JWST-TST DREAMS: Sulfur dioxide in the atmosphere of the Neptune-mass planet HAT-P-26 b from NIRSpec G395H transmission spectroscopy

We present the James Webb Space Telescope (JWST) transmission spectrum of the exoplanet HAT-P-26 b (18.6 M_⊕, 6.33 R_⊕), based on a single transit observed with the JWST NIRSpec G395H grating. We detect water vapor (ln𝐵 = 4.1), carbon dioxide (ln𝐵 = 85.6), and sulfur dioxide (ln𝐵 = 13.5) with high confidence, along with marginal indications for hydrogen sulfide and carbon monoxide (ln𝐵 < 0.5). The detection of SO₂ in a warm super-Neptune-sized exoplanet (R_P ∼ 6 R_⊕) bridges the gap between previous detections in hot Jupiters and sub-Neptunes, highlighting the role of disequilibrium photochemistry across a broad range of exoplanet atmospheres, including those cooler than 1000 K. Our precise measurements of carbon, oxygen, and sulfur indicate an atmospheric metallicity of ∼10× solar and a subsolar C/O ratio. Retrieved molecular abundances are consistent within 2σ with predictions from self-consistent models including photochemistry. The elevated CO₂ abundance and possible H₂S signal may also reflect sensitivities to the thermal structure, cloud properties, or additional disequilibrium processes such as vertical mixing. We compare the SO₂ abundance in HAT-P-26 b with that of 10 other JWST-observed giant exoplanets, and find a correlation with atmospheric metallicity. The trend is consistent with the prediction from I. J. M. Crossfield, showing a steep rise in SO₂ abundance at low metallicities, and a more gradual increase beyond 30× solar. This work is part of a series of studies by our JWST Telescope Scientist Team (JWST-TST), in which we use Guaranteed Time Observations to perform Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (DREAMS).