Molecular Excitation and Differential Gas‐Phase Depletions in the IC 5146 Dark Cloud

We present a combined near-infrared and molecular-line study of 25' x 8' areain the Northern streamer of the IC 5146 cloud. Using the technique pioneered byLada et al 1994, we construct a Gaussian smoothed map of the infraredextinction with the same resolution as the molecular line observations in orderto examine correlations of integrated intensities and molecular abundances withextinction for C17O, C34S, and N2H+. We find that over a visual extinctionrange of 0 to 40 magnitudes, there is good evidence for the presence ofdifferential gas-phase depletions in the densest portions of IC 5146. Both COand CS exhibit a statistically significant (factor of ~3) abundance reductionnear Av ~ 12 magnitudes while, in direct contrast, at the highest extinctions,Av > 10 magnitudes, N2H+ appears relatively undepleted. Moreover, for Av < 4magnitudes there exists little or no N2H+. This pattern of depletions isconsistent with the predictions of chemical theory.Comment: 36 pages (13 figures), accepted by the Astrophysical Journa