τ_dust = 1 surface at the wavelength of the HCO⁺ J = 1− 0 transition (3.4 mm, brown), J = 2−1 (1.7 mm, orange), J = 3−2 (1.2 mm, green), and J = 4−3 (0.84 mm, purple). The radial range where the water snowline is expected for various types of sources is highlighted with the blue background. The water snowline in the model for a typical Herbig Ae disk is indicated with the dashed black line.