arXiv subjects

Enrico Vesperini

Publications and source records attributed to Enrico Vesperini.

At least 37 records · Page 2Linked to original sources

arXiv2023

The structural properties of multiple populations in the dynamically young globular cluster NGC 2419

NGC 2419 is likely the globular cluster (GC) with the lowest dynamical age in the Galaxy. This makes it an extremely interesting target for studying the properties of its multiple populations (MPs), as they have been likely affected only modestly by long-term dynamical evolution effects. Here we present for the first time a detailed analysis of the structural and morphological properties of the MPs along the whole extension of this remote and massive GC by combining high-resolution HST and wide-field ground-based data. In agreement with formation models predicting that second population (SP) stars form in the inner regions of the first population (FP) system, we find that the SP is more centrally concentrated than the FP. This may provide constraints on the relative concentrations of MPs in the cluster early stages of the evolutionary phase driven by two-body relaxation. In addition, we find that the fraction of FP stars is larger than expected from the general trend drawn by Galactic GCs. If, as suggested by a number of studies, NGC 2419 formed in the Sagittarius dwarf galaxy and was later accreted by the Milky Way, we show that the observed FP fraction may be explained as due to the transition of NGC 2419 to a weaker tidal field (its current Galactocentric distance is d_gc~95 kpc) and consequently to a reduced loss rate of FP stars.

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arXiv2023

An ongoing tidal capture in the Large Magellanic Cloud: the low-mass star cluster KMK88-10 captured by the massive globular cluster NGC 1835?

In the context of a project aimed at characterizing the dynamical evolution of old globular clusters in the Large Magellanic Cloud, we have secured deep HST/WFC3 images of the massive cluster NGC 1835. In the field of view of the acquired images, at a projected angular separation of approximately 2 arcmin from the cluster, we detected the small stellar system KMK88-10. The observations provided the deepest color-magnitude diagram ever obtained for this cluster, revealing that it hosts a young stellar population with an age of 600-1000 Myr. The cluster surface brightness profile is nicely reproduced by a King model with a core radius rc = 4 arcsec (0.97 pc), an half-mass radius rhm = 12 arcsec (2.9 pc), and a concentration parameter c~1.3 corresponding to a truncation radius rt~81 arcsec (19.5 pc). We also derived its integrated absolute magnitude (MV=-0.71) and total mass (M~80-160 Msun). The most intriguing feature emerging from this analysis is that KMK88-10 presents a structure elongated in the direction of NGC 1835, with an intracluster over-density that suggests the presence of a tidal bridge between the two systems. If confirmed, this would be the first evidence of a tidal capture of a small star cluster by a massive globular.

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arXiv2023

Fast rotating Blue Stragglers prefer loose clusters

Blue stragglers are anomalously luminous core hydrogen-burning stars formed through mass-transfer in binary/triple systems and stellar collisions. Their physical and evolutionary properties are largely unknown and unconstrained. Here we analyze 320 high-resolution spectra of blue stragglers collected in eight galactic globular clusters with different structural characteristics and show evidence that the fraction of fast rotating blue stragglers (with rotational velocities larger than 40 km/s) increases for decreasing central density of the host system. This trend suggests that fast spinning blue stragglers prefer low-density environments and promises to open an unexplored route towards understanding the evolutionary processes of these stars. Since large rotation rates are expected in the early stages of both formation channels, our results provide direct evidence for recent blue straggler formation activity in low-density environments and put strong constraints on the timescale of the collisional blue straggler slow-down processes.

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arXiv2023

Internal kinematics and structure of the bulge globular cluster NGC 6569

In the context of a project aimed at characterizing the properties of star clusters in the Galactic bulge, here we present the determination of the internal kinematics and structure of the massive globular cluster NGC 6569. The kinematics has been studied by means of an unprecedented spectroscopic dataset acquired in the context of the ESO-VLT Multi-Instrument Kinematic Survey (MIKiS) of Galactic globular clusters, combining the observations from four different spectrographs. We measured the line-of-sight velocity of a sample of almost 1300 stars distributed between ~0.8" and 770" from the cluster center. From a sub-sample of high-quality measures, we determined the velocity dispersion profile of the system over its entire radial extension (from ~ 5" to ~ 200" from the center), finding the characteristic behavior usually observed in globular clusters, with a constant inner plateau and a declining trend at larger radii. The projected density profile of the cluster has been obtained from resolved star counts, by combining high-resolution photometric data in the center, and the Gaia EDR3 catalog radially extended out to ~20' for a proper sampling of the Galactic field background. The two profiles are properly reproduced by the same King model, from which we estimated updated values of the central velocity dispersion, main structural parameters (such as the King concentration, the core, half-mass, and tidal radii), total mass, and relaxation times. Our analysis also reveals a hint of ordered rotation in an intermediate region of the cluster (40"<r<90", corresponding to $ 2 r_c<r<4.5 r_c$), but additional data are required to properly assess this possibility.

