Excitation diagrams, in units of total number of detected molecules divided by degeneracy g, for H₂O (upper left), CO (upper right), and OH (lower right) emission lines detected with Herschel-PACS (circles) and Spitzer-IRS (purple squares). For the H₂O excitation diagram, PACS data are subdivided into lines that are optically-thin (blue), moderately optically-thick (green), and optically-thick (black), for the H12 model. Most of the Spitzer lines are optically-thin. Lower left: H₂O excitation diagrams obtained from RADEX models for optically thin models with T = 1500 K at high (blue) and low (red) density. At low density, subthermal excitation leads to cooler measured excitation temperatures and significant scatter in the level populations.