2012A&A...543A.145P

As a part of the Herschel key programme PRISMAS, we have used the Herschel/HIFI instrument to observe interstellar nitrogen hydrides along the sight-lines towards eight high-mass star-forming regions in order to elucidate the production pathways leading to nitrogen-bearing species in diffuse gas. Here, we report observations towards W49N of the NH N=1-0, J=2-1, and J=1-0, ortho-NH₂ N_Ka_, K_cJ=11,1_3/2–00,0_1/2, ortho-NH₃ J_K=1₀-0₀ and 2₀-1₀, para-NH₃ J_K=2₁-1₁ transitions, and unsuccessful searches for NH⁺. All detections show absorption by foreground material over a wide range of velocities, as well as absorption associated directly with the hot-core source itself. As in the previously published observations towards G10.6-0.4, the NH, NH₂ and NH₃ spectra towards W49N show strikingly similar and non-saturated absorption features. We decompose the absorption of the foreground material towards W49N into different velocity components in order to investigate whether the relative abundances vary among the velocity components, and, in addition, we re-analyse the absorption lines towards G10.6-0.4 in the same manner. Abundances, with respect to molecular hydrogen, in each velocity component are estimated using CH, which is found to correlate with H₂ in the solar neighbourhood diffuse gas. The analysis points to a co-existence of the nitrogen hydrides in diffuse or translucent interstellar gas with a high molecular fraction. Towards both sources, we find that NH is always at least as abundant as both o-NH₂ and o-NH₃, in sharp contrast to previous results for dark clouds. We find relatively constant N(NH)/N(o-NH₃) and N(o-NH₂)/N(o-NH₃) ratios with mean values of 3.2 and 1.9 towards W49N, and 5.4 and 2.2 towards G10.6-0.4, respectively. The mean abundance of o-NH₃ is ∼2x10^–9 towards both sources. The nitrogen hydrides also show linear correlations with CN and HNC towards both sources, and looser correlations with CH. The upper limits on the NH⁺ abundance indicate column densities ≲2-14% of N(NH), which is in contrast to the behaviour of the abundances of CH⁺ and OH⁺ relative to the values determined for the corresponding neutrals CH and OH. Surprisingly low values of the ammonia ortho-to-para ratio are found in both sources, ≃0.5-0.7±0.1, in the strongest absorption components. This result cannot be explained by current models as we had expected to find a value of unity or higher.