2012A&A...538A..37B

The energetics in photo-dissociation regions (PDRs) are mainly regulated by the balance between the heating from the photo-electric effect acting on dust grains, and the cooling via the copious emission of photons in far-infrared lines. The Orion Bar is a luminous and nearby PDR, which presents to the observer an ideal edge-on orientation in which to study this energy balance. Spatially resolved studies of such a nearby system are essential as they enable us to characterise the physical processes that control the energetics of the regions and can serve as templates for distant systems where these processes cannot be disentangled. We characterise the emission of the far-infrared fine-structure lines of [CII] (158µm), [OI] (63 and 145µm), and [NII] (122µm) that trace the gas local conditions, via spatially resolved observations of the Orion Bar. The observed distribution and variation of the lines are discussed in relation to the underlying geometry and linked to the energetics associated with the Trapezium stars. Herschel/PACS observations are used to map the spatial distribution of these fine-structure lines across the Bar, with a spatial resolution between 4'' and 11'' and covering a total square area of about 120''x105''. The spatial profile of the emission lines are modelled using the radiative transfer code Cloudy. The Herschel observations reveal in unprecedented detail the morphology of the Bar. The spatial distribution of the [CII] line coincides with that of the [OI] lines. The [NII] line peaks closer to the ionising star than the other three lines, but with a small region of overlap. We can distinguish several knots of enhanced emission within the Bar indicating the presence of an inhomogenous and structured medium. The emission profiles cannot be reproduced by a single PDR, clearly indicating that, besides the Bar, there is a significant contribution from additional PDR(s) over the area studied. The combination of both the [NII] and [OI] 145µm lines can be used to estimate the [CII] emission and distinguish between its ionised or neutral origin. We have calculated how much [CII] emission comes from the neutral and ionised region, and find that at least ∼82% originates from the photo-dissocciation region. Together, the [CII] 158µm and [OI] 63 and 145µm lines account for ∼90% of the power emitted by the main cooling lines in the Bar (including CO, H₂, etc.), with [OI] 63µm alone accounting for 72% of the total.