The two spectrometers on board the Infrared Space Observatory were used to observe the Herbig-Haro objects HH 80, 81, and 80N, as well as their candidate exciting source IRAS 18162-2048. The fine structure lines of [O I] 63 μm, [O I] 145 μm, and [C II] 158 μm are detected everywhere, while [N II] 122 μm and [O III] 88.3 μm are only detected toward the HH objects; line ratios confirm for the first time the collisionally excited HH nature of HH 80N. No molecular line is detected in any of the observed positions. We use a full shock code to diagnose shock velocities vs ~ 100 km s-1 toward the HH objects, as expected from the optical spectroscopy. Since proper motions suggest velocities in excess of 600 km s-1, the HH objects probably represent the interface between two flow components with velocity differing by ~vs. Aside from the flow exciting source, the [C II] 158 μm line is everywhere brighter than the [O I] 63 μm line, indicating the presence of a photodissociation region (PDR) all along the flow. Continuum emission from the HH objects and from other positions along the flow is only detected longward of ~50 μm, and its proportionality to the [C II] 158 μm line flux suggests it is PDR in origin. We propose that the far-ultraviolet continuum irradiated by the HH objects and the jet is responsible for the generation of a PDR at the walls of the flow cavity. We develop a very simple model which strengthens the plausibility of this hypothesis.
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