Water in star-forming regions with Herschel (WISH). IV. A survey of
  low-J H2O line profiles toward high-mass protostars
  To understand the origin of water line emission and absorption during
high-mass star formation, we decompose high-resolution Herschel-HIFI line
spectra toward 19 high-mass star-forming regions into three distinct physical
components. Protostellar envelopes are usually seen as narrow absorptions or
emissions in the H2O 1113 and 1669 GHz ground-state lines, the H2O 987 GHz
excited-state line, and the H2O-18 1102 GHz ground-state line. Broader features
due to outflows are usually seen in absorption in the H2O 1113 and 1669 GHz
lines, in 987 GHz emission, and not seen in H2O-18, indicating a low column
density and a high excitation temperature. The H2O 1113 and 1669 GHz spectra
show narrow absorptions by foreground clouds along the line of sight, which
have a low column density and a low excitation temperature, although their H2O
ortho/para ratios are close to 3.
  The intensities of the H2O 1113 and 1669 GHz lines do not show significant
trends with luminosity, mass, or age. In contrast, the 987 GHz line flux
increases with luminosity and the H2O-18 line flux decreases with mass.
Furthermore, appearance of the envelope in absorption in the 987 GHz and H2O-18
lines seems to be a sign of an early evolutionary stage.
  We conclude that the ground state transitions of H2O trace the outer parts of
the envelopes, so that the effects of star formation are mostly noticeable in
the outflow wings. These lines are heavily affected by absorption, so that line
ratios of H2O involving the ground states must be treated with caution. The
average H2O abundance in high-mass protostellar envelopes does not change much
with time. The 987 GHz line appears to be a good tracer of the mean weighted
dust temperature of the source, which may explain why it is readily seen in
distant galaxies.
