Near-Infrared Transmission Spectroscopy of HAT-P-18$\,$b with NIRISS: Disentangling Planetary and Stellar Features in the Era of JWST

Marylou Fournier-Tondreau; Ryan J. MacDonald; Michael Radica; David Lafrenière; Luis Welbanks; Caroline Piaulet; Louis-Philippe Coulombe; Romain Allart; Kim Morel; Étienne Artigau; Loïc Albert; Olivia Lim; René Doyon; Björn Benneke; Jason F. Rowe; Antoine Darveau-Bernier; Nicolas B. Cowan; Nikole K. Lewis; Neil James Cook; Laura Flagg; Frédéric Genest; Stefan Pelletier; Doug Johnstone; Lisa Dang; Lisa Kaltenegger; Jake Taylor; Jake D. Turner

doi:10.1093/mnras/stad3813

paper

Near-Infrared Transmission Spectroscopy of HAT-P-18b with NIRISS: Disentangling Planetary and Stellar Features in the Era of JWST

arXiv:2310.14950 · doi:10.1093/mnras/stad3813

Abstract

The JWST Early Release Observations (ERO) included a NIRISS/SOSS (0.6-2.8 $μ$ m) transit of the $\sim$ 850K Saturn-mass exoplanet HAT-P-18b. Initial analysis of these data reported detections of water, escaping helium, and haze. However, active K dwarfs like HAT-P-18 possess surface heterogeneities $-$ starspots and faculae $-$ that can complicate the interpretation of transmission spectra, and indeed, a spot-crossing event is present in HAT-P-18b's NIRISS/SOSS light curves. Here, we present an extensive reanalysis and interpretation of the JWST ERO transmission spectrum of HAT-P-18b, as well as HST/WFC3 and $Spitzer$ /IRAC transit observations. We detect H $_{2}$ O (12.5 $σ$ ), CO $_{2}$ (7.3 $σ$ ), a cloud deck (7.4 $σ$ ), and unocculted starspots (5.8 $σ$ ), alongside hints of Na (2.7 $σ$ ). We do not detect the previously reported CH $_{4}$ ( $lo g$ CH $_{4}$ $<$ -6 to 2 $σ$ ). We obtain excellent agreement between three independent retrieval codes, which find a sub-solar H $_{2}$ O abundance ( $lo g$ H $_{2}$ O $\approx - 4.4 \pm 0.3$ ). However, the inferred CO $_{2}$ abundance ( $lo g$ CO $_{2}$ $\approx - 4.8 \pm 0.4$ ) is significantly super-solar and requires further investigation into its origin. We also introduce new stellar heterogeneity considerations by fitting for the active regions' surface gravities $-$ a proxy for the effects of magnetic pressure. Finally, we compare our JWST inferences to those from HST/WFC3 and $Spitzer$ /IRAC. Our results highlight the exceptional promise of simultaneous planetary atmosphere and stellar heterogeneity constraints in the era of JWST and demonstrate that JWST transmission spectra may warrant more complex treatments of the transit light source effect.

Near-Infrared Transmission Spectroscopy of HAT-P-18b with NIRISS: Disentangling Planetary and Stellar Features in the Era of JWST

Abstract

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