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Dark skies of the slightly eccentric WASP-18 b from its optical-to-infrared dayside emission

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arxiv 2505.01544 v2 pith:X67EZVP7 submitted 2025-05-02 astro-ph.EP

Dark skies of the slightly eccentric WASP-18 b from its optical-to-infrared dayside emission

A. Deline , P. E. Cubillos , L. Carone , B.-O. Demory , M. Lendl , W. Benz , A. Brandeker , M. N. G\"unther
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A. Heitzmann S. C. C. Barros L. Kreidberg G. Bruno D. Kitzmann A. Bonfanti M. Farnir C. M. Persson S. G. Sousa T. G. Wilson D. Ehrenreich V. Singh N. Iro Y. Alibert R. Alonso T. B\'arczy D. Barrado Navascues W. Baumjohann M. Bergomi N. Billot L. Borsato C. Broeg M.-D. Busch A. Collier Cameron A. C. M. Correia Sz. Csizmadia M. B. Davies M. Deleuil L. Delrez O. D. S. Demangeon A. Derekas B. Edwards A. Erikson A. Fortier L. Fossati M. Fridlund D. Gandolfi K. Gazeas M. Gillon M. G\"udel J. Hasiba Ch. Helling K. G. Isaak L. L. Kiss J. Korth K. W. F. Lam J. Laskar A. Lecavelier des \'Etangs D. Magrin P. F. L. Maxted B. Mer\'in C. Mordasini V. Nascimbeni G. Olofsson R. Ottensamer I. Pagano E. Pall\'e G. Peter D. Piazza G. Piotto D. Pollacco D. Queloz R. Ragazzoni N. Rando F. Ratti H. Rauer I. Ribas N. C. Santos G. Scandariato D. S\'egransan A. E. Simon A. M. S. Smith M. Stalport S. Sulis Gy. M. Szab\'o S. Udry V. Van Grootel J. Venturini E. Villaver N. A. Walton K. Westerdorff
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keywords cheopsdaysideemissionplanetaryatmosphereconstraindatajwst
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We performed a joint analysis of phase-curve observations of the ultra-hot Jupiter WASP-18 b from the visible to the mid-infrared, using data from CHEOPS, TESS and Spitzer. We aim to characterise the planetary atmosphere with a consistent view over the large wavelength range covered using GCMs and retrieval analyses, and including JWST data. We obtained new ephemerides with unprecedented precisions of 1 second and 1.4 millisecond on the time of inferior conjunction and orbital period, respectively. We computed a planetary radius of $R_p = 1.1926 \pm 0.0077 R_J$ with a precision of 0.65% (or 550 km). Based on a timing inconsistency with JWST, we discuss and confirm orbital eccentricity ($e = 0.00852 \pm 0.00091$). We also constrain the argument of periastron to $\omega = 261.9^{+1.3}_{-1.4}$ deg. We show that the large dayside emission implies the presence of magnetic drag and super-solar metallicity. We find a steep thermally inverted gradient in the planetary atmosphere, which is common for UHJs. We detected the presence of strong CO emission lines at 4.5 $\mu$m from an excess of dayside brightness in the Spitzer/IRAC/Ch2 passband. Using these models to constrain the reflected contribution in the CHEOPS passband, we derived an extremely low geometric albedo of $A_g^\text{CHEOPS} = 0.027 \pm 0.011$.

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  1. Orbit Refinement of WASP-18 b and Evidence Against the Existence of WASP-18 c

    astro-ph.EP 2026-06 unverdicted novelty 3.0

    Refined ephemeris for WASP-18 b with no support found for proposed companion WASP-18 c from combined transit timing and RV analysis.