Impact of model parameters on ro-vibrational CO emission. On the l.h.s, the υ = 1 → 0R(10) line has been selected to study the effects of changing model parameters on line flux. The r.h.s. shows the R(35) /R(10) line ratio, a measure for the “CO rotational excitation temperature”. Large line ratios indicate emission from hot CO. Note that the direction of parameter impact is often reversed on the r.h.s., i.e. weak lines are usually emitted from a tiny area of hot gas, whereas strong lines are emitted from an extended area of cool gas. See Fig. 10 for further explanations.