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

Abstract

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 $K_{\mathrm{zz}}$ diffusion parameter. The role of mixing in altering the abundances of C-N-O-bearing molecules has mostly been explored for solar composition atmospheres. However, atmospheric metallicity and the C/O ratio also impact atmospheric chemistry. Therefore, we present the \texttt{Sonora Elf Owl} grid of self-consistent cloud-free 1D radiative-convective equilibrium model atmospheres for JWST observations, which includes a variation of $K_{\mathrm{zz}}$ across several orders of magnitude and also encompasses sub-solar to super-solar metallicities and C/O ratios. We find that the impact of $K_{\mathrm{zz}}$ on the $T(P)$ profile and spectra is a strong function of both $T_{\mathrm{eff}}$ and metallicity. For metal-poor objects $K_{\mathrm{zz}}$ has large impacts on the atmosphere at significantly higher $T_{\mathrm{eff}}$ compared to metal-rich atmospheres where the impact of $K_{\mathrm{zz}}$ is seen to occur at lower $T_{\mathrm{eff}}$. We identify significant spectral degeneracies between varying $K_{\mathrm{zz}}$ and metallicity in multiple wavelength windows, in particular at 3-5 $\mu$m. We use the \texttt{Sonora Elf Owl} atmospheric grid to fit the observed spectra of a sample of 9 early to late T- type objects from $T_{\mathrm{eff}}=550-1150$ K. We find evidence for very inefficient vertical mixing in these objects with inferred $K_{\mathrm{zz}}$ values lying in the range between $\sim$ 10${}^1$-10${}^4$ cm${}^2$s${}^{-1}$. Using self-consistent models, we find that this slow vertical mixing is due to the observations probing mixing in the deep detached radiative zone in these atmospheres.

Accepted for Publication in The Astrophysical Journal, 16 Figures, 3 Tables, 28 Pages

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