On the Spatial Distribution of ¹³CO Structures within ¹²CO Molecular Clouds

We look into the 2851 (CO)-C-12 molecular clouds harboring (CO)-C-13 structures to reveal the distribution of the projected angular separations and radial velocity separations between their internal (CO)-C-13 structures. The projected angular separations are determined using the minimal spanning tree algorithm. We find that similar to 50% of the angular separations fall in a narrow range of similar to 3 '-7 ' with a median of similar to 5 ', and the corresponding radial velocity separations mainly range from similar to 0.3 to 2.5 km s(-1). The mean and standard deviation of the angular separations of the internal (CO)-C-13 structures within (CO)-C-12 clouds appear to be universal, independent of the (CO)-C-12 cloud angular areas and the counts of their internal (CO)-C-13 structures. We also reveal a scaling relation between the (CO)-C-12 cloud angular area and its harbored (CO)-C-13 structure count. These results suggest there is a preferred angular separation between (CO)-C-13 structures in these (CO)-C-12 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.