[Paper Review] 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 studies88 references29 citations

TL;DR

This study presents the first optical transmission spectrum of the ultra-hot Jupiter WASP-33b using high-precision spectroscopy from the Gran Telescopio Canarias. It reports a 3.3-σ detection of aluminum oxide (AlO) in the planet's atmosphere, indicating potential non-equilibrium chemistry or enhanced abundance, while placing upper limits on TiO and VO, suggesting subsolar abundances.

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.

Motivation & Objective

• To characterize the atmospheric composition of WASP-33b, one of the hottest known exoplanets, using high-precision optical spectroscopy.
• To disentangle stellar variability from planetary transit signals, particularly due to the host star's pulsations.
• To detect and quantify trace atmospheric species such as metal oxides in ultra-hot Jupiters.
• To assess the significance of AlO as a potential opacity source in the optical transmission spectrum.
• To provide constraints on atmospheric chemistry, including TiO and VO abundances, in a thermally inverted, irradiated exoplanet.

Proposed method

• Obtained two primary transits of WASP-33b separated by 18 orbits using the OSIRIS spectrograph on the 10-meter Gran Telescopio Canarias.
• Performed simultaneous fitting of chromatic transit light curves to derive wavelength-dependent planetary radii and refine transit parameters.
• Modelled and corrected for stellar pulsations in the host star WASP-33 to isolate planetary atmospheric absorption features.
• Applied atmospheric retrieval techniques to the combined dataset to infer chemical composition and opacity sources.
• Used a physical model including Earth's atmosphere and instrumental effects to account for systematic noise.
• Constrained chemical abundances by comparing observed transmission spectra with theoretical models, focusing on AlO, TiO, and VO.

Experimental results

Research questions

• RQ1 Can aluminum oxide (AlO) be detected in the optical transmission spectrum of the ultra-hot Jupiter WASP-33b?
• RQ2 What is the significance of the spectral feature between 450 nm and 550 nm in the transmission spectrum?
• RQ3 Are TiO and VO present in the atmosphere of WASP-33b, and what are their abundance constraints?
• RQ4 How do stellar pulsations affect the interpretation of transit spectroscopy for hot Jupiters?
• RQ5 Does the retrieved AlO abundance deviate from thermochemical equilibrium predictions with solar elemental abundances?

Key findings

• A spectral feature between 450 nm and 550 nm is best explained by aluminum oxide (AlO) with a detection significance of 3.3-σ.
• The retrieved AlO abundance is approximately 1000 times higher than predicted by thermochemical equilibrium with solar elemental abundances.
• Upper limits for TiO and VO indicate subsolar abundances in the terminator region of WASP-33b.
• The transmission spectrum shows no significant evidence for clouds or hazes, and their properties remain unconstrained.
• Stellar companion and star spot contributions were ruled out as sources of the shortward-of-560-nm spectral slope, confirming the feature is due to planetary atmospheric opacity.
• The two independent transits yielded consistent transmission spectra within 1-σ uncertainties, validating the robustness of the results.