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 manybrown dwarfs and giant exoplanets is well-established. Atmosphere models forthese objects typically parameterize mixing with the highly uncertain K_ diffusion parameter. The role of mixing in altering the abundances ofC-N-O-bearing molecules has mostly been explored for solar compositionatmospheres. However, atmospheric metallicity and the C/O ratio also impactatmospheric chemistry. Therefore, we present the gridof self-consistent cloud-free 1D radiative-convective equilibrium modelatmospheres for JWST observations, which includes a variation of K_ zzacross several orders of magnitude and also encompasses sub-solar tosuper-solar metallicities and C/O ratios. We find that the impact of K_ on the T(P) profile and spectra is a strong function of both T_ and metallicity. For metal-poor objects K_ zz has large impacts onthe atmosphere at significantly higher T_ eff compared to metal-richatmospheres where the impact of K_ zz is seen to occur at lower T_. We identify significant spectral degeneracies between varying K_ and metallicity in multiple wavelength windows, in particular at 3-5μm. We use the atmospheric grid to fit the observedspectra of a sample of 9 early to late T- type objects from T_=550-1150 K. We find evidence for very inefficient vertical mixing inthese objects with inferred K_ zz values lying in the range between∼ 10^1-10^4 cm^2s^-1. Using self-consistent models, we find thatthis slow vertical mixing is due to the observations probing mixing in the deepdetached radiative zone in these atmospheres.

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