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We present spatially resolved Herschel/PACS observations of the Orion Bar. Wehave characterise the emission of the far-infrared fine-structure lines of[CII] (158um), [OI] (63 and 145um), and [NII] (122um) that trace the gas localconditions. The observed distribution and variation of the lines are discussedin relation to the underlying geometry and linked to the energetics associatedwith the Trapezium stars. These observations enable us to map the spatialdistribution of these fine-structure lines with a spatial resolution between 4"and 11" and covering a total square area of about 120"x105". The spatialprofile of the emission lines are modelled using the radiative transfer codeCloudy. We find that the spatial distribution of the [CII] line coincides withthat of the [OI] lines. The [NII] line peaks closer to the ionising star thanthe other three lines, but with a small region of overlap. We can distinguishseveral knots of enhanced emission within the Bar indicating the presence of aninhomogenous and structured medium. The emission profiles cannot be reproducedby a single photo-dissociation region, clearly indicating that, besides theBar, there is a significant contribution from additional photo-dissociationregion(s) over the area studied. The combination of both the [NII] and [OI]145um lines can be used to estimate the [CII] emission and distinguish betweenits ionised or neutral origin. We have calculated how much [CII] emission comesfrom the neutral and ionised region, and find that at least 82% originates fromthe photo-dissocciation region. Together, the [CII] 158um and [OI] 63 and 145umlines account for 90% of the power emitted by the main cooling lines in the Bar(including CO, H2, etc...), with [OI] 63um alone accounting for 72% of thetotal.

Spatial variation of the cooling lines in the Orion Bar from Herschel/PACS