SIMBAD references

We look into the 2851 ¹²CO molecular clouds harboring ¹³CO structures to reveal the distribution of the projected angular separations and radial velocity separations between their internal ¹³CO 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 ¹³CO structures within ¹²CO clouds appear to be universal, independent of the ¹²CO cloud angular areas and the counts of their internal ¹³CO structures. We also reveal a scaling relation between the ¹²CO cloud angular area and its harbored ¹³CO structure count. These results suggest there is a preferred angular separation between ¹³CO structures in these ¹²CO 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.