Relative velocities between ¹³CO structures within ¹²CO Molecular clouds

Velocity fields of molecular clouds (MCs) can provide crucial information on the merger and split between clouds, as well as their internal kinematics and maintenance, energy injection and redistribution, even star formation within clouds. Using the CO spectral lines data from the Milky Way Imaging Scroll Painting (MWISP) survey, we measure the relative velocities along the line of sight (ΔVLOS) between ¹³CO structures within ¹²CO MCs. Emphasizing MCs with double and triple ¹³CO structures, we find that approximately 70$%$ of ΔVLOS values are less than ~ 1 km s⁻¹, and roughly 10$%$ of values exceed 2 km s⁻¹, with a maximum of ~ 5 km s⁻¹. Additionally, we compare ΔVLOS with the internal velocity dispersion of ¹³CO structures (σ_ ¹³CO,in) and find that about 40$%$ of samples in either double or triple regime display distinct velocity discontinuities, i.e. the relative velocities between ¹³CO structures are larger than the internal linewidths of ¹³CO structures. Among these 40$%$ samples in the triple regime, 33$%$ exhibit signatures of combinations through the two-body motion, whereas the remaining 7$%$ show features of configurations through the multiple-body motion. The ΔVLOS distributions for MCs with double and triple ¹³CO structures are similar, as well as their ΔVLOS/σ_ ¹³CO,in distributions. This suggests that relative motions of ¹³CO structures within MCs are random and independent of cloud complexities and scales.