Characterizing dynamical states of Galactic open clusters with Gaia DR2

TL;DR

This study analyzes the dynamical states of 65 Galactic open clusters using Gaia DR2 data, revealing how internal relaxation and external tidal forces influence their structural evolution and concentration.

Contribution

It provides a uniform analysis of open clusters' dynamical parameters across a large sample, linking internal relaxation to concentration and external tides to size.

Findings

Older clusters are more centrally concentrated due to internal relaxation.

Clusters in weaker tidal fields tend to have larger tidal radii.

Inner structure shows insensitivity to external tidal variations.

Abstract

In this work, we investigate the dynamical properties of 38 Galactic open clusters: 34 of them are located at low Galactic latitudes (|b| < 10$^{\circ}$) and are projected against dense stellar fields; the other 4 comparison objects present clearer contrasts with the field population. We determine structural and time-related parameters that are associated with the clusters' dynamical evolution: core ($r_c$), tidal ($r_t$) and half-mass ($r_{hm}$) radii, ages ($t$) and crossing times ($t_{cr}$). We have also incorporated results for 27 previously studied clusters, creating a sample of 65, spanning the age and Galactocentric distance ($R_G$) ranges: 7.0 < log ($t$) < 9.7 and 6 < $R_G$ (kpc) < 13. We employ a uniform analysis method which incorporates photometric and astrometric data from the Gaia DR2 catalogue. Member stars are identified by employing a decontamination algorithm which operates on the 3D astrometric space of parallax and proper motion and attributes membership likelihoods for stars in the cluster region. Our results show that the internal relaxation causes $r_c$ to correlate negatively with the dynamical ratio $\tau_{dyn}$ = $t/t_{cr}$. This implies that dynamically older systems tend to be more centrally concentrated. The more concentrated ones tend to present smaller $r_{hm}/r_t$ ratios, which means that they are less subject to tidal disruption. The analysis of coeval groups at compatible $R_G$ suggests that the inner structure of clusters is reasonably insensitive to variations in the external tidal field. Additionally, our results confirm, on average, an increase in $r_t$ for regions with less intense Galactic gravitational field.