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The large disparity in physical conditions from the diffuse interstellarmedium (ISM) to denser clouds such as photon-dominated regions (PDRs) triggersan evolution of the dust properties (i.e. composition, size, and shape). Thegas physics and chemistry are tightly connected to these dust properties andare therefore affected by dust evolution and especially the nano-graindepletion in the outer irradiated part of PDRs. We highlight the influence ofnano-grain depletion on the gas physics and chemistry in the Horsehead nebula,a prototypical PDR. We used a model for atomic and molecular gas in PDRs, theMeudon PDR code, using diffuse ISM-like dust and Horsehead-like dust to studythe influence of nano-grain depletion on the gas physics and chemistry,focusing on the impact on photoelectric heating and H2 formation and,therefore, on the H2 gas lines. We find that nano-grain depletion in theHorsehead strongly affects gas heating through the photoelectric effect andthus the gas temperature and the H2 formation, hence the H -> H2 position.Consequently, the first four pure rotational lines of H2 (e.g. 0-0 S(0), S(1),S(2), and S(3)) vary by a factor of 2 to 14. The 0-0 S(3) line that is oftenunderestimated in models is underestimated even more when taking nano-graindepletion into account due to the decrease in gas heating through thephotoelectric effect. This strongly suggests that our understanding of theexcitation of H2 and/or of heating processes in the Horsehead, and moregenerally in PDRs, is still incomplete. Nano-grain depletion in the outer partof the Horsehead has a strong influence on several gas tracers that will beprominent in JWST observations of irradiated clouds. We therefore need to takethis depletion into account in order to improve our understanding of theHorsehead, and more generally PDRs, and to contribute to the optimal scientificreturn of the mission.

Influence of the nano-grain depletion in photon-dominated regions: Application to the gas physics and chemistry in the Horsehead