2002AJ....123.2010M

The HH 1/2 system has been observed with the two spectrometers on board the Infrared Space Observatory. Diffuse [C II] 158 µm emission indicates the presence of a photodissociation region (PDR), which is only in part due to the external far-UV (FUV) irradiation from the nearby Orion Nebula. Additional irradiation must originate locally, and we show that the FUV field produced in the recombination regions behind the shocks traced by the HH objects is sufficient to induce a PDR at the flow cavity walls. The analysis of [O I], [Si II] 34.8 µm and [Ne II] 12.8 µm lines suggests shock velocities v_s_∼100-140 km.s^–1, with preshock densities between 100 and 1000 cm^–3. H₂ pure rotational lines trace a T∼600 K gas, which is likely to be warmed up in slow, planar, molecular shocks. The coexistence in the areas subtended by the instrumental beam sizes of shocks at different velocities is consistent with the bow-shock morphology of HH 1 and, to a lesser extent, with that of HH 2 as traced by optical images.