Molecular hydrogen line ratio diagram of H₂(1–0)S(3)/H₂(1–0)S(1) vs. H₂(2–1)S(1)/H₂(1–0)S(1) (Mouri 1994; Rodríguez-Ardila et al. 2005). The grey curve shows the thermal emission at 1000−3000 K. The gray ellipse on the right represents the region in which models by Black & van Dishoeck (1987) predict non-thermal UV excitation. The grey ellipse on the left represents thermal UV excitation models by Sternberg & Dalgarno (1989). The yellow triangle represents the X-ray heating model by Draine & Woods (1990). The red star indicates the shock heating model by Brand et al. (1989).