Gaussian decomposition for the CO and isotopologues line profiles: a) two Gaussian fit for the line profiles with two different velocity components identified, such as the ¹²CO J = 10–9 spectra for the low-mass YSOs Ser SMM3 (left), the intermediate-mass Vela 19 (centre) and the ¹³CO J = 10–9 spectrum of the high-mass W33A (right). b) Single Gaussian fit of sources characterised by one component profile, such as the C¹⁸O J = 9–8 spectra of W33A. The red lines show the Gaussian fits and the green lines the baseline.