Spherical models of star clusters with potential escapers

Abstract

An increasing number of observations of the outer regions of globular clusters (GCs) have shown a flattening of the velocity dispersion profile and an extended surface density profile. Formation scenarios of GCs can lead to different explanations of these peculiarities, therefore the dynamics of stars in the outskirts of GCs are an important tool in tracing back the evolutionary history and formation of star clusters. One possible explanation for these features is that GCs are embedded in dark matter halos. Alternatively, these features are the result of a population of energetically unbound stars that can be spatially trapped within the cluster, known as potential escapers (PEs). We present a prescription for the contribution of these energetically unbound members to a family of self-consistent, distribution function-based models, which, for brevity, we call the Spherical Potential Escapers Stitched (spes) models. We show that, when fitting to mock data of bound and unbound stars from an N -body model of a tidally-limited star cluster, the spes models correctly reproduce the density and velocity dispersion profiles up to the Jacobi radius, and they are able to recover the value of the Jacobi radius itself to within 20%. We also provide a comparison to the number density and velocity dispersion profiles of the Galactic cluster 47 Tucanae. Such a case offers a proof of concept that an appropriate modeling of PEs is essential to accurately interpret current and forthcoming Gaia data in the outskirts of GCs, and, in turn, to formulate meaningful present-day constraints for GC formation scenarios in the early universe.

Radii of Young Star Clusters in Nearby Galaxies

“…In addition, one should be careful comparing the cluster mass from the models to the observed mass. The models treat mass loss from stars leaving the cluster as instantaneous, while in reality stars can remain in clusters for a long time before escaping through the Lagrangian points (Fukushige & Heggie 2000;Baumgardt 2001;Claydon et al 2019). Once stars escape, their low velocity dispersion means that unbound stars can remain near the cluster (Küpper et al 2008(Küpper et al , 2012Webb et al 2013).…”

Self-similar orbit-averaged Fokker-Planck equation for isotropic spherical dense clusters (ii) physical properties and negative heat capacity of pre-collapse core

“…More realistic arguments for finite outer halo needs to include the effect of flux at the ridge of cluster, two dimensional (anisotropic) effect, and escapers and escaping stars with high eccentricity under the influence of tidal effects. The discussions for the outer halo are found in (e.g Michie, 1962;Spitzer and Shapiro, 1972;Claydon et al, 2019)…”

A little theory of everything, with heavy neutral leptons

“…Although values of 10 3 GeV/cm 3 could be expected if globular clusters form within DM subhalos before falling into galactic halos [107], tidal stripping by subsequent mergers [108] provides a very efficient way of depleting DM in these systems, leaving them dominated today by just the stellar component [109]. The observation that the present-day dynamics of globular clusters can be explained without the need of DM suggests that these systems might form in molecular clouds in the gaseous disk of the galaxy instead of in DM overdensities [110][111][112]. It is therefore reasonable to assume that the DM density in the M4 globular cluster, which is about 2 kpc away from us in the direction towards the galactic center, could be as low as in the solar neighborhood, ∼ 0.3 GeV/cm 3 .…”