arXiv subjects

Barbara Lanzoni

Publications and source records attributed to Barbara Lanzoni.

At least 55 records · Page 3Linked to original sources

arXiv2020

Digging for relics of the past: the ancient and obscured bulge globular cluster NGC6256

We used a set of moderately-deep and high-resolution optical observations obtained with the Hubble Space Telescope to investigate the properties of the stellar population in the heavily obscured bulge globular cluster NGC 6256. The analysis of the color-magnitude diagram revealed a stellar population with an extended blue horizontal branch and severely affected by differential reddening, which was corrected taking into account color excess variations up to δE(B-V) ~ 0.51. We implemented a Monte Carlo Markov Chain technique to perform the isochrone fitting of the observed color-magnitude diagram in order to derive the stellar age, the cluster distance and the average color excess in the cluster direction. Using different set of isochrones we found that NGC 6256 is characterized by a very old stellar age around 13.0 Gyr, with a typical uncertainty of ~ 0.5 Gyr. We also found an average color excess E(B-V) = 1.19 and a distance from the Sun of 6.8 kpc. We then derived the cluster gravitational center and measured its absolute proper motion using the Gaia-DR2 catalog. All this was used to back-integrate the cluster orbit in a Galaxy-like potential and measure its integrals of motion. It turned out that NGC 6256 is currently in a low-eccentricity orbit entirely confined within the bulge and its integrals of motion are fully compatible with a cluster purely belonging to the Galaxy native globular cluster population. All these pieces of evidence suggest that NGC 6256 is an extremely old relic of the past history of the Galaxy, formed during the very first stages of its assembly.

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arXiv2019

High-resolution extinction map in the direction of the bulge globular cluster NGC 6440

We used optical images acquired with the UVIS channel of the Wide Field Camera 3 on board of the Hubble Space Telescope to construct the first high-resolution extinction map in the direction of NGC 6440, a globular cluster located in the bulge of our Galaxy. The map has a spatial resolution of 0.5" over a rectangular region of about 160" X 240" around the cluster center, with the long side in the North-West/South-East direction. We found that the absorption clouds show patchy and filamentary sub-structures with extinction variations as large as $δ{\rm E}(B-V)\sim0.5$ mag. We also performed a first-order proper motion analysis to distinguish cluster members from field interlopers. After the field decontamination and the differential reddening correction, the cluster sequences in the color-magnitude diagram appear much better defined, providing the best optical color-magnitude diagram so far available for this cluster.

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arXiv2019

Discovery of a double Blue Straggler sequence in M15: new insight into the core-collapse process

In this paper we report on the discovery of a double blue straggler star (BSS) sequence in the core of the core-collapsed cluster M15 (NGC 7078). We performed a detailed photometric analysis of the extremely dense core of the cluster using a set of images secured with the Advanced Camera for Survey in the High Resolution Channel mode on-board the Hubble Space Telescope. The proper combination of the large number of single frames in the near-UV (F220W), and blue (F435W) filters allowed us to perform a superb modeling of the Point Spread Function and an accurate deblending procedure. The Color-Magnitude diagram revealed the presence of two distinct parallel sequences of blue stragglers. In particular, the blue BSS sequence is characterized by the intriguing presence of two different branches. The first branch appears extremely narrow, it extends up to 2.5 magnitudes brighter than the cluster main-sequence turnoff (MS-TO) point, and it is nicely reproduced by a 2 Gyr-old collisional isochrone. The second branch extends up to 1.5 magnitudes from the MS-TO and it is reproduced by a 5.5 Gyr-old collisional isochrone. Our observations suggest that each of these branches is mainly constituted by a population of nearly coeval collisional BSS of different masses generated during two episodes of high collisional activity. We discuss the possibility that the oldest episode corresponds to the core-collapse event (occurred about 5.5 Gyr ago), while the most recent one (occurred about 2 Gyr ago) is associated with a core oscillation in the post-core collapse evolution. The discovery of these features provides further strong evidence in support of the connection between the BSS properties and GC dynamical evolution, and it opens new perspectives on the study of core-collapse and post core-collapse evolution.

