Origin of the enhanced atmospheric CO2 abundance

Determine the physical origin of the enhanced atmospheric CO2 abundance inferred for ZTFJ0038+2030B, which cannot be explained by near-solar bulk composition or dynamical transport and quenching alone.

Background

The synthetic spectra produced with the dynamically coupled cloud–chemistry simulations underpredict the observed CO2 absorption feature near 4.2 micrometres. The free-chemistry retrieval requires a CO2 abundance approximately three orders of magnitude higher than the disequilibrium-chemistry retrieval, while the modeled transport and quenching processes do not generate enough CO2.

The authors therefore leave unresolved which additional chemical pathways or missing physical processes could produce the enhanced CO2 abundance. Resolving this issue is necessary to explain both the observed CO2 spectral feature and the associated phase-curve behavior of ZTFJ0038+2030B.

References

The origin of this enhanced CO$_2$ abundance remains unclear. The near-solar metallicity and C/O ratio inferred by \citet{broskilaing2026} suggest that an unusual bulk composition is unlikely to be the primary explanation. Our results further indicate that dynamical transport and quenching alone do not produce sufficient CO$_2$ to match the observed feature, pointing to additional chemical pathways or other missing physical processes that are not captured in the present model.

— Dynamically Coupled Three-dimensional Cloud-Chemistry in Rapidly Rotating Irradiated Atmospheres: an Application to the Compact White Dwarf-Brown Dwarf system ZTFJ0038+2030  (2610.17714 - Lai et al., 13 Oct 2026) in Section 4, subsection “Emission spectra”