Astrophys. J., 727, 13 (2011/January-3)
Distribution of water vapor in molecular clouds.
MELNICK G.J., TOLLS V., SNELL R.L., BERGIN E.A., HOLLENBACH D.J., KAUFMAN M.J., LI D. and NEUFELD D.A.
Abstract (from CDS):
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 H2 O 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.
astrochemistry - ISM: abundances - ISM: clouds - ISM: molecules - radio lines: ISM
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