Studying Ultracool Dwarfs with Updated ATMO Atmospheric Models

A strong synergy between observations and atmospheric models is crucial for examining substellar data and enhancing the models. Here, we present an updated grid of ATMO2020 atmospheric models for ultracool dwarf objects. The models compute temperature–pressure profiles and emergent spectra for atmospheres in both radiative–convective equilibrium and non-equilibrium chemistry.The model grid includes effective temperatures ranging from 200 to 3000 K and surface gravities from log(g) = 2.5 to 5.5. We also extend the metallicity range to −1.5 ≤ [M/H] ≤ +0.5 and add extra opacities, such as MgO and SiO, which enhance the models, especially at sub-solar metallicities.We compare these models to a sample of 178 spectroscopically confirmed ultracool dwarfs (M7–T7) observed by Euclid (Domínguez-Tagle et al. 2025) and minimize chi-square to determine the best model fit. The comparison between the models and Euclid data shows robust results in obtaining effective temperature, surface gravity, and atmospheric composition, especially for late-T-type objects. We show that the metallicity and gravity of the sample are spread across all values, with most objects near solar metallicity and log(g) = 4. The wavelength coverage and updated metallicity range and opacities make the models suitable for large survey datasets such as Euclid and JWST.