The morphological stability of open clusters: a new 2D perspective

TL;DR

This study introduces a new morphological stability parameter for open clusters, revealing how their core-to-outer member ratio correlates with stability and varies with age and spatial position.

Contribution

It presents the first use of the Ncore/Nouter ratio to assess morphological stability and analyzes its variation across different spatial directions and ages of open clusters.

Findings

The Ncore/Nouter ratio correlates positively with N, indicating core density as a stability indicator.

Spatial analysis shows tidal forces impact clusters more from the Galactic center direction.

Younger clusters are less stable, with stability increasing as clusters age beyond 800 Myr.

Abstract

Open clusters (OCs) usually evolve gradually as the number of their members changes, which can be manifested in their morphological characteristics. We aim to investigate the morphological stability of 1,490 OCs and further explore the potential change of morphological stability of the OCs at different spatial positions, using the OC catalog from the literature. We define for the first time a new morphological stability parameter Ncore/Nouter, a ratio of member numbers between cluster core and outer areas within tidal radii, which has a significant positive correlation against N, with a slope of 1.140$\pm$0.039, significantly steeper than the 0.720$\pm$0.026 measured for Score/Souter. This demonstrates that the stellar density in the core is a more sensitive tracer for morphological stability than geometry. Spatially, the radial sample OCs have larger slopes of Ncore/Nouter and Score/Souter against N, with 1.083$\pm$0.116 and 0.733$\pm$0.080, respectively, whereas those in the tangential direction 1.013$\pm$0.110 and 0.529$\pm$0.075, respectively, which means that the impact on sample OCs from tidal forces directed toward the Galactic center is possibly stronger than that from the shear force caused by the differential rotation of the Galactic disk. Besides, the sample OCs younger than 30 Myr display a shallow slope of 0.751$\pm$0.166, with those older than 800 Myr (1.442$\pm$0.128), reflecting that young OCs likely endure both internal disruptions, such as early dynamical heating weakening core binding and more severe external disturbances, compared to older OCs.