Empirically Determining Substellar Cloud Compositions in the era of JWST

TL;DR

This paper models and predicts the spectral signatures of clouds in brown dwarf atmospheres, demonstrating that JWST can empirically constrain cloud compositions, especially silicates, through mid-infrared spectroscopy.

Contribution

The study introduces new models for cloud absorption features in brown dwarfs and shows how JWST can detect and analyze these features to empirically determine cloud compositions.

Findings

Small particles ($. 1  m) produce strong absorption features.

Models with small particles in upper atmospheres better match observed spectra.

JWST can detect silicate features in brown dwarf spectra.

Abstract

Most brown dwarfs have atmospheres with temperatures cold enough to form clouds. A variety of materials likely condense, including refractory metal oxides and silicates; the precise compositions and crystal structures of predicted cloud particles depend on the modeling framework used and have not yet been empirically constrained. Spitzer has shown tentative evidence of the silicate feature in L dwarf spectra and JWST can measure these features in many L dwarfs. Here, we present new models to predict the signatures of the strongest cloud absorption features. We investigate different cloud mineral species and determine how particle size, mineralogy, and crystalline structure change spectral features. We find that silicate and refractory clouds have a strong cloud absorption feature for small particle sizes ($\leq$ 1 $\mu$m). Model spectra are compared to five brown dwarfs that show evidence of the silicate feature; models that include small particles in the upper layers of the atmosphere produce a broad cloud mineral feature, and that better match the observed spectra than the Ackerman & Marley (2001) cloud model. We simulate observations with the MIRI instrument on JWST for a range of nearby, cloudy brown dwarfs, demonstrating that these features could be readily detectable if small particles are present. Furthermore, for photometrically variable brown dwarfs, our predictions suggest that with JWST, by measuring spectroscopic variability inside and outside a mineral feature, we can establish silicate (or other) clouds as the cause of variability. Mid-infrared spectroscopy is a promising tool to empirically constrain the complex cloud condensation sequence in brown dwarf atmospheres.