The line-of-sight distribution of water in the SgrB2 complex

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 SgrB2 cores M and N. Radiative transfer modeling of this feature, together with the published data set of mm and submm HDO and (H2O)-O-18 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 SgrB2 warm envelope, the (H2O)-O-18 ground-state transition detected by SWAS and KAO at 548 GHz (Neufeld et al. 2000; Zmuidzinas et al. 1995a) is instead a product of the hot, diffuse, thin gas layer lying in the foreground of the SgrB2 complex.