arXiv subjects

Enrico Vesperini

Publications and source records attributed to Enrico Vesperini.

At least 19 recordsLinked to original sources

arXiv2026

New parameters for star cluster dynamics: observational results

We recently used a large set of Monte Carlo simulations of globular clusters (GCs) to define new fully empirical parameters (named A5, P5, and S2.5) able to trace the internal dynamical evolution of dense stellar systems. These parameters are specifically designed to quantify the steepness of the cumulative radial distribution of stars in the innermost region of the host system, which tends to progressively increase with dynamical aging due to core contraction. Following the original definitions, here we measure A5 and P5 in a sample of 40 Galactic GCs homogeneously surveyed through HST photometric observations. In agreement with the predictions of our simulations, the largest values of A5 and P5 are found for the most dynamically evolved GCs, i.e., those previously classified as post-core collapse systems based on the shape of their density profile, and those characterized by the shortest central relaxation times. Moreover, the new dynamical parameters here measured strongly correlate with A+rh, another fully empirical, independent parameter that traces the dynamical age of star clusters through the level of central segregation of blue straggler stars.

astro-ph.GA↗

arXiv2026

A binary-related origin mediated by environmental conditions for blue straggler stars

Blue stragglers are anomalously massive core hydrogen-burning stars that, according to the theory of single star evolution, should not exist. They are suspected to form in mass-enhancement processes, involving binary evolution or stellar collisions. In dynamically active systems like globular clusters, the number of blue stragglers originated by collisions is expected to increase with the local density and the rate of stellar encounters. Here we analyse more than 3000 blue stragglers in 48 Galactic globular clusters with different structures, finding that their number normalized to the sampled luminosity anti-correlates (instead of correlating) with the central density, collision rate, and dynamical age of the parent cluster. Similar trends are also found for the cluster binary fraction. Once inserted in the context of the current knowledge of the BSS phenomenon, these correlations indicate that low-density regions (possibly because of a higher binary production/survival rate) are the natural habitat of both BSSs and binary systems, and the observed BSSs mostly have a binary-related origin mediated by the environmental conditions.

astro-ph.SR↗

arXiv2025

Evolution of the kinematic properties of rotating multiple-population globular clusters

Globular clusters (GCs) host multiple stellar populations differing in their chemical and dynamical properties. A number of models for the formation of multiple populations predict that the subsystem of second generation (SG) stars is characterized by a more centrally concentrated spatial distribution and a more rapid rotation than the system of first generation (FG) stars. We present the results of N-body simulations exploring the long-term dynamical evolution of rotating multiple-population GCs. We study the evolution of systems starting with four different orientations of the GC's total internal angular momentum vector relative to the orbital angular momentum. We explore the evolution driven by two-body relaxation and the effects of the GC's interaction with the galactic tidal field. We focus on the kinematic differences between the two generations and we quantify them by exploring the FG and SG rotation velocity and angular momenta. We find that kinematic differences between the generations persist for most of the GCs' lifetimes, although the strength of these differences decreases after a few relaxation times. The differences can be seen most clearly in the lowest-mass stars. We find that the GCs' internal angular momentum gradually aligns with the orbital angular momentum, although there is little difference in this alignment between the FG and SG systems. We also find that stars in the GC's outer regions align with the orbital angular momentum vector more rapidly than those in the inner regions leading to a variation of the orientation of the internal angular momentum with the clustercentric distance. The alignment between internal angular momentum and orbital angular momentum occurs more rapidly for low-mass stars. We study the evolution of the anisotropy in the velocity distribution and find the SG to be characterized by a stronger radial anisotropy than the FG.(abridged)

astro-ph.GA↗

arXiv2025

Structural parameters, chronological age and dynamical age of the LMC globular cluster NGC 1754

