Comparison of maser depths of ortho H₂O at 268.149 and 658.007 GHz (upper and lower two panels) using models in Gray et al. (2016). Strong maser emission at 268.149 and 658.007 GHz is obtained for markedly different kinetic temperatures (vertical axis on left hand-side of each panel) and dust temperatures T_d (given in the upper right-hand side of each panel). The transition frequency is given in the upper left corner of each panel with, below the letter o for ortho H₂Ο followed by the vibrational stale _fnd the rotational transition. The negative optical depth is specified in the vertical bar on the right-hand side of each panel using a log normal scale (min = −0.1 and max = −75.0); black means no inversion. The molecular H₂ density is obtained by dividing the water abundance by 3 × 10⁻⁵.)