Nitrogen hydrides in interstellar gas

Abstract

The HIFI instrument on board the <i>Herschel<i/> Space Observatory has been used to observe interstellar nitrogen hydrides along the sight-line towards G10.6-0.4 in order to improve our understanding of the interstellar chemistry of nitrogen. We report observations of absorption in NH <i>N<i/> = 1 0, <i>J<i/> = 2 1 and <i>ortho<i/>-NH<sub>2<sub/> 1<sub>1,1<sub/> 0<sub>0,0<sub/>. We also observed <i>ortho<i/>-NH<sub>3<sub/> 1<sub>0<sub/> 0<sub>0<sub/>, and 2<sub>0<sub/> 1<sub>0<sub/>, <i>para<i/>-NH<sub>3<sub/> 2<sub>1<sub/> 1<sub>1<sub/>, and searched unsuccessfully for NH<sup>+<sup/>. All detections show emission and absorption associated directly with the hot-core source itself as well as absorption by foreground material over a wide range of velocities. All spectra show similar, non-saturated, absorption features, which we attribute to diffuse molecular gas. Total column densities over the velocity range 11-54 km s<sup>-1<sup/> are estimated. The similar profiles suggest fairly uniform abundances relative to hydrogen, approximately <i>6<i/> × 10<sup>-9<sup/>, <i>3<i/> × 10<sup>-9<sup/>, and <i>3<i/> × 10<sup>-9<sup/> for NH, NH<sub>2<sub/>, and NH<sub>3<sub/>, respectively. These abundances are discussed with reference to models of gas-phase and surface chemistry.