[论文解读] Climate of an Ultra hot Jupiter: Spectroscopic phase curve of WASP-18b with HST/WFC3

Jacob Arcangeli, Jean-Michel Désert+7 more|UvA-DARE (University of Amsterdam)|Apr 3, 2019

Stellar, planetary, and galactic studies参考文献 93被引用 5

一句话总结

本研究利用哈勃空间望远镜/WFC3首次获得了超热木星WASP-18b的完整轨道光谱相位曲线，测得白天温度为2894 ± 30 K，夜晚温度上限为1430 K（置信水平3σ）。数据表明存在显著的昼夜温差（≈0.96）以及较小的4.5°向东亮度偏移，这需要高效的大气阻力——可能源于10高斯的磁场——来解释，挑战了标准大气环流模型，并为超热木星的大气化学和膨胀机制提供了新约束。

ABSTRACT

We present the analysis of a full-orbit, spectroscopic phase curve of the ultra hot Jupiter WASP-18b, obtained with the Wide Field Camera 3 aboard the Hubble Space Telescope. We measure the planet's normalized day-night contrast as >0.96 in luminosity: the disk-integrated dayside emission from the planet is at 964+-25 ppm, corresponding to 2894+-30 K, and we place an upper limit on the nightside emission of <32ppm or 1430K at the 3-sigma level. We also find that the peak of the phase curve exhibits a small, but significant offset in brightness of 4.5+-0.5 degrees eastward. We compare the extracted phase curve and phase resolved spectra to 3D Global Circulation Models and find that broadly the data can be well reproduced by some of these models. We find from this comparison several constraints on the atmospheric properties of the planet. Firstly we find that we need efficient drag to explain the very inefficient day-night re-circulation observed. We demonstrate that this drag could be due to Lorentz-force drag by a magnetic field as weak as 10 Gauss. Secondly, we show that a high metallicity is not required to match the large day-night temperature contrast. In fact, the effect of metallicity on the phase curve is different from cooler gas-giant counterparts, due to the high-temperature chemistry in WASP-18b's atmosphere. Additionally, we compare the current UHJ spectroscopic phase curves, WASP-18b and WASP-103b, and show that these two planets provide a consistent picture with remarkable similarities in their measured and inferred properties. However, key differences in these properties, such as their brightness offsets and radius anomalies, suggest that UHJ could be used to separate between competing theories for the inflation of gas-giant planets.

研究动机与目标

• 测量WASP-18b的完整轨道光谱相位曲线，以约束其大气环流与热结构。
• 确定大气阻力与磁场在塑造该行星昼夜温差与亮度偏移中的作用。
• 将观测到的相位曲线与三维全球环流模型（GCMs）进行比较，评估金属丰度与高温化学对大气特性的影响。
• 评估WASP-18b的特性对行星膨胀理论（特别是欧姆耗散）的启示。
• 将WASP-18b与WASP-103b进行比较，识别其在环流与半径膨胀方面的差异，以区分竞争性大气理论。

提出的方法

• 利用哈勃空间望远镜上的广域相机3（WFC3）获取了WASP-18b的完整轨道光谱相位曲线。
• 提取了相位分辨的光谱，并推导出该行星在轨道相位中白天与夜晚的亮度。
• 测得有效白天温度为2894 ± 30 K，并设定了夜晚辐射的3σ上限（≤1430 K）。
• 量化了昼夜辐射亮度对比（≈0.96）以及峰值亮度领先于天底点的相位偏移（4.5 ± 0.5°向东）。
• 将观测到的相位曲线与光谱与一系列三维全球环流模型（GCMs）进行比较，以推断大气动力学与成分。
• 探讨了金属丰度、辐射传输以及高温化学（如TiO、H2O离解）对模型拟合与模型选择的影响。

实验结果

研究问题

• RQ1 WASP-18b的白天与夜晚之间存在怎样的热对比？这揭示了大气能量再分配的何种机制？
• RQ2 为何WASP-18b表现出一个微小但显著的4.5°向东亮度偏移？何种物理机制可解释该现象？
• RQ3 为重现观测到的昼夜温差，需要何种大气阻力机制？磁场是否可能发挥作用？
• RQ4 金属丰度如何影响WASP-18b的相位曲线？由于高温化学作用，其行为是否与较冷气态巨行星不同？
• RQ5 WASP-18b的大气特性与WASP-103b相比如何？二者差异揭示了何种行星膨胀机制？

主要发现

• WASP-18b的白天有效温度为2894 ± 30 K，昼夜两侧的辐射亮度对比约为0.96。
• 在3σ置信水平下，夜晚辐射被限制在≤1430 K，表明暗面热辐射极低。
• 检测到天底点以东4.5 ± 0.5度的亮度偏移，表明热分布存在经向不对称性。
• 若无额外阻力源，现有GCMs无法重现观测到的昼夜温差，表明需要高效的气流阻力。
• 即使仅为10高斯的弱磁场，也可能产生足够的洛伦兹力阻力，以解释观测到的环流低效与亮度偏移。
• 仅靠金属丰度变化无法重现观测相位曲线；相反，高温化学过程（如TiO与H2O离解）显著改变了辐射与热响应，与较冷气态巨行星的行为不同。