Nitrogen hydrides in interstellar gas

Abstract

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-NH 2 1 1,1 ← 0 0,0 . We also observed ortho-NH 3 1 0 ← 0 0 , and 2 0 ← 1 0 , para-NH 3 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 × 10 −9 , 3 × 10 −9 , and 3 × 10 −9 for NH, NH 2 , and NH 3 , respectively. These abundances are discussed with reference to models of gas-phase and surface chemistry.

Odinobservations of ammonia in the Sgr A +50 km s-1cloud and circumnuclear disk

“…Our detections of NH 3 at the much lower abundance of (1 − 3) × 10 −9 in spiral arm clouds are not so easy to understand in terms of chemical models. Similarly low abundances have been derived by Persson et al (2010Persson et al ( , 2012 from Herschel observations of spiral arm NH 3 absorption regions in the directions of W49N and G10.6−0.4 (W31C), and by Wirström et al (2010) from Odin observations in the direction of Sgr B2. The NH 3 abundances in diffuse or translucent clouds, estimated by Liszt et al (2006) from 23 GHz inversion line absorption against compact extragalactic sources, are also similarly low.…”

Hydride spectroscopy of the diffuse interstellar medium: new clues on the gas fraction in molecular form and cosmic ray ionization rate in relation to H 3 +

“…Such a behaviour is consistent with the expectation from the gas phase formation of water vapour by the dissociative recombination of H 3 O + . Among neutral species, the nitrogen hydrides exhibit the unexpected result that NH 2 and NH 3 have similar column densities, whereas NH was found to be about twice as abundant as either NH 2 or NH 3 [13,14]. NH 2 [5][6], which implies that these ions reside in mostly atomic gas, with a fraction of hydrogen in molecular form f (H 2 ) smaller than 10 per cent [15,16].…”

The tight correlation of CCH and c-C3H2in diffuse and translucent clouds

“…As discussed further in Neufeld et al 2010b), the opacities of the absorption features do not scale with the HI opacities, indicating that the abundances of the different species relative to HI vary among velocity components. The recent Herschel-HIFI spectra obtained within the framework of the PRISMAS key programme show more extensive examples of molecular line profiles towards G10.6, W49N and W51 Gerin et al 2010a,b;Neufeld et al 2010a,b;Sonnentrucker et al 2010;Persson et al 2010). The Herschel data revealed that absorption features from the molecular ions CH + , OH + and H 2 O + , and from the HF molecule are better correlated with HI than those from triatomic molecules such as CCH and HCO + Gerin et al 2010a;Neufeld et al 2010a,b;Sonnentrucker et al 2010).…”