由斜压不稳定性驱动的环流型态？
• RQ4 在二次凌星观测中，相位曲线变化在多大程度上可推断非同步自转？
• RQ5 弱辐照、快速自转系外行星的环流型态与木星和土星相比如何？

主要发现

• 随着自转速率增加与恒星辐照度降低，环流从由强昼夜加热驱动的超旋转赤道急流型态，过渡为由斜压不稳定性驱动的中纬度急流型态。
• 在快速自转与低辐照条件下，每个半球均表现出多个东风急流，类似于木星与土星，表明热木星与冰/气体巨行星之间存在动力学连续性。
• 在快速自转、弱辐照条件下，赤道风速减弱甚至反向，而中纬度急流则成为主导并呈东风特征。
• 红外相位曲线仅在高恒星辐照度与慢速自转条件下表现出显著的相位变化，相位振幅随自转速率加倍而减少约两倍。
• 相位曲线中的光谱变化在吸收带（低压区域，昼夜温度对比强烈）最大，而在窗口区（高压区域）变化较小。
• 当非同步自转偏离同步自转两倍以上时，可在相位曲线中检测到，从而实现对自转速率的观测推断。