A broadband thermal emission spectrum of the ultra-hot Jupiter WASP-18b
Creators: name: Coulombe, Louis-Philippe
name: Benneke, Björn
name: Challener, Ryan
name: Piette, Anjali A. A.
name: Wiser, Lindsey S.
name: Mansfield, Megan
name: MacDonald, Ryan J.
name: Beltz, Hayley
name: Feinstein, Adina D.
name: Radica, Michael
name: Savel, Arjun B.
name: Santos, Leonardo A. Dos
name: Bean, Jacob L.
name: Parmentier, Vivien
name: Wong, Ian
name: Rauscher, Emily
name: Komacek, Thaddeus D.
name: Kempton, Eliza M. -R.
name: Tan, Xianyu
name: Hammond, Mark
name: Lewis, Neil T.
name: Line, Michael R.
name: Lee, Elspeth K. H.
name: Shivkumar, Hinna
name: Crossfield, Ian J. M.
name: Nixon, Matthew C.
name: Rackham, Benjamin V.
name: Wakeford, Hannah R.
name: Welbanks, Luis
name: Zhang, Xi
name: Batalha, Natalie M.
name: Berta-Thompson, Zachory K.
name: Changeat, Quentin
name: Désert, Jean-Michel
name: Espinoza, Néstor
name: Goyal, Jayesh M.
name: Harrington, Joseph
name: Knutson, Heather A.
name: Kreidberg, Laura
name: López-Morales, Mercedes
name: Shporer, Avi
name: Sing, David K.
name: Stevenson, Kevin B.
name: Aggarwal, Keshav
name: Ahrer, Eva-Maria
name: Alam, Munazza K.
name: Bell, Taylor J.
name: Blecic, Jasmina
name: Caceres, Claudio
name: Carter, Aarynn L.
name: Casewell, Sarah L.
name: Crouzet, Nicolas
name: Cubillos, Patricio E.
name: Decin, Leen
name: Fortney, Jonathan J.
name: Gibson, Neale P.
name: Heng, Kevin
name: Henning, Thomas
name: Iro, Nicolas
name: Kendrew, Sarah
name: Lagage, Pierre-Olivier
name: Leconte, Jérémy
name: Lendl, Monika
name: Lothringer, Joshua D.
name: Mancini, Luigi
name: Mikal-Evans, Thomas
name: Molaverdikhani, Karan
name: Nikolov, Nikolay K.
name: Ohno, Kazumasa
name: Palle, Enric
name: Piaulet, Caroline
name: Redfield, Seth
name: Roy, Pierre-Alexis
name: Tsai, Shang-Min
name: Venot, Olivia
name: Wheatley, Peter J.
Publisher: name: arXiv
Year: 2023
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: Instrumentation and Methods for Astrophysics (astro-ph.IM)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: Close-in giant exoplanets with temperatures greater than 2,000 K (''ultra-hot Jupiters'') have been the subject of extensive efforts to determine their atmospheric properties using thermal emission measurements from the Hubble and Spitzer Space Telescopes. However, previous studies have yielded inconsistent results because the small sizes of the spectral features and the limited information content of the data resulted in high sensitivity to the varying assumptions made in the treatment of instrument systematics and the atmospheric retrieval analysis. Here we present a dayside thermal emission spectrum of the ultra-hot Jupiter WASP-18b obtained with the NIRISS instrument on JWST. The data span 0.85 to 2.85 $μ$m in wavelength at an average resolving power of 400 and exhibit minimal systematics. The spectrum shows three water emission features (at $&gt;$6$σ$ confidence) and evidence for optical opacity, possibly due to H$^-$, TiO, and VO (combined significance of 3.8$σ$). Models that fit the data require a thermal inversion, molecular dissociation as predicted by chemical equilibrium, a solar heavy element abundance (''metallicity'', M/H = 1.03$_{-0.51}^{+1.11}$ $\times$ solar), and a carbon-to-oxygen (C/O) ratio less than unity. The data also yield a dayside brightness temperature map, which shows a peak in temperature near the sub-stellar point that decreases steeply and symmetrically with longitude toward the terminators.
description: JWST ERS bright star observations. Uploaded to inform JWST Cycle 2 proposals. Manuscript under review. 50 pages, 14 figures, 2 tables