Understanding the Kinetic Energy deposition within Molecular Clouds

TL;DR

This study analyzes how internal and relative motions contribute to the kinetic energy in molecular clouds, revealing that relative motions often dominate and increase with cloud scale, indicating macro-turbulence as a key energy reservoir.

Contribution

It provides a large-scale statistical analysis of gas motions within molecular clouds, highlighting the dominance of relative motions and their relation to macro-turbulence development.

Findings

Relative motions often exceed internal motions in multiple-cloud systems.

Relative velocity dispersion increases with total velocity dispersion.

Macro-turbulence becomes the main energy storage mechanism at larger cloud scales.

Abstract

According to the structures traced by $^{13}$CO spectral lines within the $^{12}$CO molecular clouds (MCs), we investigate the contributions of their internal gas motions and relative motions to the total velocity dispersions of $^{12}$CO MCs. Our samples of 2851 $^{12}$CO MCs harbor a total of 9556 individual $^{13}$CO structures, among which 1848 MCs ($\sim$ 65$\%$) have one individual $^{13}$CO structure and the other 1003 MCs ($\sim$ 35$\%$) have multiple $^{13}$CO structures. We find that the contribution of the relative motion between $^{13}$CO structures ($\sigma_{\rm ^{13}CO, re}$) is larger than that from their internal gas motion ($\sigma_{\rm ^{13}CO, in}$) in $\sim$ 62$\%$ of 1003 MCs in the `multiple' regime. In addition, we find the $\sigma_{\rm ^{13}CO, re}$ tends to increase with the total velocity dispersion($\sigma_{\rm ^{12}CO, tot}$) in our samples, especially for the MCs having multiple $^{13}$CO structures. This result provides a manifestation of the macro-turbulent within MCs, which gradually becomes the dominant way to store the kinetic energy along with the development of MC scales.