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Context. With Herschel, we can for the first time observe a wealth of high-J CO lines in the interstellar medium with a high angular resolution. These lines are specifically useful for tracing the warm and dense gas and are therefore very appropriate for a study of strongly irradiated dense photodissocation regions (PDRs). Aims. We characterize the morphology of CO J = 19–18 emission and study the high-J CO excitation in a highly UV-irradiated prototypical PDR, the Orion Bar. Methods. We used fully sampled maps of CO J = 19–18 emission with the Photoconductor Array Camera and Spectrometer (PACS) on board the Herschel Space Observatory over an area of ~110′′ × 110′′ with an angular resolution of 9′′. We studied the morphology of this high-J CO line in the Orion Bar and in the region in front and behind the Bar, and compared it with lower-J lines of CO from J = 5–4 to J = 13–12 and 13CO from J = 5–4 to J = 11–10 emission observed with the Herschel Spectral and Photometric Imaging Receiver (SPIRE). In addition, we compared the high-J CO to polycyclic aromatic hydrocarbon (PAH) emission and vibrationally excited H2. We used the CO and 13CO observations and the RADEX model to derive the physical conditions in the warm molecular gas layers. Results. The CO J = 19–18 line is detected unambiguously everywhere in the observed region, in the Bar, and in front and behind of it. In the Bar, the most striking features are several knots of enhanced emission that probably result from column and/or volume density enhancements. The corresponding structures are most likely even smaller than what PACS is able to resolve. The high-J CO line mostly arises from the warm edge of the Orion Bar PDR, while the lower-J lines arise from a colder region farther inside the molecular cloud. Even if it is slightly shifted farther into the PDR, the high-J CO emission peaks are very close to the H/H2 dissociation front, as traced by the peaks of H2 vibrational emission. Our results also suggest that the high-J CO emitting gas is mainly excited by photoelectric heating. The CO J = 19–18/J = 12–11 line intensity ratio peaks in front of the CO J = 19–18 emission between the dissociation and ionization fronts, where the PAH emission also peak. A warm or hot molecular gas could thus be present in the atomic region where the intense UV radiation is mostly unshielded. In agreement with recent ALMA detections, low column densities of hot molecular gas seem to exist between the ionization and dissociation fronts. As found in other studies, the best fit with RADEX modeling for beam-averaged physical conditions is for a density of 106 cm−3 and a high thermal pressure (P∕k = nH × T) of ~1–2 × 108 K cm−3. Conclusions. The high-J CO emission is concentrated close to the dissociation front in the Orion Bar. Hot CO may also lie in the atomic PDR between the ionization and dissociation fronts, which is consistent with the dynamical and photoevaporation effects. [ABSTRACT FROM AUTHOR]

Context. The north-west photo-dissociation region (PDR) in the reflection nebula NGC7023 displays a complex structure. Filamentlike 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. Aims. We study the spatial variation of the far-infrared fine-structure lines of [CII] (158 μm) and [OI] (63 and 145 μm). These lines trace the local gas conditions across the PDR. We also compare their emission with molecular tracers including rotational and rovibrational lines of H2 and high-rotational lines of CO. Methods. 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″ × 110″ 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 3.6 μm images that trace the emission of polycyclic aromatic hydrocarbons. We used a PDR code to model the [OI]145 μm line and infer the physical conditions in the region. Results. The [CII] (158 μm) and [OI] (63 and 145 μm) 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]63 μm (>30%), H2 contributes significantly in the PDR (~35%), as does [CII]158 μm outside the PDR (30%). Other species contribute little to the cooling ([OI]145 μm 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″ (0.015 pc) 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 [O i]145 μm emission. We find that the model with a peak density of 106 cm-3, and 2 × 104-5 cm-3 in the oxygen emitting region predicts an [OI]145 μm line that is only 30% lower than the observed emission. Finally, we did not detect emission from [NII]122 μm, suggesting that the cavity is mostly filled with non-ionised gas. [ABSTRACT FROM AUTHOR]