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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., Comment: Accepted for publication in ApJ

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"

Globular clusters (GCs) are traditionally described as simple quasi-relaxed non-rotating stellar systems, characterized by spherical symmetry and isotropy in velocity space. However, recent studies have shown deviations from isotropic velocity distributions and significant internal rotation in many GCs, suggesting that their internal structure and kinematics are more complex than previously thought. The aim of this thesis is to investigate the internal kinematics of Galactic Globular Clusters (GGCs) as part of the Multi-Instrument Kinematic Survey (MIKiS), which exploits the capabilities of different ESO-VLT spectrographs to obtain comprehensive velocity dispersion (VD) and rotation profiles of GGCs. Moreover, this thesis has the particular goal of unraveling the kinematics of GC cores, which are still largely unexplored, by taking advantage of the exceptional spatial resolution of the adaptive-optics assisted integral-field spectrograph MUSE/NFM. The thesis presents a thorough kinematic study of three GGCs NGC 1904, NGC 6440, and NGC 6569. By combining the data sets acquired with four different spectrographs, we obtained the radial velocity (RV) of more than 1000 individual stars in each cluster, sampling from the innermost to the outermost regions. This allowed us to obtain the entire VD profile of each cluster and exclude the presence of an intermediate-mass black hole in the core of NGC 1904, at odds with previous findings obtained from integrated-light spectra. The studies also revealed signatures of internal rotation in each of the GCs studied. These results, supported by those of N-body simulations, prove that GCs were born with a significant initial rotation that they gradually lost through internal two-body relaxation and angular momentum loss carried away by escaping stars. Furthermore, we derived the structural parameters of NGC 6440 and NGC 6569, obtaining a comprehensive overview of the internal kinematics and structure of these GCs, which is necessary

We used ultra-deep observations obtained with the NIRCam aboard the James Webb Space Telescope to explore the stellar population of NGC 6440: a typical massive, obscured and contaminated globular cluster formed and orbiting within the Galactic bulge. Leveraging the exceptional capabilities of this camera, we sampled the cluster down to ~5 magnitudes below the main-sequence turn-off in the (mF115W , mF115W - mF200W ) colour-magnitude diagram. After carefully accounting for differential extinction and contamination by field interlopers, we find that the main sequence splits into two branches both above and below the characteristic knee. By comparing the morphology of the colour-magnitude diagram with a suitable set of isochrones, we argue that the upper main-sequence bi-modality is likely due to the presence of a He-enriched stellar population with a helium spread of DeltaY = 0.04. The lower main-sequence bi-modality can be attributed to variations in the abundance of water (i.e., oxygen) with Delta[O/Fe] ~ -0.4. This is the first evidence of both helium and oxygen abundance variations in a globular cluster purely based on JWST observations. These results open the window for future in-depth investigations of the multiple population phenomenon in clusters located in the Galactic bulge, which were previously unfeasible with near-UV observations, due to prohibitive reddening and crowding conditions., Comment: Accepted for publication in A&A

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 ${\sigma}_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{\deg} and an amplitude of $\sim$3 km s$^{-1}$ (corresponding to Vrot/${\sigma}_0 \sim$ 0.3). Also, a flattening of the system qualitatively consistent with the rotation signal has been detected in the central region., Comment: 20 pages, 11 figures, 4 tables, accepted for publication in ApJ

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., Comment: Accepted for publication in The Astrophysical Journal; 19 pages, 9 figures and 2 tables

Questo elaborato di tesi si inserisce nel contesto della ricerca di stelle Evolved Blue Straggler Stars (E-BSS) in ammassi globulari Galattici. Le BSS sono ritenute essere significativamente più massicce delle stelle che tipicamente popolano un ammasso stellare. Questi oggetti sono facilmente identificabili durante la loro fase di bruciamento dell'idrogeno, al contrario è impossibile distinguerli fotometricamente, in modo certo, dalle altre stelle di minore massa del sistema che li ospita durante le fasi evolutive successive. In questo lavoro è stato applicato il metodo spettroscopico utilizzato da Ferraro et al. (2016) per identificare una E-BSS nell'ammasso globulare 47 Tuc e basato sul confronto tra le abbondanze chimiche derivate da righe spettrali neutre e ionizzate. Tale metodo permette di ricavare una stima della gravità a superficiale e, quindi, della massa stellare. In particolare, sono stati analizzati spettri ad alta risoluzione, acquisiti con lo spettrografo ad alta risoluzione UVES, di un campione 13 stelle candidate E-BSS dell'ammasso globulare M80 situate nella regione del CMD tra l'HB e l'attacco dell'AGB. I principali risultati di quest'analisi hanno mostrato che nel campione analizzato non vi sono E-BSS particolarmente massicce e, inoltre, hanno evidenziato una sottostima sistematica dell'abbondanza chimica ricavata da righe neutre in tutte le stelle di post-RGB, mentre le righe ionizzate risultano il diagnostico più u robusto ed affidabile per derivare le abbondanze chimiche.

Questo elaborato di tesi si inserisce nel contesto della ricerca di stelle Evolved Blue Straggler Stars (E-BSS) in ammassi globulari Galattici. Le BSS sono ritenute essere significativamente più massicce delle stelle che tipicamente popolano un ammasso stellare. Questi oggetti sono facilmente identificabili durante la loro fase di bruciamento dell'idrogeno, al contrario è impossibile distinguerli fotometricamente, in modo certo, dalle altre stelle di minore massa del sistema che li ospita durante le fasi evolutive successive. In questo lavoro è stato applicato il metodo spettroscopico utilizzato da Ferraro et al. (2016) per identificare una E-BSS nell'ammasso globulare 47 Tuc e basato sul confronto tra le abbondanze chimiche derivate da righe spettrali neutre e ionizzate. Tale metodo permette di ricavare una stima della gravità a superficiale e, quindi, della massa stellare. In particolare, sono stati analizzati spettri ad alta risoluzione, acquisiti con lo spettrografo ad alta risoluzione UVES, di un campione 13 stelle candidate E-BSS dell'ammasso globulare M80 situate nella regione del CMD tra l'HB e l'attacco dell'AGB. I principali risultati di quest'analisi hanno mostrato che nel campione analizzato non vi sono E-BSS particolarmente massicce e, inoltre, hanno evidenziato una sottostima sistematica dell'abbondanza chimica ricavata da righe neutre in tutte le stelle di post-RGB, mentre le righe ionizzate risultano il diagnostico più u robusto ed affidabile per derivare le abbondanze chimiche.