LVG predictions of the temperature versus HDO column density required to reproduce the observed velocity-integrated emission (solid curves) and to have unit line opacity (dashed curves) for densities of: 10⁷ cm^-3 (green), 10⁸ cm^-3 (magenta), 10⁹ cm^-3 (blue), and 10¹⁰ cm^-3 (red). Optical depth increases with column density. The upper panel refers to a source size of 03 and the lower panel to 006.