Probing Atmospheric Mixing with Disequilibrium Chemistry in Brown Dwarfs and Warm Exoplanets

While many aspects of atmospheric physics have seen significant improvements over the past decade, including molecular opacities and rapid radiative transfer, our understanding of mixing processes is still in its infancy. The parameter used to estimate the speed of diffusive vertical mixing, K_zz, is so poorly known that it is typically varied across a factor of a million in atmosphere models. However, we are at a turning point where model/data comparisons of spectra can finally constrain this important unknown parameter, which dictates the role of non-equilibrium chemistry, cloud particle sizes, and cloud vertical extent in substellar atmospheres. We have developed a python-based one-dimensional radiative-convective equilibrium (RCE) model which can be used both for irradiated extrasolar planets and brown dwarfs. We have included the capability of treating disequilibrium chemistry due to vertical mixing in the atmosphere self-consistently by coupling our RCE model with the open-source chemical network model VULCAN. First, we apply this model to several T-dwarfs and Y-dwarf spectra observed in Miles et al. (2020) and find that the presence of a detached convective zone between effective temperatures of 500-800 K and subsequent quenching of CO and NH3 in the intermediate radiative zones enables us to probe the magnitude of K_zz in radiative zones of brown dwarfs. We compare our model spectra with broadband photometry and available M-band spectroscopy of several T and Y-dwarfs and determine their atmospheric vertical mixing profiles. For the first time, this allows us to constrain this fundamental atmospheric parameter, K_zz, in the radiative zone at tens of bars, with significant implications for chemistry and cloud modeling. Finally, we show how JWST spectra will allow for even more significant breakthroughs in constraining vertical mixing of warm transiting giant planets and brown dwarfs, through emission and transmission spectroscopy.