Relations of the N₂H⁺ gas mass, M_gas(N₂H⁺), and envelope dust mass, M_menv, for the sample. The orange stars show multiple systems, the cyan diamonds show binary systems, and the gray circles show single protostellar systems. Each panel represents one of four ways of grouping the sample and their corresponding correlations. The lines and shaded areas show the linear regression for the data with the corresponding color. The solid lines indicate statistically significant correlations (Pearson r and Spearman ρ p-values <0.05), and the dashed lines show subsamples with p-values >0.05; see Sect. 5.5 for a discussion of the figure).