Histogram of the non-thermal velocity dispersions (in units of the sound speed) measured in C¹⁸O (1−0) (blue), ¹³CO (red), and ¹²CO (1−0) (orange) in L1517. Upper panel: non-thermal velocity dispersions directly derived from the observed linewidths (ΔV). Lower panel: non-thermal velocity dispersions obtained from the intrinsic linewidths (ΔV_int) after opacity corrections according to Eqs. (4) and (7). Although apparently supersonic, the ¹³CO and ¹²CO lines actually present sonic-line velocity dispersions.