Empirically Determining Substellar Cloud Compositions in the era of JWST

Brown dwarfs have cool atmospheres with the temperatures of giant planets. Their atmospheres are cool enough to form clouds and their temperature determines which species condense. Thick layers of dust, likely made of silicates, blanket L dwarf atmospheres, limiting the depths probed by spectra; these clouds clear dramatically at the L/T transition. Cloud chemistry and microphysics is challenging to model from first principles; clouds clearly form, but the specific species that condense are not well-constrained from theory. This uncertainty is a major barrier to understanding exoplanet atmospheres. The next key step is to empirically determine which clouds form using mid-infrared spectroscopy to identify mineral species. There is tentative evidence from Spitzer that silicate features are present in L dwarf spectra. JWST could allow us to measure these features in many L dwarfs. Before these observations are made, we need to understand in detail at what wavelengths the strongest cloud absorption features will be, and predict which objects will have the largest amplitude signals. We explore the impact of individual cloud species, including how particle sizes and cloud mineralogy change spectral features. We investigate which objects are most ideal to observe, exploring a range of temperatures and surface gravities. We find that silicate and corundum clouds have a strong cloud absorption feature for small particle sizes (<1 um). Silicate clouds strongly absorb at 10 um while corundum absorbs at 11.5 um. We simulate time-series observations with the MIRI instrument on JWST for a range of nearby, cloudy, and photometrically variable brown dwarfs. Our predictions suggest that with JWST, by measuring spectroscopic variability inside and outside a mineral feature, we can uniquely identify a range of cloud species. Mid-infrared time-series spectroscopy can be used to empirically constrain the complex cloud condensation sequence in brown dwarf atmospheres.