a) Model predictions showing the luminosity of the H₂O 448 GHz line as a function of the continuum optical depth at 448 GHz (τ₄₄₈, lower axis) and at 100 μm (τ₁₀₀, upper axis), for uniform T_dust = 50, 65, and 80 K. The models assume spherical symmetry with a radius R = 35 pc. The assumed H₂O abundance is X(H₂O) = 1.5 × 10^-6 (solid lines) and X(H₂O) = 6 × 10^-6 (dashed red line). The shadowed regions mark the favored ranges inferred from the ESO 320-G030 observations. b) Comparison between the predicted continua at 448 GHz (squares) and 244 GHz (starred symbols) and the observed values (after subtracting the free-free emission; horizontal stripes). c) Comparison between the predicted absorbing flux of the pumping H₂O 4₂₃ − 3₁₂ line at 79 μm and the observed value (− 920 Jy km s^-1 within ± 150 km s^-1; horizontal stripe). The insert compares the observed H₂O 4₂₃ − 3₁₂ absorption at 78.7 μm with the predictions of the three models encircled in the three panels. The widths of the horizontal stripes assume uncertainties of ± 10% for L_{H2O 448}, and ± 20% for the continuum flux densities and for the flux of the H₂O 79 μm line.