Ratio of free-fall to rotational timescale as a function of disk gas mass, showing most of the sources with CH₃CN velocity gradients are rotationally supported. The blue and red dots correspond to the values calculated based on the rotational velocities at the edges of the disks on the blueshifted and redshifted sides, as depicted by crosses in Fig. 2 and listed in Table 3. The black dashed line corresponds to theoretical curve of t_ff/t_rot for a disk of mass M_gas in Keplerian rotation about a star with mass M_* = 10 M_⊙ (see Eq. (4)).