2010A&A...522A..19W

Observations of transitions to the ground-state of a molecule are essential to obtain a complete picture of its excitation and chemistry in the interstellar medium, especially in diffuse and/or cold environments. For the important interstellar molecules H₂O and NH₃, these ground-state transitions are heavily absorbed by the terrestrial atmosphere, hence not observable from the ground. We attempt to understand the chemistry of nitrogen, oxygen, and their important molecular forms, NH₃ and H₂O in the interstellar medium of the Galaxy. We have used the Odin submillimetre-wave satellite telescope to observe the ground state transitions of ortho-ammonia and ortho-water, including their ¹⁵N, ¹⁸O, and ¹⁷O isotopologues, towards Sgr B2. The extensive simultaneous velocity coverage of the observations, >500km/s, ensures that we can probe the conditions of both the warm, dense gas of the molecular cloud Sgr B2 near the Galactic centre, and the more diffuse gas in the Galactic disk clouds along the line-of-sight. We present ground-state NH₃ absorption in seven distinct velocity features along the line-of-sight towards Sgr B2. We find a nearly linear correlation between the column densities of NH₃ and CS, and a square-root relation to N₂H⁺. The ammonia abundance in these diffuse Galactic disk clouds is estimated to be about 0.5-1x10^–8, similar to that observed for diffuse clouds in the outer Galaxy. On the basis of the detection of 8wat absorption in the 3kpc arm, and the absence of such a feature in the H₂¹⁷O spectrum, we conclude that the water abundance is around 10^–7, compared to ∼10^–8 for NH₃. The Sgr B2 molecular cloud itself is seen in absorption in NH₃, ¹⁵NH₃, H₂O, 8wat, and H₂¹⁷O, with emission superimposed on the absorption in the main isotopologues. The non-LTE excitation of NH₃ in the environment of Sgr B2 can be explained without invoking an unusually hot (500 K) molecular layer. A hot layer is similarly not required to explain the line profiles of the 1_1,0 ≥ 1_0,1 transition from H₂O and its isotopologues. The relatively weak ¹⁵NH₃ absorption in the Sgr B2 molecular cloud indicates a high [¹⁴N/¹⁵N] isotopic ratio >600. The abundance ratio of 8wat and H₂¹⁷O is found to be relatively low, 2.5-3. These results together indicate that the dominant nucleosynthesis process in the Galactic centre is CNO hydrogen burning.