Atmospheric characterization of HIP 67522 b with VLT/CRIRES+. VLT/CRIRES+ suggests a heavier planet and hints at deuterium fractionation

Abstract

Young transiting exoplanets provide unique opportunities to probe planetary atmospheres during the critical early phases of evolution. HIP 67522 b, a 17 Myr old hot Jupiter with an extraordinarily low bulk density, represents an ideal target for high-resolution transmission spectroscopy. We aim to characterize the atmospheric composition, thermal structure, and dynamics of HIP 67522 b using ground-based high-resolution near-infrared spectroscopy. We obtained high-resolution spectra with VLT/CRIRES+ in the K2166 band during a transit on 30 January 2025. We applied cross-correlation techniques and Bayesian nested sampling retrievals to constrain molecular abundances, temperature structure, and atmospheric dynamics. We detect H${}_2$O at 20$\sigma$ and CO at 5$\sigma$, confirming the extremely extended atmosphere of this low-mass giant. A velocity offset of $-2.9\pm 0.2$ km s${}^{-1}$ indicates day-to-night winds. The rotation velocity is constrained to <1.8 km s${}^{-1}$ at 3$\sigma$, consistent with tidal locking. Retrieval analysis suggests a planetary mass of 29.8 $\pm$ 3 Earth masses and a vertically isothermal atmosphere. This mass is two times larger than the mass estimated from JWST atmospheric observations and inconsistent at 3$\sigma$ hence leaving a doubt on the actual planetary density of the planet. Using the mass derived from the CRIRES+ data, we derive a C/O ratio of $0.83\pm 0.09$, about 1.5 times solar, and a subsolar metallicity [C+O/H] $=-0.8\pm 0.4$ which can be increased if the atmosphere is cloudy, a degeneracy our data alone cannot resolve. We report a tentative 2$\sigma$ detection of HDO with an extreme enrichment factor of $\sim$1000 relative to the protosolar D/H ratio. If confirmed, this would be the first detection of deuterium in an exoplanet atmosphere and would require intense escape rate to be explained.

Accepted for publication in A&A

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