The spatial distribution of stars in open clusters

TL;DR

This study analyzes the internal spatial structures of 16 open clusters in the Milky Way, revealing evolutionary trends from fractal to radial distributions and highlighting differences from molecular cloud structures.

Contribution

It applies the minimum spanning tree, King profile fitting, and correlation dimension methods to characterize cluster structures and their evolution, providing new insights into star cluster development.

Findings

Younger clusters tend to have fractal structures.

Older clusters show more radial density profiles.

Fractal dimensions in clusters are smaller than in molecular clouds.

Abstract

In this work we study the internal spatial structure of 16 open clusters in the Milky Way spanning a wide range of ages. For this, we use the minimum spanning tree method (the Q parameter, which enables one to classify the star distribution as either radially or fractally clustered), King profile fitting, and the correlation dimension (Dc) for those clusters with fractal patterns. On average, clusters with fractal-like structure are younger than those exhibiting radial star density profiles. There is a significant correlation between Q and the cluster age measured in crossing time units. For fractal clusters there is a significant correlation between the fractal dimension and age. These results support the idea that stars in new-born clusters likely follow the fractal patterns of their parent molecular clouds, and eventually evolve toward more centrally concentrated structures. However, there can exist stellar clusters as old as 100 Myr that have not totally destroyed their fractal structure. Finally, we have found the intriguing result that the lowest fractal dimensions obtained for the open clusters seem to be considerably smaller than the average value measured in galactic molecular cloud complexes.