Left: optical depth, cumulative line fluxes and zone where the water 538.29 μm line originates in our “standard” model. The optical depth is obtained from vertical escape probability which is a good approximation in the case of the TW Hya disk which has a low inclination of 7°. The zone of line formation outlined by black dashed contours that denote where 15 to 85% of the flux build up vertically and radially; the black solid line encloses thus 50% of the total line emission. The blue contours show total hydrogen number densities of 10⁷ and 10⁸ cm^-3. Right: density of the collision partners H, H₂ and e⁻ with the line forming region of the 538.29 μm line superimposed.