2025MNRAS.541.3561P

Ultra-hot Jupiters (dayside temperatures T$_{\mathrm{day}}$ > 2200 K) are a class of gas-giant exoplanets that, due to extreme stellar irradiation, show a peculiar combination of thermochemical properties in the form of molecular dissociation, atomic ionization, and inverted thermal structures. Atmospheric characterization of gas giants lying in the transitional regime between hot and ultra-hot Jupiters can help in understanding the physical mechanisms that cause the fundamental thermochemical transition in atmospheres between the two classes of hot gas giants. Using high-resolution cross-correlation spectroscopy with the Immersion GRating INfrared Spectrometer (IGRINS) spectrograph on Gemini South (1.4-2.5 $\mu$m), we present the dayside high-resolution spectrum of WASP-122b (T$_{\mathrm{day}}$ = 2258 $\pm$ 54 K), a gas-giant situated at this transition. We detect the signal from $\mathrm{H_2O}$, based on which we find that WASP-122b has a significantly metal-depleted atmosphere with metallicity log$_{10}$[Z$_{\mathrm{P}}$/Z$_{\odot }$] = $-1.48\ \pm$ 0.25 dex (0.033$_{-0.016}^{+0.018}$ $\times$ solar), and solar/sub-solar C/O ratio = 0.36 $\pm$ 0.22 (3$\sigma$ upper limit 0.82). Drastically low atmospheric metallicity pushes the contribution function to higher pressures, resulting in the planetary spectral lines to originate from a narrow region around 1 bar where the thermal profile is non-inverted. This is inconsistent with solar composition radiative convective equilibrium which predicts an inverted atmosphere with spectral lines in emission. The measured sub-solar metallicity and solar/sub-solar C/O ratio is inconsistent with expectations from core-accretion. We find the planetary signal to be significantly shifted in K$_{\mathrm{P}}$ and V$_{\mathrm{sys}}$, which is in tension with the predictions from global circulation models and require further investigation. Our results highlight the detailed information content of high-resolution spectroscopy data and their ability to constrain complex atmospheric thermal structures and compositions of exoplanets.