The Excitation of N2H+ in Interstellar Molecular Clouds. II. Observations

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 cm-3, 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 ~=(0.1-0.5)×105.