Abstract

Abstract We look into the 2851 12 CO molecular clouds harboring 13 CO structures to reveal the distribution of the projected angular separations and radial velocity separations between their internal 13 CO structures. The projected angular separations are determined using the minimal spanning tree algorithm. We find that ∼50% of the angular separations fall in a narrow range of ∼3′–7′ with a median of ∼5′, and the corresponding radial velocity separations mainly range from ∼0.3 to 2.5 km s −1 . The mean and standard deviation of the angular separations of the internal 13 CO structures within 12 CO clouds appear to be universal, independent of the 12 CO cloud angular areas and the counts of their internal 13 CO structures. We also reveal a scaling relation between the 12 CO cloud angular area and its harbored 13 CO structure count. These results suggest there is a preferred angular separation between 13 CO structures in these 12 CO clouds, considering the distance effects. According to that, we propose an alternative picture for the assembly and destruction of molecular clouds: there is a fundamental separation for the internal structures of molecular clouds, the build-up and destruction of molecular clouds proceeds under this fundamental unit.