Nitrogen hydrides in interstellar gas II. Analysis of Herschel/HIFI
  observations towards W49N and G10.6-0.4 (W31C)
  We have used the Herschel-HIFI instrument to observe interstellar nitrogen
hydrides along the sight-lines towards W49N and G10.6-0.4 in order to elucidate
the production pathways leading to nitrogen-bearing species in diffuse gas. 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. 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). Surprisingly low values of the
ammonia ortho-to-para ratio are found in both sources, ~0.5 - 0.7 +- 0.1. This
result cannot be explained by current models as we had expected to find a value
of unity or higher.