In the context of a new systematic study of the properties of the most compact and massive star clusters in the Large Magellanic Cloud (LMC), here we present the determination of the chronological age, the structural parameters, and the dynamical age of NGC 1754. We used high-resolution images taken with the WFC3/HST instrument, both in optical and near-ultraviolet filters. The high quality of the images made it possible to construct the star density profile from resolved star counts, and to fit the observed profile with an appropriate King model to obtain the structural parameters (e.g. core, half-mass, and tidal radii). Our findings confirm that NGC 1754 is a very compact globular cluster with a core radius of only 0.84 pc. The analysis of the same dataset allowed us to confirm a very old age ($t=12.8\pm0.4$ Gyr) for this system, thus further consolidating the indication that the process of globular cluster formation started at the same cosmic time both in the LMC and in the Milky Way, independently of the characteristics of the host environment. We have also used the empirical method called ``dynamical clock'' to estimate the dynamical age of the system. This consists of quantifying the degree of central segregation of blue straggler stars (BSSs) using the $A^+_{rh}$ parameter, which is defined as the area enclosed between the cumulative radial distribution of BSSs and that of a reference (lighter) population. This method yielded a value of $A^+_{rh}=0.31\pm0.07$, which is the highest measured so far for LMC clusters, pointing to an advanced dynamical age for the cluster, possibly on the verge of core collapse. The results presented here for NGC 1754 confirm that the natural dynamical evolution of globular clusters plays a role in shaping the age-core radius distributions observed in the LMC.

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arXiv2025

Phase-space mixing of multiple stellar populations in globular clusters

Context: Globular clusters (GCs) host multiple populations characterised by abundance variations in a number of light elements. In many cases, these populations also show spatial and/or kinematic differences, which vary in strength from cluster to cluster and tend to decrease with the clusters' dynamical ages. Aims: In this work, we aim to study the dynamical mixing of multiple populations and establish a link between the more theoretical aspects of the mixing process and various observational parameters that quantify differences between the populations' spatial concentration and velocity anisotropy. Methods: We follow the dynamical mixing of multiple populations in a set of numerical simulations through their distribution in the energy and angular momentum phase space and quantify the evolution of their degree of dynamical mixing. Results: We present the degree of dynamical mixing traced by the intrinsic differences in the phase space distribution of the populations. We compare the differences in phase-space with three observable quantities that describe the degree of mixing in the structural and kinematic differences of the populations: $A^{+}$, commonly used in the literature for spatial differences; and we introduce two new parameters, $ΔA_β$ that traces the difference in velocity anisotropy and $σ_{\text{Lz}}$, that traces the angular momentum distribution of stars. Conclusions: Our study provides new insights into the dynamics of phase space mixing of multiple populations in globular clusters. We show that differences between the 1P and the 2P observed in old clusters contain key information on the cluster's dynamics and the 1P and the 2P spatial and kinematic properties set by the formation processes, but caution is necessary in using the strength of the present-day differences to quantitatively constrain those imprinted at the time of formation.

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arXiv2024

MOCCA: Global properties of tidally filling and underfilling globular star clusters with multiple stellar populations

We explore the evolution of various properties of multiple-population globular clusters (GCs) for a broad range of initial conditions. We simulated over 200 GC models using the MOCCA Monte Carlo code and find that present-day properties (core and half-light radii, ratio of the number of second-generation (SG) stars to the total number of stars, NSG/NTOT) of these models cover the observed values of these quantities for Milky Way GCs. Starting with a relatively small value of the SG fraction (NSG/NTOT ~ 0.25) and a SG system concentrated in the inner regions of the cluster, we find, in agreement with previous studies, that systems in which the first-generation (FG) is initially tidally filling or slightly tidally underfilling best reproduce the observed ratios of NSG/NTOT and have values of the core and half-light radii typical of those of many Galactic globular clusters. Models in which the FG is initially tidally underfilling retain values of NSG/NTOT close to their initial values. These simulations expand previous investigations and serve to further constrain the viable range of initial parameters and better understand their influence on present-day GC properties. The results of this investigation also provide the basis for our future survey aimed at building specific models to reproduce the observed trends (or lack thereof) between the properties of multiple stellar populations and other clusters properties.

astro-ph.GA↗

arXiv2024

The HST Large Programme on omega Centauri -- VI. The radial gradient of the stellar populations

