Molecular excitation and differential gas-phase depletions in the IC 5146 dark cloud

We present a combined near-infrared and molecular line study of a 25' x 8' area in the northern streamer of the IC 5146 cloud. Using the technique pio neered by Lada and coworkers, we construct a Gaussian-smoothed map of the i nfrared extinction with the same resolution as the molecular line observati ons in order to examine correlations of integrated intensities and molecula r abundances with extinction for (CO)-O-17, (CS)-S-34, and N2H+. We find th at over a visual extinction range of 0-40 mag, there is good evidence for t he presence of differential gas-phase depletions in the densest portions of IC 5146. Both CO and CS exhibit a statistically significant (factor of sim ilar to3) abundance reduction near A(V) similar to 12 mag, while, in direct contrast, at the highest extinctions (A(V) > 10 mag), N2H+ appears relativ ely undepleted. Moreover, for A(V) < 4 mag, there exists little or no N2H+. This pattern of depletions is consistent with the predictions of chemical theory. Through the use of a time- and depth-dependent chemical model, we s how that the near-uniform or rising N2H+ abundance with extinction is a dir ect result of a reduction in its destruction rate at high extinction becaus e of the predicted and observed depletion of CO molecules. The observed abu ndance threshold for N2H+, A(V)(th) similar to 4 mag, is examined in the co ntext of this same model, and we demonstrate how this technique can be used to test the predictions of depth-dependent chemical models. Finally, we fi nd that cloud density gradients can have a significant effect on the excita tion and detectability of high dipole moment molecules, which are typically far from local thermodynamic equilibrium. Density gradients also cause che mical changes since reaction rates and depletion timescales are density-dep endent. Accounting for such density/excitation gradients is crucial to a co rrect determination and proper interpretation of molecular abundances.