James Webb Space Telescope Feed Post

Harvard ADS: A Tale of Two Molecules: The Underprediction of CO_2 and Overprediction of PH_3 in Late T and Y Dwarf Atmospheric Models

Paper abstract: The sensitivity and spectral coverage of JWST is enabling us to test our assumptions of ultracool dwarf atmospheric chemistry, especially with regards to the abundances of phosphine (PH_3) and carbon dioxide (CO_2). In this paper, we use NIRSpec PRISM spectra (~0.8-5.5 \mum, R~100) of four late T and Y dwarfs to show that standard substellar atmosphere models have difficulty replicating the 4.1-4.4 \mum 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 CO_2 abundances and a reduction in PH_3 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 CO_2 abundance in excess of standard atmospheric model predictions; however the signal-to-noise of the spectrum is not high enough to discriminate between these cases. We discuss atmospheric mechanisms that could explain the observed underabundance of PH_3 and overabundance of CO_2, including a vertical eddy diffusion coefficient (K_{\mathrm{zz}}) that varies with altitude, incorrect chemical pathways, or elements condensing out in forms such as NH_4H_2PO_4. However, our favored explanation for the required CO_2 enhancement is that the quench approximation does not accurately predict the CO_2 abundance, as CO_2 remains in chemical equilibrium with CO after CO quenches.