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arXiv2023

Empirical measurement of the dynamical ages of three globular clusters and some considerations on the use of the dynamical clock

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 stragglers 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 $A^+_{rh}$ now counts a gran-total of 59 clusters: 52 old GCs in the Milky Way (including the three investigated here), 5 old clusters in the Large Magellanic Cloud, and 2 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 star cluster dynamical evolution in different galactic environments. Finally, we discuss the advantages of using the dynamical clock as an indicator of star cluster dynamical ages, compared to the present-day central relaxation time.

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arXiv2023

Evolution of binary stars in the early evolutionary phases of ultra-faint dwarf galaxies

The dynamics of binary stars provides a unique avenue to gather insight into the study of the structure and dynamics of star clusters and galaxies. In this paper, we present the results of a set of $N$-body simulations aimed at exploring the evolution of binary stars during the early evolutionary phases of ultra-faint dwarf galaxies (UFD). In our simulations, we assume that the stellar component of the UFD is initially dynamically cold and evolves towards its final equilibrium after undergoing the violent relaxation phase. We show that the early evolutionary phases of the UFD significantly enhance the disruption of wide binaries and leave their dynamical fingerprints on the semi-major axis distribution of the surviving binaries as compared to models initially in equilibrium. An initially thermal eccentricity distribution is preserved except for the widest binaries for which it evolves towards a superthermal distribution; for a binary population with an initially uniform eccentricity distribution, memory of this initial distribution is rapidly lost for most binaries as wider binaries evolve to approach a thermal/superthermal distribution. The evolution of binaries is driven both by tidal effects due to the potential of the UFD dark matter halo and collisional effects associated to binary-binary/single star encounters. Collisional effects are particularly important within the clumpy substructure characterizing the system during its early evolution; in addition to enhancing binary ionization and evolution of the binary orbital parameters, encounters may lead to exchanges of either of the primordial binary components with one of the interacting stars.

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arXiv2023

Photometric binaries, mass functions, and structural parameters of 78 Galactic open clusters

Binary stars play a crucial role in our understanding of the formation and evolution of star clusters and their stellar populations. We use Gaia Data Release 3 to homogeneously analyze 78 Galactic open clusters and the unresolved binary systems they host, each composed of two main sequence (MS) stars. We first investigated the structural parameters of these clusters, such as the core radius and the central density, and determined the cluster mass function (MF) and total mass by interpolating the density profile of each cluster. We measured the fraction of binaries with a large mass ratio and the fraction of blue straggler stars (BSSs), and finally investigated possible connections between the populations of binary stars and BSSs with the main parameters of the host cluster. {Remarkably, we find that the MFs of 78 analyzed open clusters follow a similar trend and are well reproduced by two single power-law functions, with a change in slope around masses of 1$M_{\odot}$. The fraction of binary stars ranges from $\sim$15\% to more than $\sim$60\% without significant correlation with the mass and the age of the host cluster. Moreover, we detect hints of a correlation between the total fraction of binary stars and the central density of the host cluster. We compared the fraction of binary stars with that of BSSs, finding that clusters with high and low central density exhibit different trends. The fraction of binaries does not significantly change with the mass of the primary star and the mass ratio. The radial distribution of binary stars depends on cluster age. The binaries of clusters younger than $\sim$800\,Myr typically show a flat radial distribution, with some hints of a double peak. In contrast, the binaries of the remaining clusters are more centrally concentrated than the single stars, which is similar to what is observed in globular clusters.

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arXiv2023

New parameters for star cluster dynamics: the effect of primordial binaries and dark remnants

