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We study the emission of dust grains within the Orion Bar - a well-known,highly far-UV (FUV)-irradiated PDR. The Orion Bar because of its edge-ongeometry provides an exceptional benchmark for characterizing dust evolutionand the associated driving processes under varying physical conditions. Ourgoal is to constrain the local properties of dust by comparing its emission tomodels. Taking advantage of the recent JWST PDRs4All data, we follow the dustemission as traced by JWST NIRCam (at 3.35 and 4.8 micron) and MIRI (at 7.7,11.3, 15.0, and 25.5 micron), along with NIRSpec and MRS spectroscopicobservations. First, we constrain the minimum size and hydrogen content ofcarbon nano-grains from a comparison between the observed dust emission spectraand the predictions of the THEMIS dust model coupled to the numerical codeDustEM. Using this dust model, we then perform 3D radiative transfersimulations of dust emission with the SOC code and compare to data obtainedalong well chosen profiles across the Orion Bar. The JWST data allows us, forthe first time, to spatially resolve the steep variation of dust emission atthe illuminated edge of the Orion Bar PDR. By considering a dust model withcarbonaceous nano-grains and submicronic coated silicate grains, we deriveunprecedented constraints on the properties of across the Orion Bar. To explainthe observed emission profiles with our simulations, we find that thenano-grains must be strongly depleted with an abundance (relative to the gas)15 times less than in the diffuse ISM. The NIRSpec and MRS spectroscopicobservations reveal variations in the hydrogenation of the carbon nano-grains.The lowest hydrogenation levels are found in the vicinity of the illuminatingstars suggesting photo-processing while more hydrogenated nano-grains are foundin the cold and dense molecular region, potentially indicative of largergrains.

PDRs4All. V. Modelling the dust evolution across the illuminated edge of the Orion Bar