1996A&A...310..286F

The combination of interferometric and single-dish data allow us, for the first time, to determine the intimate structure of the photodissociation region associated with the prototypical reflection nebula NGC 7023. We have mapped the J=1->0 HCO⁺ line using the IRAM millimeter interferometer and the 30-m single dish telescope, and the 21cm HI line using the Very Large Array of NRAO. The interferometric HCO⁺ data show the existence of four high density molecular filaments with different velocities (1.9km/s, 2.4km/s, 2.9km/s, 4.0km/s) located at the interface between the atomic and the molecular gas. An additional filament has been tentatively detected at 5.8km/s. From the comparison between the single-dish and the interferometric HCO⁺ data with HI emission, we derive that the filament at 4.0km/s and, very likely, that at 5.8km/s are located on the inner walls of the HI clump, immersed in an atomic medium, while those at 1.9 and 2.4km/s are immersed in the molecular cloud. The filament at 2.9km/s would be located in an intermediate zone. The thickness of the filaments are ∼6" and the derived densities vary between 10⁵ and a few 10⁶cm^–3. The molecular filaments match perfectly the infrared filaments found at 2.1µm, suggesting that infrared and millimeter data are probing regions with enhanced hydrogen density.