KOALA, a new ATLAS9 database I. Model atmospheres, opacities, fluxes, bolometric corrections, magnitudes, and colours

We present the KOALA database, a new set of local thermodynamic equilibrium line-blanketed model atmospheres calculated with the code ATLAS9, together with the corresponding opacity distribution functions and emergent fluxes. The latter were also used to calculate G-band bolometric corrections and theoretical magnitudes and colours for several photometric systems, i.e. UBVRI, 2MASS, Hypparcos-Tycho, SDSS, Galex, Euclid, and Gaia DR3. With respect to previous grids of ATLAS9 model atmospheres, we adopted the solar mixture by Caffau/Lodders, and we extend the sampling in metallicity (from −5.0 to −2.5 dex, with a step of 0.5 dex, and from −2.5 dex to +0.5 dex, with a step of 0.25 dex) and in [α/Fe] (from −0.4 to +0.4 dex, with a step of 0.2 dex). We also provide a finer sampling in Teff for Teff lower than 7000 K. This finer grid allows for more accurate interpolation of colours, and in many cases it makes computing a new model atmosphere unnecessary since the atmosphere of the grid can be used directly. We computed a total of 51 663 model atmospheres and emergent fluxes. Finally, we discuss the impact of [M/H] and [α/Fe] on the thermal and pressure structures of the model atmospheres and on theoretical colours.