  'Water in Star-forming regions with Herschel' (WISH) is a Herschel Key
Programme aimed at understanding the physical and chemical structure of young
stellar objects (YSOs) with a focus on water and related species. The low-mass
protostar HH 46 was observed with the Photodetector Array Camera and
Spectrometer (PACS) on the Herschel Space Observatory to measure emission in
H2O, CO, OH, [OI], and [CII] lines located between 63 and 186 um. The
excitation and spatial distribution of emission can disentangle the different
heating mechanisms of YSOs, with better spatial resolution and sensitivity than
previously possible. Far-IR line emission is detected at the position of the
protostar and along the outflow axis. The OH emission is concentrated at the
central position, CO emission is bright at the central position and along the
outflow, and H2O emission is concentrated in the outflow. In addition, [OI]
emission is seen in low-velocity gas, assumed to be related to the envelope,
and is also seen shifted up to 170 km/s in both the red- and blue-shifted jets.
Envelope models are constructed based on previous observational constraints.
They indicate that passive heating of a spherical envelope by the protostellar
luminosity cannot explain the high-excitation molecular gas detected with PACS,
including CO lines with upper levels at >2500 K above the ground state.
Instead, warm CO and H2O emission is probably produced in the walls of an
outflow-carved cavity in the envelope, which are heated by UV photons and
non-dissociative C-type shocks. The bright OH and [OI] emission is attributed
to J-type shocks in dense gas close to the protostar. In the scenario described
here, the combined cooling by far-IR lines within the central spatial pixel is
estimated to be 2 \times 10-2 L_sun, with 60-80% attributed to J- and C-type
shocks produced by interactions between the jet and the envelope.
