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

Recent James Webb Space Telescope NIRSpec observations have revealed strong methane emission at 3.326 μ m 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 μ m 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 μ m and tentative ammonia emission features in the mid-infrared. Wavelengths beyond ∼2 μ m are significantly influenced by the inversion, except for the 4.1–5.0 μ m CO 2 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 Ch1 − m Ch2 (IRAC 3.6–4.5 μ m) 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 CO 2 and CO features that do not correlate with temperature or spectral type. We find that the thermal inversion tends to produce bluer m Ch1 − m 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.