2001ApJ...547..292M

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 v_s∼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 ∼v_s. 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.