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Barbara Lanzoni

Publications and source records attributed to Barbara Lanzoni.

At least 37 records · Page 2Linked to original sources
arXiv2023

Extended Main-Sequence Turnoff and Red Clump in intermediate-age star clusters: A study of NGC 419

With the goal of untangling the origin of extended main-sequence turnoffs (eMSTOs) and extended red clumps (eRCs) in star clusters, in this work we present the study of the intermediate-age cluster NGC 419, situated along the Bridge of the Small Magellanic Cloud. To this aim, we analyzed multi-epoch, high angular resolution observations acquired with the Hubble Space Telescope for this dynamically young cluster, which enabled the determination of precise proper motions and therefore the assessment of the cluster membership for each individual star in the field of view. With this unprecedented information at hand, we first studied the radial distribution of kinematically selected member stars in different eMSTO subregions. The absence of segregation supports the rotation scenario as the cause for the turnoff color extension and disfavors the presence of a prolonged period of star formation in the cluster. A similar analysis on the eRC of NGC 419 confirms the absence of segregation, providing further evidence against an age spread, which is at odds with previous investigations. Even so, the currently available evolutionary models including stellar rotation fail at reproducing the two photometric features simultaneously. We argue that either shortcomings in these models or a different origin for the red clump feature, such as a nonstandard differential mass loss along the red giant branch phase, are the only way to reconcile our observational findings with theoretical expectations.

astro-ph.GA↗
arXiv2023

Fast rotating Blue Straggler Stars in the Globular Cluster NGC3201

We used high resolution spectra acquired at the Magellan Telescope to measure radial and rotational velocities of approximately 200 stars in the Galactic globular cluster NGC 3201. The surveyed sample includes Blue Stragglers Stars (BSSs) and reference stars in different evolutionary stages (main sequence turn-off, sub-giant, red giant and asymptotic giant branches). The average radial velocity value ($\langle V_r\rangle = 494.5 \pm 0.5$ km s$^{-1}$) confirms a large systemic velocity for this cluster and was used to distinguish 33 residual field interlopers. The final sample of member stars counts 67 BSSs and 114 reference stars. Similarly to what is found in other clusters, the totality of the reference stars has negligible rotation ($<20$ km s$^{-1}$), while the BSS rotational velocity distribution shows a long tail extending up to $\sim 200$ km s$^{-1}$, with 19 BSSs (out of 67) spinning faster than 40 km s$^{-1}$. This sets the percentage of fast rotating BSSs to $\sim 28\%$. Such a percentage is roughly comparable to that measured in other loose systems ($ω$ Centauri, M4 and M55) and significantly larger than that measured in high-density clusters (as 47 Tucanae, NGC 6397, NGC 6752 and M30). This evidence supports a scenario where recent BSS formation (mainly from the evolution of binary systems) is occurring in low-density environments. We also find that the BSS rotational velocity tends to decrease for decreasing luminosity and surface temperature, similarly to what is observed in main sequence stars. Hence, further investigations are needed to understand the impact of BSS internal structure on the observed rotational velocities.

astro-ph.SR↗
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.

astro-ph.GA↗
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.

astro-ph.GA↗
arXiv2023

Modeling the chemical enrichment history of the Bulge Fossil Fragment Terzan 5

Terzan 5 is a heavily obscured stellar system located in the inner Galaxy. It has been postulated to be a stellar relic, a Bulge Fossil Fragment witnessing the complex history of the assembly of the Milky Way bulge. In this paper, we follow the chemical enrichment of a set of putative progenitors of Terzan 5 to assess whether the chemical properties of this cluster fit within a formation scenario in which it is the remnant of a primordial building block of the bulge. We can explain the metallicity distribution function and the runs of different element-to-iron abundance ratios as functions of [Fe/H] derived from optical-infrared spectroscopy of giant stars in Terzan 5, by assuming that the cluster experienced two major star formation bursts separated by a long quiescent phase. We further predict that the most metal-rich stars in Terzan 5 are moderately He-enhanced and a large spread of He abundances in the cluster, Y = 0.26-0.335. We conclude that current observations fit within a formation scenario in which Terzan 5 originated from a pristine, or slightly metal-enriched, gas clump about one order of magnitude more massive than its present-day mass. Losses of gas and stars played a major role in shaping Terzan 5 the way we see it now. The iron content of the youngest stellar population is better explained if the white dwarfs that give rise to type Ia supernovae (the main Fe factories) sink towards the cluster center, rather than being stripped by the strong tidal forces exerted by the Milky Way in the outer regions.

astro-ph.GA↗
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.

astro-ph.SR↗
arXiv2023

First evidence of multi-iron sub-populations in the Bulge Fossil Fragment candidate Liller 1

