Investigating dynamical properties of evolved Galactic open clusters

Abstract

Context. The stellar content of Galactic open clusters is gradually depleted during their evolution as a result of internal relaxation and external interactions. The final residues of the long-term evolution of open clusters are called open cluster remnants. These are sparsely populated structures that can barely be distinguished from the field. Aims. We aimed to characterise and compare the dynamical states of a set of 16 objects catalogued as remnants or remnant candidates. We employed parameters that are intimately associated with the dynamical evolution: age, limiting radius, stellar mass, and velocity dispersion. The sample also includes 7 objects that are catalogued as dynamically evolved open clusters for comparison purposes. Methods. We used photometric data from the 2MASS catalogue, proper motions and parallaxes from the Gaia DR2 catalogue, and a decontamination algorithm that was applied to the three-dimensional astrometric space of proper motions and parallaxes (μα, μδ) for stars in the objects' areas. The luminosity and mass functions and total masses for most open cluster remnants are derived here for the first time. Our analysis used predictions of N-body simulations to estimate the initial number of stars of the remnants from their dissolution timescales. Results. The investigated open cluster remnants present masses (M) and velocity dispersions (σv) within well-defined ranges: M between ∼10-40 M and σv between ∼1-7 km s-1. Some objects in the remnant sample have a limiting radius Rlim 2 pc, which means that they are more compact than the investigated open clusters; other remnants have Rlim between ∼2-7 pc, which is comparable to the open clusters. We suggest that cluster NGC 2180 (previously classified as an open cluster) is entering a remnant evolutionary stage. In general, our clusters show signals of depletion of low-mass stars. This confirms their dynamically evolved states. Conclusions. We conclude that the open cluster remnants we studied are in fact remnants of initially very populous open clusters (N0 ∼ 103-104 stars). The outcome of the long-term evolution is to bring the final residues of the open clusters to dynamical states that are similar to each other, thus masking out the memory of the initial formation conditions of star clusters.