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 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.

Discovery of a Hypervelocity L Subdwarf at the Star/Brown Dwarf Mass Limit

“…To further evaluate its physical properties, we compared the smoothed spectrum of J1249+3621 to the Sonora Elf Owl (Mukherjee et al 2024) and Spectral ANalog of Dwarfs (SAND; Alvarado et al 2024) atmosphere models. These models encompass the temperatures (1500 K  T eff  2400 K) and subsolar metallicities ([M/H]  −0.5) of L subdwarfs and contain up-to-date opacities and treatments for condensation and disequilibrium chemistry.…”

Multiple Clues for Dayside Aerosols and Temperature Gradients in WASP-69 b from a Panchromatic JWST Emission Spectrum

“…Bottom: the brightness temperature spectrum shows absorption signatures of H 2 O, CO 2 , and CO. The relative pressure level where each molecule has an optical depth of 1 is shown for each model in the style ofMukherjee et al (2024). We plot two models, described in Section 5, which can alternatively fit the spectrum of WASP-69 b with either significant reflection (scattering model) or a highaltitude cloud (cloud-layer model).…”