Diversity of Cold Worlds: Predicted Near- to Mid-infrared Spectral Signatures of a Cold Brown Dwarf with Potential Auroral Heating

2025/09/30 by Suárez, Genaro, Faherty, Jacqueline K., Burningham, Ben +12 · 3 citations
#Earth and Planetary Astrophysics (astro-ph.EP) #FOS: Physical sciences #Solar and Stellar Astrophysics (astro-ph.SR)

paper · doi:10.48550/arxiv.2509.26505

Abstract

Recent JWST/NIRSpec observations have revealed strong methane emission at 3.326 microns in the ≈482 K brown dwarf CWISEP J193518.59-154620.3 (W1935). Atmospheric modeling suggests the presence of a ≈300 K thermal inversion in its upper atmosphere, potentially driven by auroral activity. We present an extension of the retrieved spectra of W1935 with and without inversion spanning 1--20 microns, to identify thermal inversion-sensitive spectral features and explore the origin of the object's peculiar characteristics. Our analysis indicates that atmospheric heating contributes approximately 15% to the bolometric luminosity. The model with inversion predicts an additional similar-strength methane emission feature at 7.7 microns and tentative ammonia emission features in the mid-infrared. Wavelengths beyond ∼2 microns are significantly influenced by the inversion, except for the 4.1--5.0 microns CO₂ and CO features that originate from atmospheric layers deeper than the region where the inversion occurs. W1935 appears as an outlier in Spitzer/IRAC mid-infrared color-magnitude diagrams (CMDs) based on the m_{\rm Ch1}-m_{\rm Ch2} (IRAC 3.6 microns - 4.5 microns) color, but exhibits average behavior in all other combinations that trace clear sequences. This anomaly is likely due to the Ch2 filter probing vertical mixing-sensitive CO₂ and CO features that do not correlate with temperature or spectral type. We find that the thermal inversion tends to produce bluer m_{\rm Ch1}-m_{\rm Ch2} colors, so the overluminous and/or redder position of W1935 in diagrams involving this color cannot be explained by the thermal inversion. This analysis provides insights into the intriguing dispersion of cold brown dwarfs in mid-infrared CMDs and sheds light on their spectral diversity.

Citations

• Discovery of the Second Y+Y Dwarf Binary System: CWISEP J193518.59-154620.3
• Diversity of Cold Worlds: A Near Complete Spectral Energy Distribution for 2MASS J04151954-0935066 using JWST
• Main-oval auroral emission from a T6 brown dwarf: observations, modeling, and astrometry
• The Sonora Substellar Atmosphere Models. V. A Correction to the Disequilibrium Abundance of CO₂ for Sonora Elf Owl
• The Diversity of Cold Worlds: a blended-light binary straddling the T/Y transition in brown dwarfs
• Precise Bolometric Luminosities and Effective Temperatures of 23 late-T and Y dwarfs Obtained with JWST
• JWST/NIRSpec Observations of the Coldest Known Brown Dwarf
• Measuring and Replicating the 1-20 um Energy Distributions of the Coldest Brown Dwarfs: Rotating, Turbulent and Non-Adiabatic Atmospheres
• dynesty: a dynamic nested sampling package for estimating Bayesian posteriors and evidences

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• The Diversity of Cold Worlds: Age and Characterization of the Exoplanet COCONUTS-2b
• Mapping the Cloud-Driven Atmospheric Dynamics & Chemistry of an Isolated Exoplanet Analog with Harmonic Signatures

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