By studying the normalized cumulative radial distribution (nCRD) of the stars in the central region of a Monte Carlo-simulated globular cluster, we recently defined three parameters able to pinpoint the stage of internal dynamical evolution reached by the system: $A_5$ (i.e., the area subtended by the nCRD within 5$\%$ the half-mass radius, $r_h$), $P_5$ (the value of the nCRD at 5$\%$ $r_h$), and $S_{2.5}$ (the slope of the nCRD at 2.5$\%$ $r_h$). Here we extend the analysis and explore the effects that different fractions (0$\%$, 10$\%$, and 20$\%$) of primordial binaries and stellar-mass black holes (BHs) induce on the dynamical history of the system. As expected, the gradual contraction of the cluster becomes milder and core collapse shallower for increasing binary fraction. Nevertheless, the cluster dynamical evolution is still properly traced by the three parameters. For models with a larger initial retention of stellar mass BHs the evolution depends on the timescale of their subsequent dynamical ejection. An early dynamical ejection of BHs results in a long-term evolution of the three parameters similar to that found in systems with no initial BH retention. Conversely, in the model that retains a large number of BHs for extended time (slow dynamical ejection of BHs), the system is characterized by a less concentrated structure and by the lack of significant temporal evolution of the three parameters. The smaller values of the three parameters found in this case might be used to indirectly infer the possible presence of BHs in the cluster.

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arXiv2022

The ESO-VLT MIKiS survey reloaded: exploring the internal kinematics of NGC 6440

In the context of the ESO-VLT Multi-Instrument Kinematic Survey (MIKiS) of Galactic globular clusters, here we present the line-of-sight velocity dispersion profile of NGC 6440, a massive globular cluster located in the Galactic bulge. By combining the data acquired with four different spectrographs, we obtained the radial velocity of a sample of $\sim 1800$ individual stars distributed over the entire cluster extension, from $\sim$0.1$"$ to 778$"$ from the center. Using a properly selected sample of member stars with the most reliable radial velocity measures, we derived the velocity dispersion profile up to 250$"$ from the center. The profile is well described by the same King model that best fits the projected star density distribution, with a constant inner plateau (at $σ_0 \sim $ 12 km s$^{-1}$) and no evidence of a central cusp or other significant deviations. Our data allowed to study the presence of rotation only in the innermost regions of the cluster (r < 5$"$), revealing a well-defined pattern of ordered rotation with a position angle of the rotation axis of $\sim$132 $\pm$ 2° and an amplitude of $\sim$3 km s$^{-1}$ (corresponding to Vrot/$σ_0 \sim$ 0.3). Also, a flattening of the system qualitatively consistent with the rotation signal has been detected in the central region.

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arXiv2022

The role of rotation on the formation of second generation stars in globular clusters

By means of 3D hydrodynamic simulations, we explore the effects of rotation in the formation of second-generation (SG) stars in globular clusters (GC). Our simulations follow the SG formation in a first-generation (FG) internally rotating GC; SG stars form out of FG asymptotic giant branch (AGB) ejecta and external pristine gas accreted by the system. We have explored two different initial rotational velocity profiles for the FG cluster and two different inclinations of the rotational axis with respect to the direction of motion of the external infalling gas, whose density has also been varied. For a low (10^-24 g cm^-3) external gas density, a disk of SG helium-enhanced stars is formed. The SG is characterized by distinct chemo-dynamical phase space patterns: it shows a more rapid rotation than the FG with the helium-enhanced SG subsystem rotating more rapidly than the moderate helium-enhanced one. In models with high external gas density (10^-23 g cm^-3), the inner SG disc is disrupted by the early arrival of external gas and only a small fraction of highly enhanced helium stars preserves the rotation acquired at birth. Variations in the inclination angle between the rotation axis and the direction of the infalling gas and the velocity profile can slightly alter the extent of the stellar disc and the rotational amplitude. No significant variation has been found in the timespan of our simulations when changing the inclination angle between the rotation axis and the direction of the infalling gas, while different velocity profiles can slightly alter the extent of the stellar disc and the rotational amplitude. The results of our simulations illustrate the complex link between dynamical and chemical properties of multiple populations and provide new elements for the interpretation of observational studies and future investigations of the dynamics of multiple-population GCs.

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arXiv2022

Mass segregation and dynamics of primordial binaries in star clusters with a radially anisotropic velocity distribution

This paper is the third in a series investigating, by means of $N$-body simulations, the implications of an initial radially anisotropic velocity distribution on the dynamics of star clusters. Such a velocity distribution may be imprinted during a cluster's early evolutionary stages and several observational studies have found examples of old globular clusters in which radial anisotropy is still present in the current velocity distribution. Here we focus on its influence on mass segregation and the dynamics of primordial binary stars (disruptions, ejections, and component exchanges). The larger fraction of stars on radial/highly eccentric orbits in the outer regions of anisotropic clusters lead to an enhancement in the dynamical interactions between inner and outer stars that affects both the process of mass segregation and the evolution of primordial binaries. The results of our simulations show that the time scale of mass segregation of the initially anisotropic cluster is longer in the core and shorter in the outer regions, when compared to the initially isotropic system. The evolution of primordial binaries is also significantly affected by the initial velocity distribution and we find that the rate of disruptions, ejections and exchange events affecting the primordial binaries in the anisotropic clusters is higher than in the isotropic ones.

