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

• Published in: The Astrophysical journal Vol. 973; no. 1; pp. 60 - 69
• Main Authors: Beiler, Samuel A., Mukherjee, Sagnick, Cushing, Michael C., Kirkpatrick, J. Davy, Schneider, Adam C., Kothari, Harshil, Marley, Mark S., Visscher, Channon
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.09.2024; IOP Publishing
• Subjects: Abundance; Ammonium dihydrogen phosphate; Atmospheric chemistry; Atmospheric models; Brown dwarfs; Carbon dioxide; Chemical abundances; Diffusion coefficient; Eddy diffusion; Infrared spectra; James Webb Space Telescope; Near infrared astronomy; Near infrared radiation; Noise prediction; Phosphines; Signal to noise ratio; Spectral sensitivity; Spectroscopy; Temperature profiles; Vertical eddy diffusion; Y dwarfs
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ad6759

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 (Kzz) 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.