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

TL;DR

This paper demonstrates how mid-infrared spectroscopy with JWST can reveal surface gravity and mass of cold brown dwarfs and exoplanets, enhancing understanding of their atmospheres and physical properties.

Contribution

It introduces the use of mid-infrared spectral slopes as indicators of gravity and mass, validated by JWST data and advanced atmospheric models.

Findings

Mid-infrared flux ratios are sensitive to gravity and less affected by metallicity.

Absolute 4.5 um flux constrains temperature of cold brown dwarfs.

Identification of low-gravity, young, few-Jupiter-mass objects among T dwarfs.

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 which include disequilibrium chemistry and have a non-adiabatic pressure-temperature relationship. Preliminary fits to JWST spectroscopy of Y dwarfs show that the slope of the energy distribution from lambda = 4.5 um to lambda = 10 um is very sensitive to gravity. We explore this phenomenon using PH3-free ATMO 2020++ models and updated WISE W2 - W3 colors. We find that an absolute 4.5 um flux measurement constrains temperature, and the ratio of the 4.5 um flux to the 10 - 15 um flux is sensitive to gravity and less sensitive to metallicity. We identify 10 T dwarfs with red W2 - W3 colors which 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 to 18 um 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.