Far Infrared Spectroscopy of the HH 1 / 2 Outflow 1

The HH1/2 system has been observed with the two spectrometers on board the Infrared Space Observatory. Diffuse [Cii]158μm emission indicate the presence of a Photo-Dissociation Region (PDR) which is only in part due to the external Far-UV irradiation from the nearby Orion Nebula. Additional irradiation must be originated 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 [Oi], [Siii]34.8μm and [Neii]12.8μm lines suggests shock velocities vs ∼100÷140 km s −1 with pre-shock densities between 100 and 1000 cm. 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 beamsizes of shocks at different velocities is consistent with the bow-shock morphology of the HH1 and, to a lesser extent, HH2 as traced by optical images. Subject headings: Stars: formation (ISM:) Herbig-Haro objects ISM: individual objects: HH1/2 ISM: molecules Infrared: ISM: lines and bands ISO is an ESA project with the instruments funded by ESA Members States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.