The Sonora Substellar Atmosphere Models. IV. Elf Owl: Atmospheric Mixing and Chemical Disequilibrium with Varying Metallicity and C/O Ratios

Disequilibrium chemistry due to vertical mixing in the atmospheres of many brown dwarfs and giant exoplanets is well established. Atmosphere models for these objects typically parameterize mixing with the highly uncertain K_zz diffusion parameter. The role of mixing in altering the abundances of C-N-O-bearing molecules has mostly been explored for atmospheres with a solar composition. However, atmospheric metallicity and the C/O ratio also impact atmospheric chemistry. Therefore, we present the Sonora Elf Owl grid of self-consistent cloud-free 1D radiative-convective equilibrium model atmospheres for JWST observations, which includes a variation in K_zz across several orders of magnitude and also encompasses subsolar to supersolar metallicities and C/O ratios. We find that the impact of K_zz on the T(P) profile and spectra is a strong function of both T_eff and metallicity. For metal-poor objects, K_zz has large impacts on the atmosphere at significantly higher T_eff than in metal-rich atmospheres, where the impact of K_zz is seen to occur at lower T_eff. We identify significant spectral degeneracies between varying K_zz and metallicity in multiple wavelength windows, in particular, at 3-5 μm. We use the Sonora Elf Owl atmospheric grid to fit the observed spectra of a sample of nine early to late T-type objects from T_eff = 550-1150 K. We find evidence for very inefficient vertical mixing in these objects, with inferred K_zz values lying in the range between ∼10¹ and 10⁴ cm² s⁻¹. Using self-consistent models, we find that this slow vertical mixing is due to the observations, which probe mixing in the deep detached radiative zone in these atmospheres.