2007ApJ...667..980D

We present observations of the J=1-0, 2-1, and 3-2 rotational transitions of N₂H⁺ and N₂D⁺ toward a sample of prototypical dark clouds. The data have been interpreted using nonlocal radiative transfer models. For all sources previously studied through millimeter-continuum observations, we find a good agreement between the volume density estimated from our N₂H⁺ data and that estimated from the dust emission. This confirms that N₂H⁺ depletion is not very efficient in dark clouds for densities as large as 10⁶/cm3, and also points out that a simultaneous analysis based on millimeter-continuum, N₂H⁺ and N₂D⁺ observations should lead to reliable estimates for the temperature and density structure of cold dark clouds. From multiline modeling of N₂H⁺ and N₂D⁺, we derive the deuterium enrichment in the observed clouds. Our estimates are similar or higher than previous ones. The differences can be explained by the assumptions made on the cloud density profile and by the chemical fractionation occurring in the clouds. For two of the observed objects, L183 and TMC 2, multiposition observations have allowed us to derive the variation of the N₂D⁺/N₂H⁺ abundance ratio with the radius. We have found that it decreases by an order of magnitude for radii greater than a few 0.01 pc (i.e., outside the central cores). Inside the dense condensations, the fractionation is efficient and, compared to the abundance ratio expected from statistical considerations based on the cosmic D/H ratio, the deuterium enrichment is estimated to be ≃0.1-0.5x10⁵.