In this paper we present the analysis of Hubble Space Telescope (HST) observations of the globular cluster Omega Centauri. Our analysis combines data obtained in this work with previously published HST data from an earlier article of this series and encompasses a broad portion of the cluster's radial extension. Our findings reveal a significant radial variation in the fraction of stars within the two most populous stellar populations showing that one of the main second-population groups (referred to as bMS) is more centrally concentrated than the first-population group (referred to as rMS). Additionally, we explore the spatial variations of the other less populous stellar populations (referred to as MSa and MSd) and find a qualitatively similar, but weaker, radial decrease in the fraction of stars in these populations at larger distances from the cluster centre. Only one of the populations identified (MSe) does not show any significant radial variation.

astro-ph.GA↗

arXiv2024

Dynamics of star clusters with tangentially anisotropic velocity distribution

Recent high-precision observations with HST and Gaia enabled new investigations of the internal kinematics of star clusters (SCs) and the dependence of kinematic properties on the stellar mass. These studies raised new questions about the dynamical evolution of self-gravitating stellar systems. We aim to develop a more complete theoretical understanding of how various kinematical properties of stars affect the global dynamical development of their host SCs. We perform N-body simulations of SCs with isotropic, radially anisotropic and tangentially anisotropic initial velocity distributions. We also study the effect of an external Galactic tidal field. We find three main results. First, compared to the conventional, isotropic case, the relaxation processes are accelerated in the tangentially anisotropic models and, in agreement with our previous investigations, slower in the radially anisotropic ones. This leads to, e.g., more rapid mass segregation in the central regions of the tangential models or their earlier core collapse. Second, although all SCs become isotropic in the inner regions after several relaxation times, we observe differences in the anisotropy profile evolution in the outer cluster regions - all tidally filling models gain tangential anisotropy there while the underfilling models become radially anisotropic. Third, we observe different rates of evolution towards energy equipartition (EEP). While all SCs evolve towards EEP in their inner regions (regardless of the filling factor), the outer regions of the tangentially anisotropic and isotropic models are evolving to an "inverted" EEP (i.e., the high-mass stars having higher velocity dispersion than the low-mass ones). The extent (both spatial and temporal) of this inversion can be attributed to the initial velocity anisotropy - it grows with increasing tangential anisotropy and decreases as the radial anisotropy rises.

astro-ph.GA↗

arXiv2024

The ESO-VLT MIKiS survey reloaded: the internal kinematics of the core of M75

We present the results of a study aimed at characterizing the kinematics of the inner regions of the halo globular cluster M75 (NGC 6864) based on data acquired as part of the ESO-VLT Multi-Instrument Kinematic Survey (MIKiS) of Galactic globular clusters. Our analysis includes the first determination of the line-of-sight velocity dispersion profile in the core region of M75. By using MUSE/NFM observations, we obtained a sample of $\sim 1900$ radial velocity measurements from individual stars located within $16''$ (corresponding to about $r < 3 r_c$ where $r_c$ is the estimated core radius of the system) from the cluster center. After an appropriate selection of the most accurate velocity measures, we determined the innermost portion of the velocity dispersion profile, finding that it is characterized by a constant behavior and a central velocity dispersion of $σ_0\sim 9$ km s$^{-1}$. The simultaneous King model fitting to the projected velocity dispersion and density profiles allowed us to check and update previous determinations of the main structural parameters of the system. We also detected a mild hint of rotation in the central $\sim 7''$ from the center, with an amplitude of just $\sim 1.0$ km s$^{-1}$ and a position angle of the rotation axis of PA$_0 = 174°$. Intriguingly, the position angle is consistent with that previously quoted for the suspected rotation signal in the outer region of the cluster. Taking advantage of the high quality of the photometric catalog used for the analysis of the MUSE spectra, we also provide updated estimates of the cluster distance, age, and reddening.

astro-ph.GA↗

arXiv2024

The dynamical age of the LMC globular cluster NGC 1835 using the "dynamical clock"

