Residuals between the observed spectrum of the CND and individual CLOUDY simulations, plotted as a function of gas temperature (y axis) and density (x axis), for the best-fit abundance triplet identified in Table 2. The residuals are computed as described in Sub-section 3.3.1, with darker colors indicating a better match. A well-defined minimum is found around T ∼ 10^4.4 K, with weaker constraints on the density. Secondary minima are found at lower temperatures.