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arXiv2017

Modelling the Observed Stellar Mass Function and its Radial Variation in Galactic Globular Clusters

We measure how the slope $α$ of the stellar mass function (MF) changes as a function of clustercentric distance $r$ in five Galactic globular clusters and compare $α(r)$ to predictions from direct $N$-body star cluster simulations. Theoretical studies predict that $α(r)$ (which traces the degree of mass segregation in a cluster) should steepen with time as a cluster undergoes two-body relaxation and that the amount by which the global MF can evolve from its initial state due to stellar escape is directly linked to $α(r)$. We find that the amount of mass segregation in M10, NGC 6218, and NGC 6981 is consistent with their dynamical ages, but only the global MF of M10 is consistent with its degree of mass segregation as well. NGC 5466 and NGC 6101 on the other hand appear to be less segregated than their dynamical ages would indicate. Furthermore, despite the fact that the escape rate of stars in non-segregated clusters is independent of stellar mass, both NGC 5466 and NGC 6101 have near-flat MFs. We discuss various mechanisms which could produce non-segregated clusters with near-flat MFs, including higher mass-loss rates and black hole retention, but argue that for some clusters (NGC 5466 and NGC 6101) explaining the present-day properties might require either a non-universal IMF or a much more complex dynamical history.

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arXiv2017

A complete census of $Hα$ emitters in NGC 6397

We used a dataset of archival Hubble Space Telescope images obtained through the F555W, F814W and F656N filters, to perform a complete search for objects showing $Hα$ emission in the globular cluster NGC 6397. As photometric diagnostic, we used the $(V-Hα)_0$ color excess in the $(V-Hα)_0$-$(V-I)_0$ color-color diagram. In the analysed field of view, we identified 53 $Hα$ emitters. In particular, we confirmed the optical counterpart to 20 X-ray sources (7 cataclysmic variables, 2 millisecond pulsars and 11 active binaries) and identified 33 previously unknown sources, thus significantly enlarging the population of known active binaries in this cluster. We report the main characteristics for each class of objects. Photometric estimates of the equivalent width of the $Hα$ emission line, were derived from the $(V-Hα)_0$-excess and, for the first time, compared to the spectroscopic measurements obtained from the analysis of MUSE spectra. The very good agreement between the spectroscopic and photometric measures fully confirmed the reliability of the proposed approach to measure the $Hα$ emission. The search demonstrated the efficiency of this novel approach to pinpoint and measure $Hα$-emitters, thus offering a powerful tool to conduct complete census of objects whose formation and evolution can be strongly affected by dynamical interactions in star clusters.

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arXiv2016

Investigating mass segregation process in globular clusters with Blue Straggler Stars: the impact of dark remnants

We present the results of a set of N-body simulations aimed at exploring how the process of mass segregation (as traced by the spatial distribution of blue straggler stars, BSSs) is affected by the presence of a population of heavy dark remnants (as neutron stars and black holes). To this end, clusters characterized by different initial concentrations and different fractions of dark remnants have been modeled. We find that an increasing fraction of stellar-mass black holes significantly delays the mass segregation of BSSs and the visible stellar component. In order to trace the evolution of BSS segregation, we introduce a new parameter ($A^+$) that can be easily measured when the cumulative radial distribution of these stars and a reference population are available. Our simulations show that $A^+$ might also be used as an approximate indicator of the time remaining to the core collapse of the visible component.

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arXiv2015

Deep multi-telescope photometry of NGC 5466. II. The radial behaviour of the mass function slope

