Energy budget of forming clumps in numerical simulations of collapsing clouds

TL;DR

This study investigates the energy dynamics of dense regions in simulated collapsing molecular clouds, revealing gravitational contraction as the main driver and highlighting the role of external turbulence and stellar feedback in clump evolution.

Contribution

It demonstrates that clumps follow a gravitational contraction-based energy relation, challenging the virial equilibrium assumption, and analyzes how different selection criteria affect observed scalings.

Findings

Clumps follow a gravitational contraction energy relation.

External turbulence often assembles rather than disperses clumps.

High-density cores with stars tend to be in virial equilibrium.

Abstract

We analyze the physical properties and energy balance of density enhancements in two SPH simulations of the formation, evolution, and collapse of giant molecular clouds. In the simulations, no feedback is included, so all motions are due either to the initial, decaying turbulence, or to gravitational contraction. We define clumps as connected regions above a series of density thresholds. The resultingfull set of clumps follows the generalized energy-equipartition relation $\sigma_{v}/R^{1/2} \propto \Sigma^{1/2}$, where $\sigma_{v}$ is the velocity dispersion, $R$ is the "radius", and $\Sigma$ is the column density. We interpret this as a natural consequence of gravitational contraction at all scales, rather than virial equilibrium. Nevertheless, clumps with low $\Sigma$ tend to show a large scatter around equipartition. In more than half of the cases, this scatter is dominated by external turbulent compressions that {\it assemble} the clumps, rather than by small-scale random motions that would disperse them. The other half does actually disperse. Moreover, clump sub-samples selected by means of different criteria exhibit different scalings. Sub-samples with narrow $\Sigma$ ranges follow Larson-like relations, although characterized by their respective value of $\Sigma$. Finally, we find that: i) clumps lying in filaments tend to appear sub-virial; ii) high-density cores ($n \ge 10^5$ cm$^3$) that exhibit moderate kinetic energy excesses often contain sink ("stellar") particles, and the excess disappears when the stellar mass is taken into account in the energy balance; iii) cores with kinetic energy excess but no stellar particles are truly in a state of dispersal.