Molecular hydrogen line ratio diagrams. The ratios of H₂(2−1)S(1)/H₂(1−0)S(1) versus H₂(1−0)S(2)/H₂(1−0)S(0) are shown in a). The ratios of H₂(2−1)S(1)/H₂(1−0)S(1) versus H₂(1−0)S(3)/H₂(1−0)S(1) are shown in b). The curves represent the thermal emission at 1000−3000 K. Vertical stripes represent the region where models by Black & van Dishoeck (1987) predict non-thermal UV excitation. Horizontal stripes are thermal UV excitation models by Sternberg & Dalgarno (1989). The open, magenta triangle represents thermal X-ray models by Draine & Woods (1990) and the open, turquoise circle represents a shock model from Brand et al. (1989). The filled dark bullets connected with a dotted line in a) are the predicted line ratios from a mixture of thermal and low-density fluorescence models of Black & van Dishoeck (1987). The first bullet from the left represents 10% non-thermal and 90% thermal UV-fluorescence, the second 20% non-thermal and 80% thermal UV-fluorescence and so on.