H₂ formation rates on for different values of G₀ and n_H as a function of T_gas. Solid lines indicate H₂ formation by PAHs and dashed lines represent dust grain H₂ formation. For each datapoint, the color of the marker corresponds to the dust temperature of that datapoint. Both the gas and dust temperatures have been derived from the models in Hocuk & Spaans (2011). The black markers indicate the H₂ formation rates reported by Habart et al. (2004) for different PDRs. These formation rates have been estimated using H₂ line intensity ratios from the H₂ fluorescent lines.