In the context of the study of the size-age relationship observed in star clusters in the Large Magellanic Cloud and the investigation of its origin, here we present the determination of the structural parameters and the dynamical age of the massive cluster NGC 1835. We have used a powerful combination of optical and near-ultraviolet images acquired with the WFC3 onboard the HST to construct the star density profile from resolved star counts, determining the values of the core, half-mass and tidal radii through the comparison with the King model family. The same data also allowed us to evaluate the dynamical age of the cluster by using the 'dynamical clock'. This is an empirical method that quantifies the level of central segregation of blue stragglers stars (BSSs) within the cluster half-mass radius by means of the A+ parameter, which is defined as the area enclosed between the cumulative radial distribution of BSSs and that of a reference (lighter) population. The results confirm that NGC 1835 is a very compact cluster with a core radius of only 0.84 pc. The estimated value of A+ ($0.30\pm 0.04$) is the largest measured so far in the LMC clusters, providing evidence of a highly dynamically evolved stellar system. NGC 1835 nicely fits into the correlation between A+ and the central relaxation time and in the anti-correlation between A+ and the core radius defined by the Galactic and the Magellanic Cloud clusters investigated to date.

astro-ph.GA↗

arXiv2024

New Parameters for Star Cluster Dynamics: the role of clusters initial conditions

We recently introduced three new parameters that describe the shape of the normalized cumulative radial distribution (nCRD) of the innermost stars in globular clusters and trace the clusters dynamical evolution. Here we extend our previous investigations to the case of a large set of Monte Carlo simulations of globular clusters, started from a broad range of initial conditions. All the models are analyzed at the same age of 13 Gyr, when they have reached different evolutionary phases. The sample of models is well representative of the structural properties of the observed population of Galactic globular clusters. We confirm that the three nCRD parameters are powerful tools to distinguish systems in early stages of dynamical evolution, from those that already experienced core collapse. They might also help disentangle clusters hosting a low-mass intermediate-mass black hole of a few hundred solar masses, from cases with large concentrations of dark remnants in their centers. With respect to other dynamical indicators, the nCRD parameters offer the advantage of being fully empirical and easier to measure from observational data.

astro-ph.GA↗

arXiv2024

Discovery of an extended Horizontal Branch in the Large Magellanic Cloud globular cluster NGC1835

We present a high angular resolution multi-wavelength study of the massive globular cluster NGC 1835 in the Large Magellanic Cloud. Thanks to a combination of optical and near ultraviolet images acquired with the WFC3 on board the HST, we performed a detailed inspection of the stellar population in this stellar system adopting a ``UV-guided search'' to optimize the detection of relatively hot stars. This allowed us to discover a remarkably extended horizontal branch (HB), spanning more than 4.5 magnitudes in both magnitude and colour from the region redder than the instability strip, up to effective temperatures of 30,000 K, and including a large population of RR Lyrae (67 confirmed variables, and 52 new candidates). This is the first time that such a feature has been detected in an extra-Galactic cluster, demonstrating that the physical conditions responsible for the formation of extended HBs are ubiquitous. The acquired dataset has been also used to redetermine the cluster distance modulus, reddening, and absolute age, yielding $(m-M)_0=18.58$, $E(B-V)=0.08$, and $t=12.5$ Gyr, respectively.

astro-ph.GA↗

arXiv2024

The structural properties of multiple populations in globular clusters: the instructive case of NGC 3201