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arXiv2022

Long-term evolution of multimass rotating star clusters

We investigate the long-term dynamical evolution of the internal kinematics of multimass rotating star clusters. We have performed a set of N-body simulations to follow the internal evolution of clusters with different degrees of initial rotation and have explored the evolution of the rotational velocity, the degree of energy equipartition, and anisotropy in the velocity distribution. Our simulations show that: 1) as the cluster evolves, the rotational velocity develops a dependence on the stellar mass with more massive stars characterised by a more rapid rotation and a peak in the rotation curve closer to the cluster centre than low-mass stars; 2) the degree of energy equipartition in the cluster's intermediate and outer regions depends on the component of the velocity dispersion measured; for more rapidly rotating clusters, the evolution towards energy equipartition is more rapid in the direction of the rotational velocity; 3) the anisotropy in the velocity distribution is stronger for massive stars; 4) both the degree of mass segregation and energy equipartition are characterised by spatial anisotropy; they have a dependence on both $R$ and $z$, correlated with the flattening in the spatial variation of the cluster's density and velocity dispersion, as shown by 2D maps of the mass segregation and energy equipartition on the ($R$-$z$) meridional plane.

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arXiv2022

Early dynamical evolution of rotating star clusters in a tidal field

In order to explore how the early internal rotational properties of star clusters are affected by the external potential of their host galaxies, we have run a suite of $N$-body simulations following the early dynamical evolution and violent relaxation of rotating star clusters embedded in a tidal field. Our study focuses on models for which the cluster's rotation axis has a generic orientation relative to the torque of the tidal field. The interaction between the violent relaxation process, angular momentum of the cluster, and the external torque creates a complex kinematic structure within the cluster, most prominently a radial variation in the position of the rotation axis along both the polar and azimuthal directions. We also examine the cluster's velocity dispersion anisotropy and show that the projected anisotropy may be affected by the variation of the rotation axis directions within the cluster; the combination of projection effects and the complex kinematical features may result in the measurement of tangential anisotropy in the cluster's inner regions. We also characterize the structural properties of our clusters as a function of their initial rotation and virial ratio and find that clusters may develop a triaxial morphology and a radial variation of the minor axis not necessarily aligned with the rotation axis. Finally, we examine the long-term evolution of these complex kinematic features.

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arXiv2022

HST observations of the globular cluster NGC 6402 (M14) and its peculiar multiple populations

We present Hubble Space Telescope (HST) photometric results for NGC 6402, a highly reddened very luminous Galactic globular cluster (GC). Recent spectroscopic observations of its red giant stars have shown a quite peculiar behavior in the chemistry of its multiple populations. These results have prompted UV and optical HST observations aimed at obtaining the cluster's "Chromosome map" (ChM), an efficient tool to classify GCs and characterize their multiple populations. We find that the discontinuity in the abundance distributions of O, Mg, Al and Na inferred from spectroscopy is more nuanced in the ChM, which is mostly sensitive to nitrogen. Nevertheless, photometry in optical bands reveals a double main sequence, indicating a discontinuity in the helium content of the populations. The population with the largest chemical anomalies (extreme) peaks at a helium mass fraction Y~0.31. This helium content is consistent with results from the analysis of the distribution of horizontal-branch stars and the spectrophotometry of the red giants. The ChM and the color magnitude diagrams are compared with those in NGC 2808, a prototype GC with helium abundances up to Y > 0.35, and both confirm that NGC 6402 does not host stellar populations with such extreme helium content. Further, the ChM reveals the presence of a group of stars with larger metallicity, thus indicating that NGC 6402 is a Type II cluster. The modalities of formation of the multiple populations in NGC 6402 are briefly surveyed, with main attention on the Asymptotic Giant Branch and Supermassive star models, and on possible clusters' merging.

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arXiv2021

Evolution towards energy equipartition in star clusters: effects of the tidal field, primordial binaries, and internal velocity anisotropy

This paper is the second in a series investigating the evolution of star clusters towards energy equipartition (EEP). Here, we focus on the effects of the external tidal field of the host galaxy, initial anisotropy in the velocity distribution, and primordial binary star population. The results of our $N$-body simulations show that regardless of the strength of the tidal field or the fraction of primordial binaries: (i) the evolution towards EEP in the intermediate and outer regions of initially anisotropic systems is more rapid than for isotropic systems; (ii) this evolution also proceeds at different rates for the tangential and radial components of the velocity dispersion; and (iii) the outer regions of the initially isotropic systems show a tendency to evolve towards a state of `inverted' EEP in which low-mass stars have smaller velocity dispersion than high-mass stars. We also find that the clusters with primordial binaries stay even farther from EEP than systems containing only single stars. Finally, we show that all these results also hold when the degree of EEP is calculated using quantities measured in projection as it is done in observational studies, and that our findings could be tested with current and upcoming observational data.

