A Tale of Two Molecules: The Underprediction of CO2 and Overprediction of PH3 in Late T and Y Dwarf Atmospheric Models

The sensitivity and spectral coverage of JWST are enabling us to test our assumptions of ultracool dwarf atmospheric chemistry, especially with regards to the abundances of phosphine (PH₃) and carbon dioxide (CO₂). In this paper, we use Near Infrared Spectrograph PRISM spectra (∼0.8‑5.5 μm, R ∼ 100) of four late T and Y dwarfs to show that standard substellar atmosphere models have difficulty replicating the 4.1‑4.4 μm wavelength range, as they predict an overabundance of phosphine and an underabundance of carbon dioxide. To help quantify this discrepancy, we generate a grid of models using PICASO, based on the Elf Owl chemical and temperature profiles, where we include the abundances of these two molecules as parameters. The fits to these PICASO models show a consistent preference for orders-of-magnitude higher CO₂ abundances and a reduction in PH₃ abundance as compared to the nominal models. This tendency means that the claimed phosphine detection in UNCOVER‑BD‑3 could instead be explained by a CO₂ abundance in excess of standard atmospheric model predictions; however, the signal-to-noise ratio of the spectrum is not high enough to discriminate between these cases. We discuss atmospheric mechanisms that could explain the observed underabundance of PH₃ and overabundance of CO₂, including a vertical eddy diffusion coefficient (K _zz) that varies with altitude, incorrect chemical pathways, or elements condensing out in forms such as NH₄H₂PO₄. However, our favored explanation for the required CO₂ enhancement is that the quench approximation does not accurately predict the CO₂ abundance, as CO₂ remains in chemical equilibrium with CO after CO quenches.