Empirical Measurement of the Dynamical Ages of Three Globular Clusters and Some Considerations on the Use of the Dynamical Clock ^*

Abstract

We have used the “dynamical clock” to measure the level of dynamical evolution reached by three Galactic globular clusters (namely, NGC 3201, NGC 6316, and NGC 6440). This is an empirical method that quantifies the level of central segregation of blue straggler stars (BSSs) within the cluster half-mass radius by means of the A rh + parameter, defined as the area enclosed between the cumulative radial distribution of BSSs and that of a lighter population. The total sample with homogeneous determinations of A rh + currently includes 59 clusters: 52 old GCs in the Milky Way (including the three investigated here), five old clusters in the Large Magellanic Cloud, and two young systems in the Small Magellanic Cloud. The three objects studied here nicely nest into the correlation between A rh + and the central relaxation time defined by the previous sample, thus proving and consolidating the use of the dynamical clock as an excellent tracer of the stage of dynamical evolution of a star cluster in different galactic environments. Finally, we discuss the advantages of using the dynamical clock as an indicator of the dynamical ages of star clusters, compared to the present-day central relaxation time.

An Ongoing Tidal Capture in the Large Magellanic Cloud: The Low-mass Star Cluster KMK 88-10 Captured by the Massive Globular Cluster NGC 1835?*

“…These images already revealed the presence of the small stellar system KMK 88-10 at just 2 ′ from the center of NGC 1835, suggesting the exciting possibility that it has been captured by the close massive GC and it is on the verge of tidal disruption (Giusti et al 2023). In a companion paper (Giusti et al, in preparation), we will present the determination of the density profile and the structural parameters of NGC 1835, as well as its dynamical age measured from the sedimentation level of blue straggler stars following the dynamical clock prescription (Ferraro et al 2018(Ferraro et al , 2020(Ferraro et al , 2023.…”

“…In this paper, we provide the first tentative evidence of an ongoing tidal capture of the small star cluster KMK 88-10 by the massive globular cluster NGC 1835 in the LMC. This study is part of a project aimed at characterizing the dynamical age of old stellar clusters in the LMC (Ferraro et al 2019;Lanzoni et al 2019) by using the so-called "dynamical clock" (see Ferraro et al 2012;Lanzoni et al 2016;Ferraro et al 2018Ferraro et al , 2020Ferraro et al , 2023. To this end, we secured a set of multiband Hubble Space Telescope (HST) images of NGC 1835, a very massive (∼6 × 10 5 M e ; Mackey & Gilmore 2003;McLaughlin & van der Marel 2005), old (t ∼ 13 Gyr; Olsen et al 1998), and metal-poor ([Fe/H] ∼ −1.7 dex; Mucciarelli et al 2021) globular cluster lying close to the central bar of the LMC.…”

“…This investigation is part of a project aimed at determining the structural parameters, chronological and dynamical age (i.e., the level of dynamical evolution) of the oldest stellar clusters in the LMC (Ferraro et al 2019;Lanzoni et al 2019). In particular the dynamical age is determined by using the so-called "dynamical clock" that quantifies the central segregation of blue straggler stars (see Ferraro et al 2012Ferraro et al , 2018Ferraro et al , 2020Ferraro et al , 2023Lanzoni et al 2016). As part of this project, we secured a set of multiband Hubble Space Telescope (HST) images of NGC 1835, a very massive (∼ 6 × 10 5 M ⊙ ) and old (t ∼ 13 Gyr) globular cluster lying close to the central bar of the LMC (Mackey & Gilmore 2003).…”

New Parameters for Star Cluster Dynamics: The Role of Clusters’ Initial Conditions

“…In fact, they just require high-resolution photometry down to ∼0.5 mag below the main-sequence turnoff point and within a projected distance equal to 0.5 − 1 × r h from the center. This makes them suitable to recognize highly evolved systems also in cases where the central density cusp is not detectable from observations (e.g., Ferraro et al 2023), either because of too scarce Figure 7. A 5 parameter measured in the analyzed snapshots plotted as a function of the adopted initial conditions (see Table 1): from the top left to the bottom right, the total number of stars N, the BH kick velocity prescription (k), the galactocentric distance (R g ), the TF parameter, and the initial King dimensionless potential (W 0 ).…”

“…For this reason, additional indicators have been proposed in recent years. These indicators are either based on peculiar populations of heavy stars (e.g., blue straggler stars) that are sensitive tracers of the dynamical friction efficiency (Ferraro et al 2012(Ferraro et al , 2018(Ferraro et al , 2019(Ferraro et al , 2020(Ferraro et al , 2023Lanzoni et al 2016), or based on the radial variations of the stellar-mass function, kinematics, or distribution of stars in the cluster (Baumgardt & Makino 2003;Bianchini et al 2016Bianchini et al , 2018Tiongco et al 2016;Webb & Vesperini 2017;Leveque et al 2021;Bhat et al 2022).…”