2001/03/30 by Edwin A. Bergin, E. A. Bergin, David R. Ciardi +5 · 161 citations
Chemistry · Earth and Planetary Sciences · Physics and Astronomy · #Abundance (ecology) #Astrophysics #Astrophysics and Star Formation Studies #Atmospheric Ozone and Climate #Biology #Chemistry #Context (archaeology) #Ecology #Excitation #Extinction (optical mineralogy) #Line (geometry) #Mineralogy #Physics #Spectroscopy and Laser Applications #astro-ph
paper · pdf · doi:10.1086/321625
published in The Astrophysical Journal 557(1), 209-225 (IOP Publishing) · 36 pages (13 figures), accepted by the Astrophysical Journal
arxiv created 2001/03/30 · openalex publication_date 2001/08/10 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
We present a combined near-infrared and molecular line study of a 25' × 8' area in the northern streamer of the IC 5146 cloud. Using the technique pioneered by Lada and coworkers, we construct a Gaussian-smoothed map of the infrared extinction with the same resolution as the molecular line observations in order to examine correlations of integrated intensities and molecular abundances with extinction for C 17 O, C 34 S, and N 2 H + . We find that over a visual extinction range of 0-40 mag, there is good evidence for the presence of differential gas-phase depletions in the densest portions of IC 5146. Both CO and CS exhibit a statistically significant (factor of ~3) abundance reduction near A V ~ 12 mag, while, in direct contrast, at the highest extinctions ( A V > 10 mag), N 2 H + appears relatively undepleted. Moreover, for A V < 4 mag, there exists little or no N 2 H + . This pattern of depletions is consistent with the predictions of chemical theory. Through the use of a time- and depth-dependent chemical model, we show that the near-uniform or rising N 2 H + abundance with extinction is a direct result of a reduction in its destruction rate at high extinction because of the predicted and observed depletion of CO molecules. The observed abundance threshold for N 2 H + , A ~ 4 mag, is examined in the context of this same model, and we demonstrate how this technique can be used to test the predictions of depth-dependent chemical models. Finally, we find that cloud density gradients can have a significant effect on the excitation and detectability of high dipole moment molecules, which are typically far from local thermodynamic equilibrium. Density gradients also cause chemical changes since reaction rates and depletion timescales are density-dependent. Accounting for such density/excitation gradients is crucial to a correct determination and proper interpretation of molecular abundances.