The kinetic and magnetic energy budget of hub-filament systems during the gravitational fragmentation of molecular clouds

Abstract

We present a numerical study of the balance between the gravitational (Eg), kinetic (Ek), and magnetic (Em) energies of structures within a hub-filament system in a simulation of the formation and global hierarchical collapse (GHC) of a giant molecular cloud. For structures defined by various density thresholds, and at different evolutionary stages, we investigate the scaling of the virial parameter, $\alpha$, with mass $M$, and of the Larson ratio, ${\cal{L}}\equiv\sigma_v/R^{1/2}$, with column density $\Sigma$, where $\sigma_v$ is the 1D velocity dispersion, and $R$ is an effective radius. We also investigate these scalings for the corresponding magnetic parameters $\alpha_m$ and ${\cal{L}}_m$. Finally, we compare our numerical results with an observational sample of massive clumps. We find that: 1) $\alpha_m$ and ${\cal{L}}_m$ follow similar scalings as their kinetic counterparts, although the ratio Em/Ek decreases as |Eg| increases. 2) The largest objects, defined by the lowest thresholds, tend to appear gravitationally bound (and magnetically supercritical), while their internal substructures tend to appear unbound (and subcritical). This suggests that the latter are being compressed by the infall of their parent structures, and supports earlier suggestions that the measured mass-to-magnetic flux ratio $\mu$ decreases inwards in a centrally-peaked cloud under ideal MHD. 3)~The scatter in the $\alpha$-$M$ and ${\cal{L}}$-$\Sigma$ plots is reduced when Ek and Em are plotted directly against Eg, suggesting that the scatter is due to an ambiguity between mass and size. 4) The clumps in our GHC simulation follow the same trends as the observational sample of massive clumps in the $\alpha$-$M$ and ${\cal{L}}$-$\Sigma$ diagrams. We conclude that the main controlling parameter of the energy budget in the structures is Eg, with the kinetic and magnetic energies being derived from it.