Distribution of Water Vapor in Molecular Clouds

TL;DR

This study maps water vapor distribution in the Orion Molecular Cloud, revealing that gas-phase water decreases with depth, supporting models with molecular freeze-out affecting water vapor abundance in dense clouds.

Contribution

It provides the first large-area, depth-dependent analysis of water vapor in a molecular cloud, highlighting the limitations of traditional tracers and supporting freeze-out models.

Findings

Water vapor correlates strongly with surface tracers like CN and HCN.

Gas-phase water abundance decreases at higher visual extinctions (Av > 15 mag).

Results challenge assumptions based on traditional cloud tracers.

Abstract

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.