(2023) The Roasting Marshmallows program with IGRINS on Gemini South I : composition and climate of the ultrahot Jupiter WASP-18 b. The Astronomical Journal, 165 (3). 91. doi:10.3847/1538-3881/acaf5c ISSN 0004-6256.

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Abstract

We present high-resolution dayside thermal emission observations of the exoplanet WASP-18 b using IGRINS on Gemini South. We remove stellar and telluric signatures using standard algorithms, and we extract the planet signal via cross-correlation with model spectra. We detect the atmosphere of WASP-18 b at a signal-to-noise ratio (S/N) of 5.9 using a full chemistry model, measure H2O (S/N = 3.3), CO (S/N = 4.0), and OH (S/N = 4.8) individually, and confirm previous claims of a thermal inversion layer. The three species are confidently detected (>4σ) with a Bayesian inference framework, which we also use to retrieve abundance, temperature, and velocity information. For this ultrahot Jupiter (UHJ), thermal dissociation processes likely play an important role. Retrieving abundances constant with altitude and allowing the temperature–pressure profile to adjust freely results in a moderately super-stellar carbon-to-oxygen ratio (C/O = ${0.75}_{-0.17}^{+0.14}$) and metallicity ([M/H] = ${1.03}_{-1.01}^{+0.65}$). Accounting for undetectable oxygen produced by thermal dissociation leads to C/O = ${0.45}_{-0.10}^{+0.08}$ and [M/H] = ${1.17}_{-1.01}^{+0.66}$. A retrieval that assumes radiative–convective–thermochemical equilibrium and naturally accounts for thermal dissociation constrains C/O < 0.34 (2σ) and [M/H] = ${0.48}_{-0.29}^{+0.33}$, in line with the chemistry of the parent star. Looking at the velocity information, we see a tantalizing signature of different Doppler shifts at the level of a few kilometers per second for different molecules, which might probe dynamics as a function of altitude and/or location on the planet disk. Our results demonstrate that ground-based, high-resolution spectroscopy at infrared wavelengths can provide meaningful constraints on the compositions and climate of highly irradiated planets. This work also elucidates potential pitfalls with commonly employed retrieval assumptions when applied to the spectra of UHJs.

Item Type:	Journal Article
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Faculty of Science, Engineering and Medicine > Science > Physics
Library of Congress Subject Headings (LCSH):	Extrasolar planets, Extrasolar planets -- Atmopsheres, High resolution spectroscopy , Infrared spectroscopy
Journal or Publication Title:	The Astronomical Journal
Publisher:	Institute of Physics Publishing, Inc.
ISSN:	0004-6256
Official Date:	8 February 2023
Dates:	Date Event 8 February 2023 Published 29 December 2022 Accepted
Volume:	165
Number:	3
Number of Pages:	22
Article Number:	91
DOI:	10.3847/1538-3881/acaf5c
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Open Access (Creative Commons)
Date of first compliant deposit:	8 March 2023
Date of first compliant Open Access:	9 March 2023
RIOXX Funder/Project Grant:	Project/Grant ID RIOXX Funder Name Funder ID 80NSSC19K0293 [NASA] National Aeronautics and Space Administration http://dx.doi.org/10.13039/100000104 ST/T000406/1 [STFC] Science and Technology Facilities Council http://dx.doi.org/10.13039/501100000271 HST-HF2-51485.001-A [NASA] National Aeronautics and Space Administration http://dx.doi.org/10.13039/100000104 805445 [ERC] Horizon 2020 Framework Programme http://dx.doi.org/10.13039/100010661

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