Detection of CO2, CO, and H2O in the atmosphere of the warm sub-Saturn HAT-P-12 b

Context. The chemical composition of warm gas giant exoplanet atmospheres (with T<SUB>eq</SUB> < 1000 K) is not well known due to the lack of observational constraints. Aims. HAT-P-12 b is a warm, sub-Saturn-mass transiting exoplanet that is ideal for transmission spectroscopy. We aim to characterise its atmosphere and probe the presence of carbonaceous species using near-infrared observations. Methods. One transit of HAT-P-12 b was observed in spectroscopy with JWST NIRSpec in the 2.87─5.10 µm range with a resolving power of ~1000. The JWST data are combined with archival observations from HST WFC3 covering the 1.1─1.7 µm range. The data were analysed using two data reduction pipelines and two atmospheric retrieval tools. Atmospheric simulations using chemical forward models were performed to interpret the spectra. Results. CO<SUB>2</SUB>, CO, and H<SUB>2</SUB>O are detected at 12.2, 4.1, and 6.0 σ confidence, respectively. Their volume mixing ratios are consistent with an atmosphere of ~10× solar metallicity and production of CO<SUB>2</SUB> by photochemistry. CH<SUB>4</SUB> is not detected and seems to be lacking, which could be due to a high intrinsic temperature with strong vertical mixing or other phenomena. SO<SUB>2</SUB> is also not detected and its production seems limited by low upper atmosphere temperatures (~500 K at P ≲ 10<SUP>−3</SUP> bar derived from one-dimensional retrievals), insufficient to produce it in detectable quantities (≳ 800 K required according to photochemical models). H<SUB>2</SUB>S is marginally detected using one data analysis method, but not by the other. Retrievals indicate the presence of clouds between 2 and 11 mbar using one data analysis method, and between 5 and 269 mbar using the other. The derived C/O ratio is below unity, but is not well constrained. Conclusions. This study points towards an atmosphere for HAT-P-12 b that could be enriched in carbon and oxygen with respect to its host star, a possibly cold upper atmosphere that may explain the non-detection of SO<SUB>2</SUB>, and a CH<SUB>4</SUB> depletion that is yet to be fully understood. When including the production of CO<SUB>2</SUB> via photochemistry, an atmospheric metallicity that is close to Saturn's can explain the observations. Metallicities inferred for other gas giant exoplanets based on their CO<SUB>2</SUB> mixing ratios may need to account for its photochemical production pathways. This may impact studies on mass-metallicity trends and links between exoplanet atmospheres, interiors, and formation history.