SIMBAD references

The north-west photo-dissociation region (PDR) in the reflection nebula NGC7023 displays a complex structure. Filament-like condensations at the edge of the cloud can be traced via the emission of the main cooling lines, offering a great opportunity to study the link between the morphology and energetics of these regions. We study the spatial variation of the far-infrared fine-structure lines of [CII] (158µm) and [OI] (63 and 145µm). These lines trace the local gas conditions across the PDR. We also compare their emission with molecular tracers including rotational and ro-vibrational lines of H₂ and high-rotational lines of CO. We used observations from the Herschel/PACS instrument to map the spatial distribution of these fine-structure lines. The observed region covers a square area of about 110"x110" with an angular resolution that varies from 4" to 11". We compared this emission with ground-based and Spitzer observations of H₂ lines, Herschel/SPIRE observations of CO lines, and Spitzer/IRAC 3.6µm images that trace the emission of polycyclic aromatic hydrocarbons. We used a PDR code to model the [OI]145µm line and infer the physical conditions in the region. The [CII] (158µm) and [OI] (63 and 145µm) lines arise from the warm cloud surface where the PDR is located and the gas is warm, cooling the region. We find that although the relative contribution to the cooling budget over the observed region is dominated by [OI]63µm (>30%), H₂ contributes significantly in the PDR (∼35%), as does [CII]158µm outside the PDR (30%). Other species contribute little to the cooling ([OI]145µm 9%, and CO 4%). Enhanced emission of these far-infrared atomic lines trace the presence of condensations, where high-excitation CO rotational lines and dust emission in the submillimetre are detected as well. The [OI] maps resolve these condensations into two structures and show that the peak of [OI] is slightly displaced from the molecular H₂ emission. The size of these structures is about 8" (0.015pc) and in surface cover about 9% of the PDR emission. We have tested whether the density profile and peak densities that were derived in previous studies to model the dust and molecular emission can predict the [OI]145 µm emission. We find that the model with a peak density of 10⁶cm^–3, and 2x10^4–5cm^–3 in the oxygen emitting region predicts an [OI]145µm line that is only 30% lower than the observed emission. Finally, we did not detect emission from [NII]122µm, suggesting that the cavity is mostly filled with non-ionised gas.