All multiple population (MP) formation models in globular clusters (GCs) predict that second population (SP) stars form more centrally concentrated than the first population (FP). As dynamical evolution proceeds, differences are progressively erased, and only dynamically young clusters are expected to still retain a partial memory of the initial structural differences. In recent years, this picture has been supported by observations of the MP radial distributions of both Galactic and extragalactic GCs. However, recent observations have suggested that in some systems, FPs might actually form more centrally segregated, with NGC 3201 being one significant example of such a possibility. Here we present a morphological and kinematic characterization of the MPs in NGC 3201 based on a combination of photometric and astrometric data. We show that the distribution of the SP is bimodal. Specifically, the SP is significantly more centrally concentrated than the FP within ~1.3 cluster's half-mass radius. Beyond this point, the SP fraction increases again, likely due to asymmetries in the spatial distributions of the two populations. The central concentration of the SP observed in the central regions implies that it formed more centrally concentrated than the FP, even more so than what is observed in the present-day. This interpretation is supported by the MP kinematic properties. Indeed, we find that the FP is isotropic across all the sampled cluster extension, while the velocity distribution of the SP becomes radially anisotropic in the cluster's outer regions, as expected for the dynamical evolution of SP stars formed more centrally concentrated than the FP. The combination of spatial and kinematic observations provide key insights into the dynamical properties of this cluster and lend further support to scenarios in which the SP forms more centrally concentrated than the FP.

astro-ph.GA↗

arXiv2023

Young, wild and free: the early expansion of star clusters

Early expansion plays a fundamental role in the dynamical evolution of young star clusters. However, until very recently most of our understanding of cluster expansion was based only on indirect evidence or on statistically limited samples of clusters. Here we present a comprehensive kinematic analysis of virtually all known young ($t<300$ Myr) Galactic clusters based on the improved astrometric quality of the Gaia DR3 data. Such a large sample provides the unprecedented opportunity to robustly constrain the fraction of clusters and the timescale during which expansion has a prominent impact on the overall kinematics. We find that a remarkable fraction (up to $80\%$) of clusters younger than $\sim30$ Myr is currently experiencing significant expansion, whereas older systems are mostly compatible with equilibrium configurations. We observe a trend where the expansion speed increases with the clustercentric distance, suggesting that clusters undergoing expansion will likely lose a fraction of their present-day mass. Also, most young expanding clusters show large sizes, possibly due to the expansion itself. A comparison with a set of N-body simulations of young star clusters shows that the observed expansion pattern is in general qualitative agreement with that found for systems undergoing violent relaxation and evolving toward a final virial equilibrium state. However, we also note that additional processes likely associated with residual gas expulsion and mass loss due to stellar evolution are also likely to play a key role in driving the observed expansion.

astro-ph.GA↗

arXiv2023

Is the M81 Fast Radio Burst Host Globular Cluster Special?

We use multiband archival HST observations to measure the photometric and structural parameters of the M81 globular cluster that hosts the Fast Radio Burst FRB 20200120E. Our best-fitting King model has an effective radius $r_h = 3.06$ pc with a moderate King model concentration of $c = 53$, and an inferred core radius of 0.81 pc. We revisit the exact astrometric location of the FRB within the cluster, and find that FRB 20200120E is located 1.92 pc from the center, but within the projected half-light radius. We estimate the relative encounter rate of the FRB host, along with the corresponding rates of 210 other globular clusters in M81, and compare these values with the encounter rates of Galactic globular clusters. The FRB resides in a globular cluster with an encounter rate that is moderately higher than the median stellar encounter rate in our two comparison samples. While the estimated encounter rate of the FRB host cluster (e.g., $\sim50\%$ of a cluster like 47 Tuc) is sufficient to allow the possibility that the FRB formed dynamically, our results do not place strong constraints on this scenario due to the limitations of the available HST data and the possible systematic uncertainties and selection effects in the comparison data.

astro-ph.HE↗

arXiv2023

Exploring the Structures and Substructures of the Andromeda Satellite Dwarf Galaxies Cassiopeia III, Perseus I, and Lacerta I

