Normalized level populations of HDO computed by RATRAN, as a function of the radius of the protostar envelope. To avoid confusion, the population of levels 2_0,2, 2_1,1, and 3_1,2 are indicated by dashed lines. The HDO 1_1,1 level population jump visible at  ~1300 AU is an artifact due to the change of the velocity field at the interface between the infalling envelope and the static envelope.