Evolution of fractality in centrally concentrated young clusters

TL;DR

This study uses advanced simulations to explore how young star clusters develop their fractal structures over time, highlighting the roles of feedback, dynamics, and initial cloud conditions in shaping cluster morphology.

Contribution

It provides a comprehensive analysis of the evolution of fractality in young clusters, incorporating self-consistent gas dynamics, star formation, and feedback effects using the Torch framework.

Findings

Clusters inherit initial fractal substructure from parental clouds.

Fractal substructure is typically erased within about 2.5 free-fall times.

Feedback from massive stars can create secondary subclusters and influence fractality.

Abstract

We investigate the structural evolution of young star clusters forming within centrally concentrated molecular clouds. Our simulations use the Torch framework, which integrates the FLASH magnetohydrodynamics code with the AMUSE environment, enabling a self-consistent treatment of gas dynamics, star formation, stellar evolution, radiative transfer, and gravitational interactions. We quantify cluster structure using the $Q$ parameter for fractality and compute fractal dimensions via two methods: box-counting and correlation dimension. Our results show that clusters generally inherit fractal substructure from their parental clouds, which is typically erased within $\sim 2.5\,t_\mathrm{ff}$ through dynamical relaxation. Massive stars can induce the formation of secondary subclusters via feedback, with outcomes strongly dependent on stellar mass and formation timing. Interactions among subclusters, including mergers and dispersal, can extend fractal structure beyond $4\,t_\mathrm{ff}$. We also find systematic correlations between the fractality parameter $Q$ and the fractal dimension: fractality is positively correlated with both the correlation and box-counting dimensions, with the correlation dimension exhibiting a stronger correlation. These results demonstrate how stellar feedback and internal dynamics jointly shape the measurable fractal properties of embedded star clusters.