Evolution of dust in the Orion Bar with Herschel

Context. Interstellar dust is a key element in our understanding of the interstellar medium and star formation. The manner in which dust populations evolve with the excitation and the physical conditions is a first step in the comprehension of the evolution of interstellar dust. Aims. Within the framework of the Evolution of interstellar dust Herschel key program, we have acquired PACS and SPIRE spectrophotometric observations of various photodissociation regions, to characterise this evolution. The aim of this paper is to trace the evolution of dust grains in the Orion Bar photodissociation region. Methods. We use Herschel/PACS (70 and 160 μm) and SPIRE (250, 350 and 500 μm) together with Spitzer/IRAC observations to map the spatial distribution of the dust populations across the Bar. Brightness profiles are modelled using the DustEM model coupled with a radiative transfer code. Results. Thanks to Herschel, we are able to probe finely the dust emission of the densest parts of the Orion Bar with a resolution from 5.6′′ to 35.1′′. These new observations allow us to infer the temperature of the biggest grains at different positions in the Bar, which reveals a gradient from ∼ 80 K to 40 K coupled with an increase of the spectral emissivity index from the ionization front to the densest regions. Combining Spitzer/IRAC observations, which are sensitive to the dust emission from the surface, with Herschel maps, we have been able to measure the Orion Bar emission from 3.6 to 500 μm. We find a stratification in the different dust components which can be reproduced quantitatively by a simple radiative transfer model without dust evolution (diffuse ISM abundances and optical properties). However including dust evolution is needed to explain the brightness in each band. PAH abundance variations, or a combination of PAH abundance variations with an emissivity enhancement of the biggest grains due to coagulation give good results. Another hypothesis is to consider a different length along the line of sight of the Bar at the ionization front than in the densest parts.