Redshifting the Study of Cold Brown Dwarfs and Exoplanets: The Mid-infrared Wavelength Region as an Indicator of Surface Gravity and Mass

Abstract

JWST is opening many avenues for exploration. For cold brown dwarfs and exoplanets, JWST has opened the door to the mid-infrared wavelength region, where such objects emit significant energy. For the first time, astronomers have access to mid-infrared spectroscopy for objects colder than 600 K. The first spectra appear to validate the model suite known as ATMO 2020++: atmospheres that include disequilibrium chemistry and have a nonadiabatic pressure–temperature relationship. Preliminary fits to JWST spectroscopy of Y dwarfs show that the slope of the energy distribution from λ ≈ 4.5 μ m to λ ≈ 10 μ m is very sensitive to gravity. We explore this phenomenon using PH _3 -free ATMO 2020++ models and updated Wide-field Infrared Survey Explorer W2−W3 colors. We find that an absolute 4.5 μ m flux measurement constrains temperature, and the ratio of the 4.5 μ m flux to the 10–15 μ m flux is sensitive to gravity and less sensitive to metallicity. We identify 10 T dwarfs with red W2−W3 colors that are likely to be very-low-gravity, young, few-Jupiter-mass objects; one of these is the previously known COCONUTS-2b. The unusual Y dwarf WISEPA J182831.08+265037.8 is blue in W2−W3, and we find that the 4–18 μ m JWST spectrum is well reproduced if the system is a pair of high-gravity 400 K dwarfs. Recently published JWST colors and luminosity-based effective temperatures for late-T and -Y dwarfs further corroborate the ATMO 2020++ models, demonstrating the potential for significant improvement in our understanding of cold, very-low-mass bodies in the solar neighborhood.

Keywords

• Brown dwarfs
• Exoplanet astronomy
• Fundamental parameters of stars
• Infrared photometry