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

Gandhi, S; Madhusudhan, N; Mandell, A

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

Published version

Type

Article

Authors

Gandhi, S

https://orcid.org/0000-0001-9552-3709

Madhusudhan, N

https://orcid.org/0000-0002-4869-000X

Mandell, A

https://orcid.org/0000-0002-8119-3355

Abstract

Atmospheres of a number of ultra-hot Jupiters (UHJs) with temperatures $\gtrsim $2000 K h a v e b e e n o b s e r v e d r e c e n t l y . M a n y o f t h e s e p l a n e t s s h o w l a r g e l y f e a t u r e l e s s t h e r m a l s p e c t r a i n t h e n e a r − i n f r a r e d o b s e r v e d w i t h t h e H S T W F C 3 s p e c t r o g r a p h (1.1−1.7$ \mu $m) e v e n t h o u g h t h i s s p e c t r a l r a n g e c o n t a i n s s t r o n g H$ _2 $O o p a c i t y . R e c e n t w o r k s h a v e p r o p o s e d t h e p o s s i b i l i t y o f H − o p a c i t y m a s k i n g t h e H$ _2 $O f e a t u r e a n d / o r t h e r m a l d i s s o c i a t i o n o f H$ _2 $O c a u s i n g i t s a p p a r e n t d e p l e t i o n a t t h e h i g h t e m p e r a t u r e s o f U H J s . I n t h i s w o r k w e t e s t t h e s e h y p o t h e s e s u s i n g o b s e r v a t i o n s o f t h e e x o p l a n e t W A S P −18 b a s a c a s e s t u d y . W e r e p o r t d e t a i l e d a t m o s p h e r i c r e t r i e v a l s o f t h e p l a n e t u s i n g t h e H y D R A r e t r i e v a l c o d e, e x t e n d e d t o i n c l u d e t h e e f f e c t s o f H − o p a c i t y a n d t h e r m a l d i s s o c i a t i o n . W e r e p o r t c o n s t r a i n t s o n t h e H$ _2 $O, C O a n d H − a b u n d a n c e s a s w e l l a s t h e p r e s s u r e − t e m p e r a t u r e p r o f i l e o f t h e d a y s i d e a t m o s p h e r e f o r r e t r i e v a l s w i t h a n d w i t h o u t H − / d i s s o c i a t i o n f o r e a c h d a t a s e t . W e f i n d t h a t t h e H$ _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.

Keywords

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

Journal Title

Astronomical Journal

Journal ISSN

0004-6256
1538-3881

Volume Title

159

Publisher

American Astronomical Society

Publisher DOI

https://doi.org/10.3847/1538-3881/ab845e

Rights

Sponsorship

Science and Technology Facilities Council (ST/N000927/1)

Collections

Cambridge University Research Outputs