We use a combination of data acquired with the Advanced Camera for Survey (ACS) on board the Hubble Space Telescope and the Large Binocular Camera (LBC-blue) mounted on the Large Binocular Telescope, to sample the main sequence stars of the globular cluster NGC~5466 in the mass range $0.3<M/M_\odot<0.8$. We derive the cluster's Luminosity Function in several radial regions, from the center of the cluster out to the tidal radius. After corrections for incompleteness and field-contamination, this has been compared to theoretical Luminosity Functions, obtained by multiplying a simple power law Mass Function in the form dN/dm$ \propto m^α$ by the derivative of the mass-luminosity relationship of the best-fit isochrone. We find that $α$ varies from -0.6 in the core region to -1.9 in the outer region. This fact allows us to observationally prove that the stars in NGC 5466 have experienced the effects of mass segregation. We compare the radial variation of $α$ from the center out to 5 core radii (r$_c$) in NGC 5466 and the globular cluster M10, finding that the gradient of $α$ in the first 5r$_c$ is more than a factor of 2 shallower in NGC 5466 than in M10, in line with the differences in the clusters' relaxation timescales. NGC 5466 is dynamically younger than M10, with two-body relaxation processes only recently starting to shape the distribution of main sequence stars. This result fully agrees with the conclusion obtained in our previous works on the radial distribution of Blue Straggler Stars, further confirming that this can be used as an efficient clock to measure the dynamical age of stellar systems.

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arXiv2015

Searching for IMBHs in Galactic globular clusters through radial velocities of individual stars

I present an overview of our ongoing project aimed at building a new generation of velocity dispersion profiles ad rotation curves for a representative sample of Galactic globular clusters, from the the radial velocity of hundreds individual stars distributed at different distances from the cluster center. The innermost portion of the profiles will be used to constrain the possibile presence of intermediate-mass black holes. The adopted methodology consists in combining spectroscopic observations acquired with three different instruments at the ESO-VLT: the adaptive-optics assisted, integral field unit (IFU) spectrograph SINFONI for the innermost and highly crowded cluster cores, the multi-IFU spectrograph KMOS for the intermediate regions, and the multi-fiber instrument FLAMES/GIRAFFE-MEDUSA for the outskirts. The case of NGC 6388, representing the pilot project that motivated the entire program, is described in some details.

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arXiv2014

Radial velocities from VLT-KMOS spectra of giant stars in the globular cluster NGC 6388

We present new radial velocity measurements for 82 stars, members of the Galactic globular cluster NGC 6388, obtained from ESO-VLT KMOS spectra acquired during the instrument Science Verification. The accuracy of the wavelength calibration is discussed and a number of tests of the KMOS response are presented. The cluster systemic velocity obtained (81.3 +/- 1.5 km/sec) is in very good agreement with previous determinations. While a hint of ordered rotation is found between 9'' and 20'' from the cluster centre, where the distribution of radial velocities is clearly bimodal, more data are needed before drawing any firm conclusions. The acquired sample of radial velocities has been also used to determine the cluster velocity dispersion profile between ~9'' and 70'', supplementing previous measurements at r < 2'' and r > 60'' obtained with ESO-SINFONI and ESO-FLAMES spectroscopy, respectively. The new portion of the velocity dispersion profile nicely matches the previous ones, better defining the knee of the distribution. The present work clearly shows the effectiveness of a deployable Integral Field Unit in measuring the radial velocities of individual stars for determining the velocity dispersion profile of Galactic globular clusters. It represents the pilot project for an ongoing large program with KMOS and FLAMES at the ESO-VLT, aimed at determining the next generation of velocity dispersion and rotation profiles for a representative sample of globular clusters.

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arXiv2014

NLTE effects on AGB stars in 47 Tucanae

We present the iron abundance of 24 asymptotic giant branch (AGB) stars members of the globular cluster 47 Tucanae, obtained with high-resolution spectra collected with the FEROS spectrograph at the MPG/ESO-2.2m Telescope. We find that the iron abundances derived from neutral lines (with mean value [Fe I/H] = -0.94 +/- 0.01, σ = 0.08 dex) are systematically lower than those derived from single ionized lines ([Fe II/H] = -0.83 +/- 0.01, σ = 0.05 dex). Only the latter are in agreement with those obtained for a sample of red giant branch (RGB) cluster stars, for which Fe I and Fe II lines provide the same iron abundance. This finding suggests that Non Local Thermodynamical Equilibrium (NLTE) effects driven by overionization mechanisms are present in the atmosphere of AGB stars and significantly affect Fe I lines, while leaving Fe II features unaltered. On the other hand, the very good ionization equilibrium found for RGB stars indicates that these NLTE effects may depend on the evolutionary stage. We discuss the impact of this finding both on the chemical analysis of AGB stars, and on the search for evolved blue stragglers.

