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-NH _ { 2 } N _ { K _ { a } ,K _ { c } } J = 1 _ { 1 , 1 } 3 / 2 – 0 _ { 0 , 0 } 1/2 , ortho-NH _ { 3 } J _ { K } = 1 _ { 0 } – 0 _ { 0 } and 2 _ { 0 } – 1 _ { 0 } , para-NH _ { 3 } J _ { K } = 2 _ { 1 } – 1 _ { 1 } 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 , NH _ { 2 } and NH _ { 3 } 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 H _ { 2 } 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-NH _ { 2 } and o-NH _ { 3 } , in sharp contrast to previous results for dark clouds .
We find relatively constant N ( NH ) / N ( o-NH _ { 3 } ) and N ( o-NH _ { 2 } ) / N ( o-NH _ { 3 } ) 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-NH _ { 3 } is \sim 2 \times 10 ^ { -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 \lesssim 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 , \approx 0.5 – 0.7 \pm 0.1 , in the strongest absorption components .
This result can not be explained by current models as we had expected to find a value of unity or higher .