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arXiv · 2205.10765

The Hubble PanCET Program: A Featureless Transmission Spectrum for WASP-29b and Evidence of Enhanced Atmospheric Metallicity on WASP-80b

Ian Wong·Yayaati Chachan·Heather A. Knutson·Gregory W. Henry·Danica Adams·Tiffany Kataria·Björn Benneke·Peter Gao·Drake Deming·Mercedes López-Morales·David K. Sing·Munazza K. Alam·Gilda E. Ballester·Joanna K. Barstow·Lars A. Buchhave·Leonardo A. dos Santos·Guangwei Fu·Antonio García Muñoz·Ryan J. MacDonald·Thomas Mikal-Evans·Jorge Sanz-Forcada·Hannah R. Wakeford
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Abstract

We present a uniform analysis of transit observations from the Hubble Space Telescope and Spitzer Space Telescope of two warm gas giants orbiting K-type stars - WASP-29b and WASP-80b. The transmission spectra, which span 0.4-5.0 $μ$m, are interpreted using a suite of chemical equilibrium PLATON atmospheric retrievals. Both planets show evidence of significant aerosol opacity along the day-night terminator. The spectrum of WASP-29b is flat throughout the visible and near-infrared, suggesting the presence of condensate clouds extending to low pressures. The lack of spectral features hinders our ability to constrain the atmospheric metallicity and C/O ratio. In contrast, WASP-80b shows a discernible, albeit muted H$_2$O absorption feature at 1.4 $μ$m, as well as a steep optical spectral slope that is caused by fine-particle aerosols and/or contamination from unocculted spots on the variable host star. WASP-80b joins the small number of gas-giant exoplanets that show evidence for enhanced atmospheric metallicity: the transmission spectrum is consistent with metallicities ranging from $\sim$30-100 times solar in the case of cloudy limbs to a few hundred times solar in the cloud-free scenario. In addition to the detection of water, we infer the presence of CO$_2$ in the atmosphere of WASP-80b based on the enhanced transit depth in the Spitzer 4.5 $μ$m bandpass. From a complementary analysis of Spitzer secondary eclipses, we find that the dayside emission from WASP-29b and WASP-80b is consistent with brightness temperatures of $937 \pm 48$ and $851 \pm 14$ K, respectively, indicating relatively weak day-night heat transport and low Bond albedo.

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Ian Wong, Yayaati Chachan, Heather A. Knutson, Gregory W. Henry, Danica Adams, Tiffany Kataria, Björn Benneke, Peter Gao, Drake Deming, Mercedes López-Morales, David K. Sing, Munazza K. Alam, Gilda E. Ballester, Joanna K. Barstow, Lars A. Buchhave, Leonardo A. dos Santos, Guangwei Fu, Antonio García Muñoz, Ryan J. MacDonald, Thomas Mikal-Evans, Jorge Sanz-Forcada, Hannah R. Wakeford. 2022-07-01. The Hubble PanCET Program: A Featureless Transmission Spectrum for WASP-29b and Evidence of Enhanced Atmospheric Metallicity on WASP-80b. https://doi.org/10.3847/1538-3881%2Fac7234

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