OH 127.8+0.0 H₂O emission spectrum modeling results for the temperature law and mass-loss rate of Models 1 and 2 in Table 4 in red and black, respectively. ψ_H2O and its uncertainty is determined for a wide range of (ortho + para) H₂O vapor abundances. From the modeling of the IR continuum and the CO data, a value of ψ_dens = 0.01 is determined. The expected H₂O vapor abundance from chemical models is 3 × 10^-4 (Cherchneff 2006). Both values are indicated by the dashed black lines. The dark gray area indicates the lower limit defined by the critical H₂O vapor abundance derived from the H₂O ice fraction of Model 2, see Sect. 4.2.5. For comparison, the light gray area indicates the lower limit found for Model 1.