Deciphering Star Cluster Evolution by Shape Morphology

TL;DR

This study investigates how the shapes of open star clusters evolve over time, revealing correlations between ellipticity, age, mass, and environmental influences, and providing shape parameters for 265 clusters.

Contribution

It offers the first comprehensive analysis of morphological evolution in open clusters, including shape parameters and correlations with age, mass, and external forces.

Findings

Clusters become more elliptical with age.

Core regions tend to remain circular or become more circular.

Ellipticity negatively correlates with number of members and mass.

Abstract

We analyze the morphological evolution of open clusters and provide shape parameters for 265 open clusters. The results show that the overall shape of sample clusters becomes more elliptical as they grow older, while their core remains circular or slightly trend to circularize. There is a negative correlation of the ellipticities with the number of members of the sample clusters. A significant negative correlation between the overall ellipticities and masses is also detected for the sample clusters with log(age/year) $\geq$ 8, suggesting that the overall shapes of the clusters are possibly influenced by the number of members and masses, in addition to the external forces and the surrounding environment. For most young sample clusters, the radial stratification degree of the short axis direction is greater than that of the long, implying that the radial stratification degree in the two directions within the young sample cluster may be unevenly affected by an internal evolutionary process. Older sample clusters exhibit lower stratification in the tangential direction, which possibly means those clusters may continue to survive for a long time at a low level of stratification. Our analysis shows that the overall shape of the sample clusters may be more susceptible to the influence of Galactic tides toward the Galactic center than the shear forces embedded in Galactic differential rotation. By analyzing the distribution of the ages and number of members of star clusters, we suggest that NGC 6791 may originate from superclusters.