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We present a new set of solar metallicity atmosphere and evolutionary modelsfor very cool brown dwarfs and self-luminous giant exoplanets, which we termATMO 2020. Atmosphere models are generated with our state-of-the-art 1Dradiative-convective equilibrium code ATMO, and are used as surface boundaryconditions to calculate the interior structure and evolution of$0.001-0.075\,\mathrm{M_{\odot}}$ objects. Our models include several keyimprovements to the input physics used in previous models available in theliterature. Most notably, the use of a new H-He equation of state including abinitio quantum molecular dynamics calculations has raised the mass by$\sim1-2\%$ at the stellar-substellar boundary and has altered the coolingtracks around the hydrogen and deuterium burning minimum masses. A second keyimprovement concerns updated molecular opacities in our atmosphere model ATMO,which now contains significantly more line transitions required to accuratelycapture the opacity in these hot atmospheres. This leads to warmer atmospherictemperature structures, further changing the cooling curves and predictedemission spectra of substellar objects. We present significant improvement forthe treatment of the collisionally broadened potassium resonance doublet, andhighlight the importance of these lines in shaping the red-optical andnear-infrared spectrum of brown dwarfs. We generate three different grids ofmodel simulations, one using equilibrium chemistry and two usingnon-equilibrium chemistry due to vertical mixing, all three computedself-consistently with the pressure-temperature structure of the atmosphere. Weshow the impact of vertical mixing on emission spectra and in colour-magnitudediagrams, highlighting how the $3.5-5.5\,\mathrm{\mu m}$ flux window can beused to calibrate vertical mixing in cool T-Y spectral type objects.

A new set of atmosphere and evolution models for cool T-Y brown dwarfs and giant exoplanets