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Atomic diffusion and mixing processes in stellar interiors influence thestructure and the surface composition of stars. Some of these processes cannotyet be modelled from the first principles. This limits their applicability instellar models used for studies of stellar populations and Galactic evolution.Our main goal is to put constrains on the stellar structure and evolutionmodels using new refined measurements of chemical composition in stars ofGalactic open cluster. We use medium-resolution, 19 200 <= R <= 21 500, opticalspectra of the stars in the open cluster NGC 2420 obtained within the Gaia-ESOsurvey. The sample covers all evolutionary stages from the main-sequence to redgiant branch. Stellar parameters are derived using a combined Bayesian analysisof spectra, 2MASS photometry, and astrometric data from Gaia DR2. Theabundances of Mg, Ca, Fe, and Li are determined from non-local thermodynamicequilibrium (NLTE) synthetic spectra, computed using one-dimensional (1D) andaveraged three-dimensional (3D) model atmospheres. We compare our results witha grid of Code d'Evolution Stellaire Adaptatif et Modulaire (CESTAM) stellarevolution models, which include atomic diffusion, turbulent and rotationalmixing. We find prominent evolutionary trends in the abundances of Fe, Ca, Mg,and Li with the mass of the stars in the cluster. Fe, Mg, and Ca show adepletion at the cluster turn-off, but the abundances gradually increase andflatten near the base of the RGB. The abundance trend for Li displays asignature of rotational mixing on the main-sequence and abrupt depletion on thesubgiant branch, which is caused by advection of Li-poor material to thesurface. The analysis of abundances combined with the CESTAM model predictionsallows us to place limits on the parameter space of the models and to constrainthe zone in the stellar interior where turbulent mixing takes place.

astronomy and astrophysics

The Gaia-ESO survey: 3D NLTE abundances in the open cluster NGC 2420  suggest atomic diffusion and turbulent mixing at the origin of chemical  abundance variations