Origin of the hot gas in low-mass protostars - Herschel-PACS spectroscopy of HH 46

摘要

Aims. ``Water In Star-forming regions with Herschel'' (WISH) is a Herschelkey programme aimed at understanding the physical and chemicalstructure of young stellar objects (YSOs) with a focus on water andrelated species. Methods. The low-mass protostar HH46 was observed with the Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory to measure emission in H2O, CO, OH, [O I], and [C II] lines located between 63 and 186m.The excitation and spatial distribution of emission can disentangle thedifferent heating mechanisms of YSOs, with better spatial resolutionand sensitivity than previously possible. Results. Far-IR line emission is detected at the position of theprotostar and along the outflow axis. The OH emission is concentratedat the central position, CO emission is bright at the central positionand along the outflow, and H2O emission is concentrated in the outflow. In addition, [O I] emission is seen in low-velocity gas, assumed to be related to the envelope, and is also seen shifted up to 170kms-1in both the red- and blue-shifted jets. Envelope models are constructedbased on previous observational constraints. They indicate that passiveheating of a spherical envelope by the protostellar luminosity cannotexplain the high-excitation molecular gas detected with PACS, includingCO lines with upper levels at >2500K above the ground state.Instead, warm CO and H2O emission is probably produced inthe walls of an outflow-carved cavity in the envelope, which are heatedby UV photons and non-dissociative C-type shocks. The bright OH and [O I] emission is attributed to J-typeshocks in dense gas close to the protostar. In the scenario describedhere, the combined cooling by far-IR lines within the central spatialpixel is estimated to be ,with 60-80% attributed to J- and C-type shocks produced by interactions between the jet and the envelope. Key words: astrochemistry - stars: formation - ISM: molecules - ISM: jets and outflows - ISM: individual objects: HH46