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H- And Dissociation in Ultra-hot Jupiters: A Retrieval Case Study of WASP-18b

Published version
Peer-reviewed

Type

Article

Change log

Abstract

Atmospheres of a number of ultra-hot Jupiters (UHJs) with temperatures $\gtrsim2000 Khavebeenobservedrecently.ManyoftheseplanetsshowlargelyfeaturelessthermalspectrainthenearinfraredobservedwiththeHSTWFC3spectrograph(1.1−1.7 \mum)eventhoughthisspectralrangecontainsstrongH_2Oopacity.RecentworkshaveproposedthepossibilityofHopacitymaskingtheH_2Ofeatureand/orthermaldissociationofH_2OcausingitsapparentdepletionatthehightemperaturesofUHJs.InthisworkwetestthesehypothesesusingobservationsoftheexoplanetWASP−18basacasestudy.WereportdetailedatmosphericretrievalsoftheplanetusingtheHyDRAretrievalcode,extendedtoincludetheeffectsofHopacityandthermaldissociation.WereportconstraintsontheH_2O,COandHabundancesaswellasthepressuretemperatureprofileofthedaysideatmosphereforretrievalswithandwithoutH/dissociationforeachdataset.WefindthattheH_2$O and H- abundances are relatively unconstrained given the featureless WFC3 spectra. We do not conclusively detect H- in the planet contrary to previous studies which used equilibrium models to infer its presence. The constraint on the CO abundance depends on the combination of WFC3 and Spitzer data, ranging from solar to super-solar CO values. We additionally see signs of a thermal inversion from two of the datasets. Our study demonstrates the potential of atmospheric retrievals of UHJs including the effects of H- and thermal dissociation of molecules.

Description

Keywords

Exoplanet atmospheres, Spectroscopy, Radiative transfer, Hot Jupiters, Exoplanets

Journal Title

Astronomical Journal

Conference Name

Journal ISSN

0004-6256
1538-3881

Volume Title

159

Publisher

American Astronomical Society

Rights

All rights reserved
Sponsorship
Science and Technology Facilities Council (ST/N000927/1)