Molecular hydrogen diagnostic diagram with 2-1S(1)/1-0S(1) vs. 1-0S(2)/1-0S(1) (Mouri 1994). The locations of the apertures are indicated. The location of thermal UV excitation (Sternberg & Dalgarno 1989) and non-thermal models (Black & van Dishoeck 1987), as well as the thermal emission curve from 1000 K to 3000 K are plotted in grey. The X-ray heating models (Draine & Woods 1990) are marked by a yellow triangle, the shock-heating model (Brand et al. 1989) by a green circle.