Peak intensities of the observed molecular lines compared to the bolometric luminosity L_bol (L_⊙) of each system. The dashed lines are linear fits to the data for the cases where a correlation is found. Circles, diamonds, and stars show single, close binary, and wide multiple protostellar systems, respectively. C₂ H and c-C₃H₂ show somewhat higher peak intensities in systems with relatively higher luminosities.