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arXiv2014

Dynamical friction in multi-component evolving globular clusters

We use the Chandrasekhar formalism and direct N-body simulations to study the effect of dynamical friction on a test object only slightly more massive than the field stars, orbiting a spherically symmetric background of particles with a mass spectrum. The main goal is to verify whether the dynamical friction time (t_DF) develops a non-monotonic radial-dependence that could explain the bimodality of the Blue Straggler radial distributions observed in globular clusters. In these systems, in fact, relaxation effects lead to a mass and velocity radial segregation of the different mass components, so that mass-spectrum effects on t_DF are expected to be dependent on radius. We find that, in spite of the presence of different masses, t_DF is always a monotonic function of radius, at all evolutionary times and independently of the initial concentration of the simulated cluster. This because the radial dependence of t_DF is largely dominated by the total mass density profile of the background stars (which is monotonically decreasing with radius). Hence, a progressive temporal erosion of the BSS population at larger and larger distances from the cluster center remains the simplest and the most likely explanation of the shape of the observed BSS radial distributions, as suggested in previous works. We also confirm the theoretical expectation that approximating a multi-mass globular cluster as made of (averaged) equal-mass stars can lead to significant overestimates of t_DF within the half-mass radius.

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arXiv2014

Blue Straggler Stars in Globular Clusters: a powerful tool to probe the internal dynamical evolution of stellar systems

This chapter presents an overview of the main observational results obtained to date about Blue Straggler Stars (BSSs) in Galactic Globular Clusters (GCs). The BSS specific frequency, radial distribution, chemical composition and rotational properties are presented and discussed in the framework of using this stellar population as probe of GC internal dynamics. In particular, the shape of the BSS radial distribution has been found to be a powerful tracer of the dynamical age of stellar systems, thus allowing the definition of the first empirical "dynamical clock".

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arXiv2013

TERZAN 5: the remnant of a pristine fragment of the Galactic Bulge?

Terzan 5 is a stellar system in the Galactic bulge commonly catalogued as a globular cluster. Through dedicated NIR photometry and spectroscopy we have discovered that it harbors two main stellar populations defining two distinct red clumps (RCs) in the colour-magnitude diagram, and displaying different iron content: [Fe/H] = -0.2 and [Fe/H]=+0.3 for the faint and the bright red clumps, respectively. In addition, a third minor population with significantly lower metallicity ([Fe/H]=-0.79) has been recently detected, thus enlarging the metallicity range covered by Terzan 5 to Delta[Fe/H] ~ 1 dex. This evidence demonstrates that, similarly to omega Centauri in the Galactic halo, Terzan 5 is not a genuine globular cluster, but a stellar system that experienced a much more complex star formation and chemical enrichment history. Moreover the striking chemical similarity with the bulge stars suggests that Terzan 5 could be the relic of one of the massive clumps that contributed through strong dynamical interactions with other similar sub-structures) to the formation of the Galactic bulge.

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arXiv2013

HST absolute Proper Motions of NGC 6681 (M70) and the Sagittarius Dwarf Spheroidal Galaxy

We have measured absolute proper motions for the three populations intercepted in the direction of the Galactic globular cluster NGC 6681: the cluster itself, the Sagittarius dwarf spheroidal galaxy and the field. For this we used Hubble Space Telescope ACS/WFC and WFC3/UVIS optical imaging data separated by a temporal baseline of 5.464 years. Five background galaxies were used to determine the zero point of the absolute-motion reference frame. The resulting absolute proper motion of NGC 6681 is ($μ_α\cosδ, μ_δ$)=($1.58\pm0.18, -4.57\pm0.16$) \masyr. This is the first estimate ever made for this cluster. For the Sgr dSph we obtain ($μ_α\cosδ, μ_δ)=(-2.54\pm0.18, -1.19\pm0.16$) \masyr, consistent with previous measurements and with the values predicted by theoretical models. The absolute proper motion of the Galaxy population in our field of view is ($μ_α\cosδ, μ_δ)=(-1.21\pm0.27, -4.39\pm0.26$) \masyr. In this study we also use background Sagittarius Dwarf Spheroidal stars to determine the rotation of the globular cluster in the plane of the sky and find that NGC 6681 is not rotating significantly:\ $v_{\rm rot}=0.82\pm1.02$ km$\,$s$^{-1}$ at a distance of 1 arcmin from the cluster center.

