JWST observations of photodissociation regions

Context . The interpretation of infrared (IR) measurements of photon-dominated regions (PDRs) relies on understanding the properties of dust. Additionally, the dependence of dust properties on the environment provides key insights into dust composition, evolution, as well as formation and destruction processes. This work is conducted as part of the Physics and Chemistry of PDR Fronts program dedicated to the study of dust and gas in PDRs with the James Webb Space Telescope (JWST). Aims . A significant component of interstellar dust consists of carbonaceous nano-grains which often dominate the mid-IR output of PDRs. In this paper, we study the evolution of the nano-grains across the illuminated edge of the Horsehead nebula and especially their abundance and size properties. Methods . We used NIRCam (3.0, 3.35, and 4.3 μm) and MIRI (5.6, 7.7, 10.0, 11.3, 12.8, 15.0, 18.0, 21.0, and 25.5 μm) photometric bands, along with NIRSpec and MRS spectroscopic observations to map the illuminated edge of the Horsehead. We modeled dust emission, including the aromatic and aliphatic IR bands, using the THEMIS interstellar dust model together with the 3D radiative transfer code SOC, in order to fit the photometric bands. Results . A detailed modeling of high angular resolution JWST data (~6 times higher than that of former observations) allowed us to obtain quantitative constraints on the size distribution of nano-grains. In addition, original constraints on the optical properties of these nano-grains were derived from the JWST NIRSpec spectroscopic data. We find that the diffuse interstellar medium (DISM) dust cannot account for the observed data, and it is necessary to use evolved grains. A sharp increase in density is observed at the illuminated edge, consistent with recent ALMA observations, which reveal a very sharp transition between molecular and ionized gas. Although the PDR length along the line of sight ( l PDR ) could not be directly determined from this study, we estimate an upper limit of ~0.015 pc based on geometric considerations and low extinction measured in the IR. This constraint implies a lower limit on the abundance of small grains (M a–C /M H , > 0.003), showing that small grains are not depleted at the external edge of the Horsehead nebula, unlike in other PDRs such as the Orion Bar. Conclusions . Our findings indicate a high-density environment and a less steep size distribution for nano-grains at the illuminated edge, in contrast with the DISM. This implies that nano-grain destruction mechanisms, such as UV-induced destruction, might be less efficient in the Horsehead’s moderate-UV field than in PDRs with more intense radiation, such as the Orion Bar. These results support a model where nano-grain population recovery, potentially through grain reformation due to the fragmentation of larger grains, is slower in moderate-ultraviolet (UV) environments, leading to a unique dust size distribution at the edge of the Horsehead nebula.