Value of Σ₃₀₀ and p for the three sources DM Tau (circle), LkCa 15 (square), and MWC 480 (triangle). Our observations are plotted in filled black markers, the results from the PDR chemical modeling are in open markers. In the DM Tau case, we also plot the results with the photodissociation rates calculated for a black body at 4000 K from van Dishoeck et al. (2006). Results from the literature are marked with stars; red: Aikawa & Herbst (1999); green: Aikawa et al. (2002); blue: van Zadelhoff et al. (2003); cyan: Aikawa & Nomura (2006); magenta: Willacy & Langer (2000); and black: Willacy et al. (2006) (a, b and c correspond to their K = 0,K = 10¹⁶ and K = 10¹⁸ cases). The error bars show the power-law fitting errors (1σ).