Kavli Affiliate: Sara Seager
| First 5 Authors: Amélie Gressier, Amélie Gressier, , ,
| Summary:
We present the James Webb Space Telescope (JWST) transmission spectrum of the
exoplanet HAT-P-26 b (18.6 Earth masses, 6.33 Earth radii), based on a single
transit observed with the JWST NIRSpec G395H grating. We detect water vapor (ln
B = 4.1), carbon dioxide (ln B = 85.6), and sulfur dioxide (ln B = 13.5) with
high confidence, along with marginal indications for hydrogen sulfide and
carbon monoxide (ln B < 0.5). The detection of SO2 in a warm super-Neptune
sized exoplanet (radius about 6 Earth radii) 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 about 10 times solar and a
sub-solar C/O ratio. Retrieved molecular abundances are consistent within 2
sigma 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 ten other JWST-observed giant exoplanets, and find a
correlation with atmospheric metallicity. The trend is consistent with the
prediction from Crossfield (2023), showing a steep rise in SO2 abundance at low
metallicities, and a more gradual increase beyond 30 times 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).
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