Exploration of morphological coherence in open clusters with a "core-shell'' structure

TL;DR

This paper investigates the 3D morphological coherence of open clusters with core-shell structures using ellipsoid fitting and introduces new insights into how cluster shape relates to member count and external influences.

Contribution

It applies a novel ellipsoid fitting method and defines morphological dislocation and ellipticity ratio to analyze open cluster structures and their stability.

Findings

Clusters with more members tend to have more elliptical external shapes.

Morphological dislocation decreases slightly as the number of members increases.

No significant correlation between morphology and age or orbital parameters.

Abstract

The study of their morphological coherence allows us to obtain a better understanding of the morphological evolution of open clusters. We employed the ellipsoid fitting method to delineate the 3D spatial structure of the sample clusters while using the morphological dislocation (MD) defined in our previous work and the ellipticity ratio (ER) of the clusters' inner and outer structures to characterize the morphological coherence of the sample clusters. The results show an inverse correlation between the ER of the sample clusters and the number of their members, indicating that sample clusters with a much more elliptical external morphology than internal shape generally tend to host a large number of members. Meanwhile, a slight shrinking of the MD of the sample clusters with their members' number may shed light on the significant role of the gravitational binding of the sample clusters in maintaining their morphological stability. Moreover, there are no correlations between the MD and ER of the sample clusters and their age. They are also not significantly correlated with the X-axis, the Y-axis, their orbital eccentricities, and the radial and vertical forces on them. However, the ER of the sample clusters displays some fluctuations in the distributions between it and the above covariates, implying that the morphologies of the sample clusters are sensitive to the external environment if sample effects are not taken into account. Finally, the analysis of the 3D spatial shapes of sample clusters with a small ER or a large ER demonstrates that the number of members lays an important foundation for forming a dense internal system for sample clusters. At the same time, the MD of the sample clusters can serve well as an indicator of their morphological stability, which is built upon a certain amount of member stars.