Water in star-forming regions with Herschel: highly excited molecular
  emission from the NGC 1333 IRAS 4B outflow
  During the embedded phase of pre-main sequence stellar evolution, a disk
forms from the dense envelope while an accretion-driven outflow carves out a
cavity within the envelope. Highly excited H2O emission in spatially unresolved
Spitzer/IRS spectra of a low-mass Class 0 object, NGC 1333 IRAS 4B, has
previously been attributed to the envelope-disk accretion shock but could
instead be produced in an outflow. As part of the survey of low-mass sources in
the Water in Star Forming Regions with Herschel (WISH-LM) program, we used
Herschel/PACS to obtain a far-IR spectrum and several Nyquist-sampled spectral
images with to determine the origin of excited H2O emission from NGC 1333 IRAS
4B. The spectrum has high signal-to-noise in a rich forest of H2O, CO, and OH
lines, providing a near-complete census of far-IR molecular emission from a
Class 0 protostar. The excitation diagrams for the three molecules all require
fits with two excitation temperatures, indicating the presence of two physical
components. The highly excited component of H2O emission is characterized by
subthermal excitation of 1500 K gas with a density of 10^5 - 10^7 cm-3,
conditions that also reproduce the mid-IR H2O emission detected by Spitzer. On
the other hand, a high density, low temperature gas can reproduce the H2O
spectrum observed by Spitzer but underpredicts the H2O lines seen by Herschel.
Nyquist-sampled spectral maps of several lines show two spatial components of
H2O emission, one centered at 1200 AU south of the central source at the
position of the blueshifted outflow lobe and a second centered on-source. Both
spatial components of the far-IR H2O emission are consistent with emission from
the outflow. The gas cooling from the IRAS 4B envelope cavity walls is
dominated by far-IR H2O emission, in contrast to stronger [O I] and CO cooling
from more evolved protostars. [one sentence truncated]
