Outflow momentum against L/M, for the SiO (2−1) and HCO⁺ (1−0) outflows. Data for SiO (5−4) outflows are indicated as open symbols. Triangles and circles correspond to sources classified as IRL and IRD, respectively (see Table 1). The black dotted line shows the result of a least squares fit to the data, with the Pearson correlation coefficient (ρ) shown in the bottom right corner.