Heddy Arab (IAS): Dust in photodissociation regions with Herschel

Understanding dust properties and evolution is a key issue to comprehend the physics of interstellar medium and galaxies. Photodissociation regions (PDRs) are ideal laboratories to study how the dust evolves in response to the excitation and physical conditions of the gas. Within the framework of the 'Evolution of the interstellar dust' Herschel key project, we have obtained PACS and SPIRE spectro-photometric data of several PDRs of various characteristics. The example of the Orion Bar, a famous edge-on PDR, located in the Orion nebula is presented. We use SPIRE/FTS maps in combination with ancillary data from Spitzer/IRAC to perform a spatially detailed comparison between the CO, the PAH and the Big Grain emission. The evolution of the dust properties with respect to the local conditions is traced by analyzing the spectral energy distribution at different positions across the cloud. We also present a complete modeling of the dust emission, from the exciting star to the molecular region, using the numerical dust model dustEM. This is used to put new constraints on both the physical conditions on the Bar and the abundances of dust particles in the PDR, which reveals a dust evolution from dense to diffuse interstellar medium.