2003A&A...402..635C

We report the detection, with the Caltech Submillimeter Observatory, of the 894-GHz HDO(1_1,1-0_0,0) transition, observed in absorption against the background continuum emission of the Sgr B2 cores M and N. Radiative transfer modeling of this feature, together with the published data set of mm and submm HDO and H₂¹⁸O transitions, suggests that ground-state absorption features from deuterated and non-deuterated water trace different gas components along the line of sight. In particular, while the HDO line seems to be produced by the large column densities of gas located in the Sgr B2 warm envelope, the H₂¹⁸O ground-state transition detected by SWAS and KAO at 548GHz (Neufeld et al., 2000ApJ...539..111D; Zmuidzinas et al., 1995, Proc. of the Airborne Astronomy Symp. on the Galactic Ecosystem: From Gas to Stars to Dust, ed. M.R. Haas, J.A. Davidson, & E.F. Erickson (San Francisco: ASP)) is instead a product of the hot, diffuse, thin gas layer lying in the foreground of the Sgr B2 complex.