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 _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 μ m. 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 ∼10 ^1 and 10 ^4 cm ^2 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.

Keywords

• Brown dwarfs
• T dwarfs
• Y dwarfs
• Atmospheric composition
• Extrasolar gaseous giant planets