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

Abstract

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 (PH _3 ) and carbon dioxide (CO _2 ). In this paper, we use Near Infrared Spectrograph PRISM spectra (∼0.8−5.5 μ m, R ∼ 100) of four late T and Y dwarfs to show that standard substellar atmosphere models have difficulty replicating the 4.1−4.4 μ m 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 ratio 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 _zz ) that varies with altitude, incorrect chemical pathways, or elements condensing out in forms such as NH _4 H _2 PO _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.

Keywords

• Brown dwarfs
• Carbon dioxide
• Chemical abundances
• Near infrared astronomy
• Spectroscopy
• Y dwarfs