Upper panel: rotation diagrams for CH₃CHO, derived using Cycle 1 (black points) and Cycle 4 (red) data. A total emitting region size of 0.′′18 is assumed (see text). The parameters N_u, g_u, and E_up are the column density, degeneracy, and energy of the upper level, respectively. The derived values of the rotational temperature and total column density are reported. Lower panel: LVG predictions of the temperature vs. HDO column density required to reproduce the observed velocity-integrated emission (the portion of the plot delimited by solid curves; see Table 1) densities of 10⁸ cm⁻³ (blue), 10⁹ cm⁻³ (magenta), and 10¹⁰ cm⁻³ (red). Dashed lines are for the optical depth (τ = 0.3 and 0.4), increasing with column density. A source size of 0.′′18 is assumed. The grey zone shows the temperature derived from CH₃CHO (see upper panel).