[论文解读] An optical transmission spectrum of the ultra-hot Jupiter WASP-33b. First indication of AlO in an exoplanet

C. von Essen, M. Mallonn+5 more|arXiv (Cornell University)|Nov 6, 2018

Stellar, planetary, and galactic studies参考文献 88被引用 29

一句话总结

本研究利用加那利大望远镜的高精度光谱仪，首次获得了超热木星WASP-33b的光学传输光谱。研究报告了在行星大气中检测到铝氧化物（AlO）的3.3-σ信号，表明可能存在非平衡化学过程或元素丰度增强；同时对TiO和VO设定了上限，提示其丰度可能低于太阳值。

ABSTRACT

There has been increasing progress toward detailed characterization of exoplanetary atmospheres, in both observations and theoretical methods. Improvements in observational facilities and data reduction and analysis techniques are enabling increasingly higher quality spectra, especially from ground-based facilities. The high data quality also necessitates concomitant improvements in models required to interpret such data. In particular, the detection of trace species such as metal oxides has been challenging. Extremely irradiated exoplanets (~3000 K) are expected to show oxides with strong absorption signals in the optical. However, there are only a few hot Jupiters where such signatures have been reported. Here we aim to characterize the atmosphere of the ultra-hot Jupiter WASP-33b using two primary transits taken 18 orbits apart. Our atmospheric retrieval, performed on the combined data sets, provides initial constraints on the atmospheric composition of WASP-33b. We report a possible indication of aluminum oxide (AlO) at 3.3-sigma significance. The data were obtained with the long slit OSIRIS spectrograph mounted at the 10-meter Gran Telescopio Canarias. We cleaned the brightness variations from the light curves produced by stellar pulsations, and we determined the wavelength-dependent variability of the planetary radius caused by the atmospheric absorption of stellar light. A simultaneous fit to the two transit light curves allowed us to refine the transit parameters, and the common wavelength coverage between the two transits served to contrast our results. Future observations with HST as well as other large ground-based facilities will be able to further constrain the atmospheric chemical composition of the planet.

研究动机与目标

• 利用高精度光学光谱学表征WASP-33b的大气成分，该行星是目前已知最热的系外行星之一。
• 区分恒星耀变与系外行星凌星信号，特别是由于其宿主恒星脉动引起的干扰。
• 检测并量化超热木星中金属氧化物等痕量大气成分。
• 评估AlO作为光学传输光谱中潜在消光源的重要性。
• 为热层反转、受恒星强烈照射的系外行星提供大气化学约束，包括TiO和VO的丰度。

提出的方法

• 利用10米加那利大望远镜上的OSIRIS光谱仪，获取了相隔18个轨道的两次WASP-33b主凌星观测。
• 对色散凌星光曲线进行联合拟合，以推导波长依赖的行星半径并优化凌星参数。
• 对宿主恒星WASP-33的恒星脉动进行建模与校正，以隔离系外行星大气的吸收特征。
• 将大气反演技术应用于合并数据集，以推断化学成分与消光来源。
• 采用包含地球大气与仪器效应的物理模型，以解释系统性噪声。
• 通过将观测到的传输光谱与理论模型对比，约束化学丰度，重点关注AlO、TiO和VO。

实验结果

研究问题

• RQ1 能否在超热木星WASP-33b的光学传输光谱中检测到铝氧化物（AlO）？
• RQ2 传输光谱中450 nm至550 nm之间的光谱特征具有何种意义？
• RQ3 TiO和VO是否存在于WASP-33b的大气中？其丰度受到何种约束？
• RQ4 恒星脉动如何影响对热木星凌星光谱学的解释？
• RQ5 所反演出的AlO丰度是否偏离了基于太阳元素丰度的热力学平衡预测？

主要发现

• 450 nm至550 nm之间的光谱特征最可能由铝氧化物（AlO）引起，检测显著性为3.3-σ。
• 反演得到的AlO丰度约为基于太阳元素丰度的热力学平衡预测值的1000倍。
• TiO和VO的上限表明其在WASP-33b晨昏圈区域的丰度低于太阳值。
• 传输光谱未显示云层或霾层的显著证据，其性质仍无法确定。
• 排除了恒星伴星与星 spots 作为560 nm短波长光谱斜率的来源，确认该特征源于行星大气的消光。
• 两次独立凌星观测得到的传输光谱在1-σ不确定度范围内一致，验证了结果的稳健性。