Distribution of Water Vapor in Molecular Clouds
  We report the results of a large-area study of water vapor along the Orion
Molecular Cloud ridge, the purpose of which was to determine the
depth-dependent distribution of gas-phase water in dense molecular clouds. We
find that the water vapor measured toward 77 spatial positions along the
face-on Orion ridge, excluding positions surrounding the outflow associated
with BN/KL and IRc2, display integrated intensities that correlate strongly
with known cloud surface tracers such as CN, C2H, 13CO J =5-4, and HCN, and
less well with the volume tracer N2H+. Moreover, at total column densities
corresponding to Av < 15 mag., the ratio of H2O to C18O integrated intensities
shows a clear rise approaching the cloud surface. We show that this behavior
cannot be accounted for by either optical depth or excitation effects, but
suggests that gas-phase water abundances fall at large Av. These results are
important as they affect measures of the true water-vapor abundance in
molecular clouds by highlighting the limitations of comparing measured water
vapor column densities with such traditional cloud tracers as 13CO or C18O.
These results also support cloud models that incorporate freeze-out of
molecules as a critical component in determining the depth-dependent abundance
of water vapor.