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arXiv2013

Blue straggler stars in Globular Clusters: chemical and kinematical properties

Blue Straggler stars are present in all the properly observed Globular Clusters. They mimic a rejuvenated stellar population and their existence has been a puzzle for many years. We performed an extensive spectroscopic survey of the surface abundances and the rotational velocities of Blue Straggler stars in different Globular Clusters, by using the high-resolution spectrograph FLAMES@VLT in the UVES+GIRAFFE combined mode. In this contribution we show our main results obtained for M4, ω Centauri and M30.

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arXiv2012

High resolution reddening map in the direction of the stellar system Terzan 5

We have used optical images acquired with the Hubble Space Telescope to construct the first high-resolution extinction map in the direction of Terzan 5, a peculiar stellar system in the inner bulge of our Galaxy. The map has a spatial resolution of 8" X 8", over a total FoV of 200" X 200". The absorption clouds show a patchy structure on a typical scale of 20" and extinction variations as large as delta E(B-V) = 0.67 mag, especially in the direction of the center of the system. These correspond to an absolute color excess ranging from E(B-V)=2.15 mag, up to 2.82 mag. After the correction for differential reddening, two distinct red giant branches become clearly visible in the color magnitude diagram of Terzan 5 and they well correspond to the two sub-populations with different iron abundances recently discovered in this system.

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arXiv2012

UV Properties of Galactic Globular Clusters with GALEX II. Integrated colors

We present ultraviolet (UV) integrated colors of 44 Galactic globular clusters (GGCs) observed with the Galaxy Evolution Explorer (GALEX) in both FUV and NUV bands. This data-base is the largest homogeneous catalog of UV colors ever published for stellar systems in our Galaxy. The proximity of GGCs makes it possible to resolve many individual stars even with the somewhat low spatial resolution of GALEX. This allows us to determine how the integrated UV colors are driven by hot stellar populations, primarily horizontal branch stars and their progeny. The UV colors are found to be correlated with various parameters commonly used to define the horizontal branch morphology. We also investigate how the UV colors vary with parameters like metallicity, age, helium abundance and concentration. We find for the first time that GCs associated with the Sagittarius dwarf galaxy have (FUV-V) colors systematically redder than GGCs with the same metallicity. Finally, we speculate about the presence of an interesting trend, suggesting that the UV color of GCs may be correlated with the mass of the host galaxy, in the sense that more massive galaxies possess bluer clusters.

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arXiv2012

The central Blue Straggler population in four outer-halo globular clusters

Using HST/WFPC2 data, we have performed a comparative study of the Blue Straggler Star (BSS) populations in the central regions of the globular clusters AM 1, Eridanus, Palomar 3, and Palomar 4. Located at distances RGC > 50 kpc from the Galactic Centre, these are (together with Palomar 14 and NGC 2419) the most distant clusters in the Halo. We determine their colour-magnitude diagrams and centres of gravity. The four clusters turn out to have similar ages (10.5-11 Gyr), significantly smaller than those of the inner-Halo globulars, and similar metallicities. By exploiting wide field ground based data, we build the most extended radial density profiles from resolved star counts ever published for these systems. These are well reproduced by isotropic King models of relatively low concentration. BSSs appear to be significantly more centrally segregated than red giants in all globular clusters, in agreement with the estimated core and half-mass relaxation times which are smaller than the cluster ages. Assuming that this is a signature of mass segregation, we conclude that AM 1 and Eridanus are slightly dynamically more evolved than Pal 3 and Pal 4.

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