In the context of a project aimed at characterizing the properties of the so-called Bulge Fossil Fragments (the fossil remnants of the bulge formation epoch), here we present the first determination of the metallicity distribution of Liller 1. For a sample of 64 individual member stars we used ESO- MUSE spectra to measure the equivalent width of the CaII triplet and then derive the iron abundance. To test the validity of the adopted calibration in the metal-rich regime, the procedure was first applied to three reference bulge globular clusters (NGC 6569, NGC 6440, and NGC 6528). In all the three cases, we found single-component iron distributions, with abundance values fully in agreement with those reported in the literature. The application of the same methodology to Liller 1 yielded, instead, a clear bimodal iron distribution, with a sub-solar component at $\text{[Fe/H]}= -0.48\,$dex ($σ= 0.22$) and a super-solar component at $\text{[Fe/H]}= +0.26\,$dex ($σ= 0.17$). The latter is found to be significantly more centrally concentrated than the metal-poor population, as expected in a self-enrichment scenario and in agreement with what found in another bulge system, Terzan 5. The obtained metallicity distribution is astonishingly similar to that predicted by the reconstructed star formation history of Liller 1, which is characterized by three main bursts and a low, but constant, activity of star formation over the entire lifetime. These findings provide further support to the possibility that, similar to Terzan 5, also Liller 1 is a Bulge Fossil Fragment.

astro-ph.GA↗
arXiv2023

The "canonical" White Dwarf Cooling Sequence of M5

Recently, a new class of white dwarfs (dubbed ``slowly cooling WDs'') has been identified in two globular clusters (namely M13 and NGC 6752) showing a horizontal branch (HB) morphology with an extended blue tail. The cooling rate of these WDs is reduced by stable thermonuclear hydrogen burning in their residual envelope, and they are thought to be originated by stars that populate the blue tail of the HB and then skip the asymptotic giant branch phase. Consistently, no evidence of such kind of WDs has been found in M3, a similar cluster with no blue extension of the HB. To further explore this phenomenon, we took advantage of deep photometric data acquired with the Hubble Space Telescope in the near-ultraviolet and investigate the bright portion of the WD cooling sequence in M5, another Galactic globular cluster with HB morphology similar to M3. The normalized WD luminosity function derived in M5 turns out to be impressively similar to that observed in M3, in agreement with the fact that the stellar mass distribution along the HB of these two systems is almost identical. The comparison with theoretical predictions is consistent with the fact that the cooling sequence in this cluster is populated by canonical (fast cooling) WDs. Thus, the results presented in this paper provide further support to the scenario proposing a direct causal connection between the slow cooling WD phenomenon and the horizontal branch morphology of the host stellar cluster.

astro-ph.SR↗
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.

astro-ph.GA↗
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.

astro-ph.GA↗
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.

astro-ph.GA↗
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.

astro-ph.GA↗
arXiv2022

Digging into the Galactic Bulge: stellar population and structure of the poorly studied cluster NGC 6316

High-resolution Hubble Space Telescope optical observations have been used to analyze the stellar population and the structure of the poorly investigated bulge globular cluster NGC 6316. We constructed the first high-resolution reddening map in the cluster direction, which allowed us to correct the evolutionary sequences in the color magnitude diagram (CMD) for the effects of differential reddening. A comparison between the CMDs of NGC 6316 and 47 Tucanae revealed strikingly similar stellar populations, with the two systems basically sharing the same turn-off, sub-giant branch, and horizontal branch morphologies, indicating comparable ages. The red giant branch in NGC 6316 appears slightly bluer than in 47 Tucanae, suggesting a lower metal content. This has been confirmed by the isochrone fitting of the observed CMD, which provided us with updated values of the cluster age, distance, average color excess, and metallicity. We estimated an absolute age of 13.1 $\pm$ 0.5 Gyr, consistent with the age of 47 Tucanae, an average color excess E(B-V) = 0.64 $\pm$ 0.01, and a true distance modulus (m-M)0 = 15.27 $\pm$ 0.03 that sets the cluster distance at 11.3 kpc from the Sun. In addition, the photometric estimate of the cluster metallicity suggests [Fe/H]$\approx$ -0.9, which is $\sim$ 0.2 dex smaller than that of 47 Tucanae. We also determined the gravitational center and the density profile of the system from resolved stars. The latter is well reproduced by a King model. Our results confirm that NGC 6316 is another extremely old relic of the assembly history of the Galaxy.

astro-ph.GA↗
arXiv2022

Discovery of a double sequence of blue straggler stars in the core-collapsed globular cluster NGC 6256

We used a combination of high-resolution optical images acquired with the Hubble Space Telescope and near-IR wide-field data to investigate the stellar density profile and the population of blue straggler star (BSS) in the Galactic globular cluster NGC6256, with the aim of probing its current stage of internal dynamical evolution. We found that the inner stellar density profile significantly deviates from a King model while is well reproduced by a steep cusp with a power-law slope alpha=-0.89, thus implying that the cluster is currently in the post core-collapse (PCC) phase. This is also confirmed by the very high segregation level of the BSS population measured through the A+ parameter. We also found that the distribution of BSSs in the color-magnitude diagram is characterized by a collimated blue sequence and a red more sparse component, as already observed in other three PCC clusters. The comparison with appropriate collisional models demonstrates that the vast majority of the BSSs lying along the collimated blue sequence is consistent with a generation of coeval (1 Gyr-old) stars with different masses originated by an event that highly enhanced the collisional rate of the system (i.e. the core collapse). This study confirms that the segregation level of BSSs is a powerful dynamical diagnostic also of star cluster in a very advanced stage of dynamical evolution. Moreover, it pushes forward the possibility of using the morphology of the BSS in the color-magnitude diagram as a tracer of the core-collapse and subsequent dynamical evolutionary phases.