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arXiv2021

On the role of Type Ia supernovae in the second generation star formation in globular clusters

By means of 3D hydrodynamic simulations, we study how Type Ia supernovae (SNe) explosions affect the star formation history and the chemical properties of second generation (SG) stars in globular clusters (GC). SG stars are assumed to form once first generation asymptotic giant branch (AGB) stars start releasing their ejecta; during this phase, external gas is accreted by the system and SNe Ia begin exploding, carving hot and tenuous bubbles. Given the large uncertainty on SNe Ia explosion times, we test two different values for the 'delay time'. We run two different models for the external gas density: in the low-density scenario with short delay time, the explosions start at the beginning of the SG star formation, halting it in its earliest phases. The external gas hardly penetrates the system, therefore most SG stars present extreme helium abundances (Y > 0.33). The low-density model with delayed SN explosions has a more extended SG star formation epoch and includes SG stars with modest helium enrichment. On the contrary, the high-density model is weakly affected by SN explosions, with a final SG mass similar to the one obtained without SNe Ia. Most of the stars form from a mix of AGB ejecta and pristine gas and have a modest helium enrichment. We show that gas from SNe Ia may produce an iron spread of $\sim 0.14$ dex, consistent with the spread found in about 20% of Galactic GCs, suggesting that SNe Ia might have played a key role in the formation of this sub-sample of GCs.

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arXiv2021

A new identity card for the bulge globular cluster NGC 6440 from resolved star counts

We present a new identity card for the cluster NGC 6440 in the Galactic Bulge. We have used a combination of high-resolution Hubble Space Telescope images, wide-field ground-based observations performed with the ESO-FORS2, and the public survey catalog Pan-STARRS, to determine the gravitational center, projected density profile and structural parameters of this globular from resolved star counts. The new determination of the cluster center differs by ~ 2" (corresponding to 0.08 pc) from the previous estimate, which was based on the surface brightness peak. The star density profile, extending out to 700" from the center and suitably decontaminated from the Galactic field contribution, is best-fitted by a King model with significantly larger concentration ($c=1.86\pm0.06$) and smaller core radius ($r_c=6.4"\pm0.3"$) with respect to the literature values. By taking advantage of high-quality optical and near-infrared color-magnitude diagrams, we also estimated the cluster age, distance and reddening. The luminosity of the RGB-bump was also determined. This study indicates that the extinction coefficient in the bulge, in the direction of the cluster has a value ($R_V=2.7$) that is significantly smaller than that traditionally used for the Galaxy ($R_V=3.1$). The corresponding best-fit values of the age, distance and color excess of NGC 6440 are 13 Gyr, 8.3 kpc and $E(B-V)\sim 1.27$, respectively. These new determinations also allowed us to update the values of the central ($t_{rc}=2.5\ 10^7$ yr) and half-mass ($t_{rh}=10^9$ yr) relaxation times, suggesting that NGC 6440 is in a dynamically evolved stage.

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arXiv2021

Light Element Abundances and Multiple Populations in M53

We present results from a study of 94 red giant stars in the globular cluster M53. We use low-resolution spectra to measure the strength of CN and CH features at $\sim$3800 and 4300 Å, respectively. The strengths of these features are used to classify stars into a CN-enhanced and CN-normal population and to measure C and N abundances in all 94 stars. We find the red giant branch stars to be evenly split between the two populations identified, and observe the presence of CN-enhanced stars on the asymptotic giant branch. In addition, we identify 5 CH star candidates based on the strength of their CN and CH band features, and the presence of a P-branch in their CH band. We compare our identification of multiple populations to those based on the Na-O anti-correlation and pseudo-color indices in HST UV photometry, and find general agreement between all three methods. Our large sample size also allows us to study the radial distribution of each population, and we find that the CN-enhanced population is more centrally concentrated. We use our C and N measurements to compare the evolutionary changes in these elements as a function of magnitude between the two populations, and show that both populations experience similar evolutionary changes to the surface abundances of C and N. Finally, we calculate C+N+O abundances for each population and compare them to similar measurements made in M10; we find that in both clusters CN-enhanced stars have a slightly enhanced C+N+O ($Δ$(C+N+O) $\sim$ 0.2 dex).

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