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Arxiv: JWST observations of the Horsehead photon-dominated region I. First results from multi-band near- and mid-infrared imaging Published: 4/24/2024 2:39:42 PM Updated: 4/24/2024 2:39:42 PM

Paper abstract: The JWST has captured the sharpest IR images ever taken of the Horseheadnebula, a prototypical moderately irradiated PDR that is fully representativeof most of the UV-illuminated molecular gas in the Milky Way and star-forminggalaxies. We investigate the impact of FUV radiation of a molecular cloud andconstrain the structure of the edge of the PDR and its illumination conditions.We used NIRCam and MIRI to obtain 17 broadband and 6 narrowband maps from 0.7to 28 \mum. We mapped the dust emission, scattered light, and several gasphase lines. We also used HST-WFC3 maps at 1.1 and 1. 6 \mum, along withHST-STIS spectroscopic observations of the H\alpha line. We probed thestructure of the edge of the Horsehead and resolved its spatial complexity. Wedetected a network of faint striated features extending perpendicularly to thePDR front into the H\,II region in filters sensitive to nano-grain emission andlight scattered by larger grains. This may indeed figure as the first detectionof the entrainment of dust particles in the evaporative flow. The map of the1-0 S(1) line of H_2 presents sharp sub-structures on scales as small as 1.5arcsec. The ionization and dissociation fronts appear at distances 1-2 arcsecbehind the edge of the PDR and seem to spatially coincide, indicating athickness of the neutral atomic layer below 100 au. All broadband maps presentstrong color variations which can be explained by dust attenuation. Deviationsof the emissions in the H\alpha, Pa\alpha, and Br\alpha lines alsoindicate dust attenuation. With a very simple model, we derive the mainfeatures of the extinction curve. A small excess of extinction at 3 \mum maybe attributed to icy H_2O mantles onto grains. In all lines of sight crossingthe inner regions of the Horsehead, it appears that dust attenuation isnon-negligible over the entire spectral range.