Grid of H₂O LVG models for different gas temperatures and densities. Bottom: contour levels of the p-H₂O 6₀₆–5₁₅/5₁₅–4₀₄ (83.284/95.627 μm) line ratio for N(p-H₂O) = 10¹⁶ cm^-2. Note the gas temperature dependence of this ratio. The black continuous curve shows the observed intensity line ratio of 0.33. Top: contour levels of the o-H₂O 3₃₀–3₀₃/6₁₅–3₀₅ (67.269/82.031 μm) ratio for N(o-H₂O) = 3  ×  10¹⁶ cm^-2. Note the gas density dependence of this ratio. The black continuous curve shows the observed intensity line ratio of 0.14 while the black dotted curve shows the observed p-H₂O 6₀₆–5₁₅/5₁₅–4₀₄ ratio. The intersection of both curves is marked with a star (see text).