Example of the Keplerian fitting approach based on Seifried et al. (2016) to the 6σ outer edge of the PV plot of CH₃CN (12₄−11₄) for the model simulation inclined to 60° at a distance of 800 pc in order to determine the protostellar mass. The fits to the inner, outer, and entire regions yield protostellar mass estimates of 12.4, 21.4, and 15.4 M_⊙, respectively. The inner region traces the disc that follows a Keplerian rotation curve better than the outer region, which has contributions from the rotating and infalling envelope. The bottom panel shows the residuals. The protostar has a mass of ~10 M_⊙ while the disc has a mass of ~8 M_⊙ .