On the Spatial Distribution of ${}^{13}$CO Structures within ${}^{12}$CO Molecular Clouds

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 $\sim$ 50$\%$ of the angular separations fall in a narrow range of $\sim$ 3 - 7 arcmin with a median of $\sim$ 5 arcmin, and the corresponding radial velocity separations mainly range from $\sim$ 0.3 km s${}^{-1}$ 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.

14 pages, 12 figures, accepted for publication in ApJ