Understanding the Energy Input Required for Methane Emission on CWISEP J193518.59-154620.3: A Comprehensive Analysis

Abstract

The Y dwarf WISE 1935 exhibits a thermal inversion in its radiative atmosphere, producing methane emission features in its JWST spectrum, but the physical mechanism responsible for this inversion remains unknown. Using the open-source radiative--convective equilibrium code PICASO, we model atmospheric heating with Chapman energy deposition profiles to reproduce the observed thermal inversion and methane emission feature. Our models require heating rates of approximately 10^5-10^6 erg cm^-2 s^-1. We show that the atmospheric response depends primarily on the integrated heating deposited in the observable atmosphere, revealing a degeneracy between heating magnitude, vertical extent, and emitting surface fraction. Disequilibrium chemistry lowers the required energy input by lowering CH$_4$ opacity and strengthening the inversion. Comparison with recent electron-beam heating models indicates that reproducing the thermal inversion in W1935 requires substantially greater energy deposition than currently predicted for brown dwarf auroral heating, while the observed methane emission favors energy deposition near 10^-3-10^-2 bar. Our models also predict a prominent methane emission feature near 7.8 microns, along with energy-sensitive ammonia features near 6 microns, implying a bolometric luminosity greater than that yet measured. Finally, we investigate potential sources of the inferred upper-atmospheric heating. We find that Joule heating would require a strong magnetic field and large electron densities, the latter supported by external ionization from an unidentified source. We also consider cometary impacts as a possible source of atmospheric heating.