astro-ph.SR↗
arXiv2022

Blue Stragglers as tracers of the dynamical state of two clusters in the Small Magellanic Cloud: NGC 339 and NGC 419

The level of central segregation of Blue Straggler stars proved to be an excellent tracer of the dynamical evolution of old star clusters (the so-called "dynamical clock"), both in the Milky Way and in the Large Magellanic Cloud. The $A^{+}$ parameter, used to measure the Blue Stragglers degree of segregation, has in fact been found to strongly correlate with the parent cluster central relaxation time. Here we studied the Blue-Straggler population of two young stellar systems in the Small Magellanic Cloud, namely NGC 339 (which is 6 Gyr old) and NGC 419 (with an age of only 1.5 Gyr), in order to study their dynamical state. Thanks to multi-epoch, high angular resolution Hubble Space Telescope observations available for both clusters, we took advantage of the stellar proper motions measured in the regions of the two systems and we selected a population of likely cluster members, removing the strong contamination from Small Magellanic Cloud stars. This enabled us to study, with unprecedented accuracy, the radial distribution of Blue Stragglers in these two extragalactic clusters and to measure their dynamical age. As expected for such young clusters, we found that both systems are poorly evolved from the dynamical point of view, also fully confirming that the $A^{+}$ parameter is a sensitive "clock hand" even in the dynamically-young regime.

astro-ph.GA↗
arXiv2021

High-resolution extinction map in the direction of the strongly obscured bulge fossil fragment Liller 1

We used optical images acquired with the Wide Field Camera of the Advanced Camera for Surveys onboard the Hubble Space Telescope and near-infrared data from GeMS/GSAOI to construct a high-resolution extinction map in the direction of the bulge stellar system Liller 1. In spite of its appearance of a globular cluster, Liller 1 has been recently found to harbor two stellar populations with remarkably different ages, and it is the second complex stellar system with similar properties (after Terzan5) discovered in the bulge, thus defining a new class of objects: the Bulge Fossil Fragments. Because of its location in the inner bulge of the Milky Way, very close to the Galactic plane, Liller 1 is strongly affected by large and variable extinction. The simultaneous study of both the optical and the near-infrared color-magnitude diagrams revealed that the extinction coefficient R$_V$ in the direction of Liller 1 has a much smaller value than commonly assumed for diffuse interstellar medium (R$_V=2.5$, instead of 3.1), in agreement with previous findings along different light paths to the Galactic bulge. The derived differential reddening map has a spatial resolution ranging from $1''$ to $3''$ over a field of view of about $90''$X$90''$. We found that the absorption clouds show patchy sub-structures with extinction variations as large as $δ{\rm E}(B-V)\sim0.9$ mag.

astro-ph.GA↗
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.

astro-ph.GA↗
arXiv2020

PSR J1641+3627F: a low-mass He white dwarf orbiting a possible high-mass neutron star in the globular cluster M13

We report on the discovery of the companion star to the millisecond pulsar J1631+3627F in the globular cluster M13. By means of a combination of optical and near-UV high-resolution observations obtained with the Hubble Space Telescope, we identified the counterpart at the radio source position. Its location in the color-magnitude diagrams reveals that the companion star is a faint (V \sim 24.3) He-core white dwarf. We compared the observed companion magnitudes with those predicted by state-of-the-art binary evolution models and found out that it has a mass of 0.23 \pm 0.03 Msun, a radius of 0.033^+0.004_-0.005 Rsun and a surface temperature of 11500^+1900_-1300 K. Combining the companion mass with the pulsar mass function is not enough to determine the orbital inclination and the neutron star mass; however, the last two quantities become correlated: we found that either the system is observed at a low inclination angle, or the neutron star is massive. In fact, assuming that binaries are randomly aligned with respect to the observer line of sight, there is a \sim 70% of probability that this system hosts a neutron star more massive than 1.6 Msun. In fact, the maximum and median mass of the neutron star, corresponding to orbital inclination angles of 90 deg and 60 deg, are M_NS,max = 3.1 \pm 0.6 Msun and M_NS,med = 2.4 \pm 0.5 Msun, respectively. On the other hand, assuming also an empirical neutron star mass probability distribution, we found that this system could host a neutron star with a mass of 1.5 \pm 0.1 Msun if orbiting with a low-inclination angle around 40 deg.

astro-ph.HE↗