Exploration of models with X-ray heating only (no dust and no CR). The predicted line fluxes (model/observation) are plotted against each model parameter with from top to bottom, the dust mass and H⁰ mass (log  M_⊙), minimum temperature (K) and maximum density (log  cm^-3) in the radiation-bounded sector, and the X-ray luminosity (log  L_⊙). Each point represents a model, and the symbol size is inversely proportional to the residuals between model predictions and observations for [C ii], [O i], and [Si ii]. The light gray zone corresponds to the observed values and their associated error bar. The dark gray zone corresponds to the best models.