A survey of N₂H⁺ in dense clouds: Implications for interstellar nitrogen and ion-molecule chemistry

Spectra of the N₂H⁺ J = 1 → 0, J = 3 → 2 and ¹⁵NNH⁺ and N¹⁵NH⁺ J = 1 → 0 rotational transitions have been obtained toward a sample of star-forming and cold dark clouds in the Galaxy. Toward the starforming regions, line profiles are relatively narrow (typically 1-5 km s^-1) and show no evidence of line wings, in contrast to the spectra of HCO⁺. The apparent absence of N₂H⁺ in hot, shocked gas suggests that this ion may be a selective tracer of extended, quiescent material. Column densities of N₂H⁺ were found to be N_tot ∼ 5 × 10¹² cm^-2 toward cold clouds and N_tot ∼ 10¹⁴ cm^-2 toward warm clouds. These values correspond to fractional abundances, relative to H₂, of f ∼ 4 × 10^-10 toward both the warm and cold clouds. Such abundances agree well with predictions of quiescent cloud ion-molecule chemistry models, provided "steady state," not "early time" values are used. Thus, the abundance and distribution of N₂H⁺ is well-explained by ion-molecule chemistry, provided it has reached steady state, as is found for interstellar NH₃. In contrast, most abundances of simple interstellar molecules are better reproduced by models calculated for early times. The N₂H⁺ results thus are further evidence that interstellar nitrogen chemistry is anomalous.