Nitrogen hydrides in interstellar gas Herschel/HIFI observations towards G10.6-0.4 (W31C)

The HIFI instrument on board the Herschel 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 N = 1 <- 0, J = 2 <- 1 and ortho-NH2 1(1,1) <- 0(0,0). We also observed ortho-NH3 1(0) <- 0(0) and 2(0) <- 1(0), para-NH3 2(1) <- 1(1), and searched unsuccessfully for NH+. 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(-1) are estimated. The similar profiles suggest fairly uniform abundances relative to hydrogen, approximately 6 x 10(-9), 3 x 10(-9), and 3 x 10(-9) for NH, NH2, and NH3, respectively. These abundances are discussed with reference to models of gas-phase and surface chemistry.