2019A&A...624A.112A

Aims. We study the spatial distribution and kinematics of water emission in a ∼8x8pc² region of the Galactic center (GC) that covers the main molecular features around the supermassive black hole Sagittarius A* (Sgr A*). We also analyze the water excitation to derive the physical conditions and water abundances in the circumnuclear disk (CND) and the "quiescent clouds".
Methods. We presented the integrated line intensity maps of the ortho 1₁₀-1₀₁, and para 2₀₂-1₁₁ and 1₁₁-0₀₀ water transitions observed using the On the Fly mapping mode with the Heterodyne Instrument for the Far Infrared (HIFI) on board Herschel. To study the water excitation, we used HIFI observations of the ground state ortho and para H₂¹⁸O transitions toward three selected positions in the vicinity of Sgr A*. In our study, we also used dust continuum measurements of the CND, obtained with the Spectral and Photometric Imaging REceiver (SPIRE) instrument. Using a non-local thermodynamical equilibrium (LTE) radiative transfer code, the water line profiles and dust continuum were modeled, deriving H₂O abundances (X_H2O), turbulent velocities (V_t), and dust temperatures (T_d). We also used a rotating ring model to reproduce the CND kinematics represented by the position velocity (PV) diagram derived from para 2₀₂-1₁₁ H₂O lines.
Results. In our H₂O maps we identify the emission associated with known features around Sgr A*: CND, the Western Streamer, and the 20 and 50km/s clouds. The ground-state ortho water maps show absorption structures in the velocity range of [-220,10]km/s associated with foreground sources. The PV diagram reveals that the 2₀₂-1₁₁ H₂O emission traces the CND also observed in other high-dipole molecules such as SiO, HCN, and CN. Using the non-LTE code, we derive high X_H2O of ~(0.1-1.3)x10^–5, V_t of 14-23km/s , and T_d of 15-45K for the CND, and the lower X_H2O of 4x10^–8 and V_t of 9km/s for the 20km/s cloud. Collisional excitation and dust effects are responsible for the water excitation in the southwest lobe of the CND and the 20km/s cloud, whereas only collisions can account for the water excitation in the northeast lobe of the CND. We propose that the water vapor in the CND is produced by grain sputtering by shocks of 10-20km/s, with some contribution of high temperature and cosmic-ray chemistries plus a photon-dominated region chemistry, whereas the low X_H2O derived for the 20km/s cloud could be partially a consequence of the water freeze-out on grains.