We present results from wide-field imaging of the resolved stellar populations of the dwarf spheroidal galaxies Cassiopeia III (And XXXII) and Perseus I (And XXXIII), two satellites in the outer stellar halo of the Andromeda galaxy (M31). Our WIYN pODI photometry traces the red giant star population in each galaxy to ~2.5-3 half-light radii from the galaxy center. We use the Tip of the Red Giant Branch (TRGB) method to derive distances of (m-M)_0 = 24.62+/-0.12 mag (839 (+48,-450) kpc, or 156 (+16,-13) kpc from M31) for Cas III and 24.47+/-0.13 mag (738 (+48,-45) kpc, or 351 (+17,-16) kpc from M31) for Per I. These values are consistent within the errors with TRGB distances derived from a deeper Hubble Space Telescope study of the galaxies' inner regions. For each galaxy, we derive structural parameters, total magnitude, and central surface brightness. We also place upper limits on the ratio of neutral hydrogen gas mass to optical luminosity, confirming the gas-poor nature of both galaxies. We combine our data set with corresponding data for the M31 satellite galaxy Lacerta I (And XXXI) from earlier work, and search for substructure within the RGB star populations of Cas III, Per I, and Lac I. We find an overdense region on the west side of Lac I at a significance level of 2.5-3-sigma and a low-significance filament extending in the direction of M31. In Cas III, we identify two modestly significant overdensities near the center of the galaxy and another at two half-light radii. Per I shows no evidence for substructure in its RGB star population, which may reflect this galaxy's isolated nature.

astro-ph.GA↗

arXiv2023

Multiple Stellar Populations in Metal-Poor Globular Clusters with JWST: a NIRCam view of M92

Recent work on metal-intermediate globular clusters (GCs) with [Fe/H]=$-1.5$ and $-0.75$ has illustrated the theoretical behavior of multiple populations in photometric diagrams obtained with the James Webb Space Telescope (JWST). These results are confirmed by observations of multiple populations among M-dwarfs of 47 Tucanae. Here, we explore the multiple populations in metal-poor GCs with [Fe/H]=$-$2.3. We take advantage of synthetic spectra and isochrones that account for the chemical composition of multiple populations to identify photometric diagrams that separate the distinct stellar populations of GCs. We derived high-precision photometry and proper motion for main-sequence stars in the metal-poor GC M 92 from JWST and Hubble Space Telescope (HST) images. We identified a first generation (1G) and two main groups of second-generation stars (2G$_{\rm A}$ and 2G$_{\rm B}$) and investigated their kinematics and chemical composition. We find isotropic motions with no differences among the distinct populations. The comparison between the observed colors of M 92 stars and the colors derived by synthetic spectra reveals that helium abundance of 2G$_{\rm A}$ and 2G$_{\rm B}$ stars are higher than that of the 1G by $ΔY \sim 0.01$ and $0.04$, respectively. The $m_{\rm F090W}$ vs. $m_{\rm F090W}-m_{\rm F277W}$ color-magnitude diagram shows that below the knee, MS stars exhibit a wide color broadening due to multiple populations. We constrain the amount of oxygen variation needed to reproduce the observed MS width, which is consistent with results on red-giant branch stars. We conclude that multiple populations with masses of $\sim$0.1-0.8$M_{\odot}$ share similar chemical compositions.

astro-ph.SR↗

arXiv2023

Effects of massive central objects on the degree of energy equipartition of globular clusters

We present an analysis of the degree of energy equipartition in a sample of 101 Monte Carlo numerical simulations of globular clusters (GCs) hosting either a system of stellar-mass black holes (BHS), an intermediate-mass black hole (IMBH) or neither of them. For the first time, we systematically explore the signatures that the presence of BHS or IMBHs produces on the degree of energy equipartition and if these signatures could be found in current observations. We show that a BHS can halt the evolution towards energy equipartition in the cluster centre. We also show that this effect grows stronger with the number of stellar-mass black holes in the GC. The signatures introduced by IMBHs depend on how dominant their masses are to the GCs and for how long the IMBH has co-evolved with its host GCs. IMBHs with a mass fraction below 2% of the cluster mass produce a similar dynamical effect to BHS, halting the energy equipartition evolution. IMBHs with a mass fraction larger than 2% can produce an inversion of the observed mass-dependency of the velocity dispersion, where the velocity dispersion grows with mass. We compare our results with observations of Galactic GCs and show that the observed range of the degree of energy equipartition in real clusters is consistent with that found in our analysis. In particular, we show that some Galactic GCs fall within the anomalous behaviour expected for systems hosting a BHS or an IMBH and are promising candidates for further dynamical analysis.

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