arXiv subjects

Emanuele Dalessandro

Publications and source records attributed to Emanuele Dalessandro.

At least 55 records · Page 3Linked to original sources

arXiv2021

MUSE narrow field mode observations of the central kinematics of M15

We present observations of the stellar kinematics of the centre of the core collapsed globular cluster M15 obtained with the MUSE integral field spectrograph on the VLT operating in narrow field mode. Thanks to the use of adaptive optics, we obtain a spatial resolution of 0.1arcsec and are able to reliably measure the radial velocities of 864 stars within 8 arcsec of the centre of M15 thus providing the largest sample of radial velocities ever obtained for the innermost regions of this system. Combined with previous observations of M15 using MUSE in wide field mode and literature data, we find that the central kinematics of M15 are complex with the rotation axis of the core of M15 offset from the rotation axis of the bulk of the cluster. While this complexity has been suggested by previous work, we confirm it at higher significance and in more detail.

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arXiv2020

Radial variation of the stellar mass functions in the globular clusters M15 and M30: clues of a non-standard IMF?

We exploit a combination of high-resolution Hubble Space Telescope and wide-field ESO-VLT observations to study the slope of the global mass function (alphaG) and its radial variation (alpha(r)) in the two dense, massive and post core-collapse globular clusters M15 and M30. The available data-set samples the clusters' Main Sequence down to 0.2 Msun and the photometric completeness allows the study of the mass function between 0.40 Msun and 0.75 Msun from the central regions out to their tidal radii. We find that both clusters show a very similar variation in alpha(r) as a function of clustercentric distance. They both exhibit a very steep variation in alpha(r) in the central regions, which then attains almost constant values in the outskirts. Such a behavior can be interpreted as the result of long-term dynamical evolution of the systems driven by mass-segregation and mass-loss processes. We compare these results with a set of direct N-body simulations and find that they are only able to reproduce the observed values of alpha(r) and alphaG at dynamical ages (t/trh) significantly larger than those derived from the observed properties of both clusters. We investigate possible physical mechanisms responsible for such a discrepancy and argue that both clusters might be born with a non-standard (flatter/bottom-lighter) initial mass function.

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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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arXiv2020

Photometric characterization of multiple populations in star clusters: The impact of the first dredge-up

The existence of star-to-star light-element abundance variations (multiple populations, MPs) in massive Galactic and extragalactic star clusters older than about 2 Gyr is by now well established. Photometry of red giant branch (RGB) stars has been and still is instrumental in enabling the detection and characterization of cluster MPs, through the appropriate choices of filters, colours and colour combinations, that are mainly sensitive to N and --to a lesser degree-- C stellar surface abundances. An important issue not yet properly addressed is that the translation of the observed widths of the cluster RGBs to abundance spreads must account for the effect of the first dredge-up on the surface chemical patterns, hence on the spectral energy distributions of stars belonging to the various MPs. We have filled this gap by studying theoretically the impact of the dredge-up on the predicted widths of RGBs in clusters hosting MPs. We find that for a given initial range of N abundances, the first dredge up reduces the predicted RGB widths in N-sensitive filters compared to the case when its effect on the stellar spectral energy distributions is not accounted for. This reduction is a strong function of age and has also a dependence on metallicity. The net effect is an underestimate of the initial N-abundance ranges from RGB photometry if the first dredge-up is not accounted for in the modelling, and also the potential determination of spurious trends of N-abundance spreads with age.

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arXiv2019

The peculiar kinematics of the multiple populations in the globular cluster Messier 80 (NGC 6093)

We combine MUSE spectroscopy and Hubble Space Telescope ultraviolet (UV) photometry to perform a study of the chemistry and dynamics of the Galactic globular cluster Messier 80 (M80, NGC 6093). Previous studies have revealed three stellar populations that not only vary in their light-element abundances, but also in their radial distributions, with concentration decreasing with increasing nitrogen enrichment. This remarkable trend, which sets M80 apart from the other Galactic globular clusters, points towards a complex formation and evolutionary history. To better understand how M80 formed and evolved, revealing its internal kinematics is key. We find that the most N-enriched population rotates faster than the other two populations at a 2 sigma confidence level. While our data further suggest that the intermediate population shows the least amount of rotation, this trend is rather marginal (1 - 2 sigma). Using axisymmetric Jeans models, we show that these findings can be explained from the radial distributions of the populations if they possess different angular momenta. Our findings suggest that the populations formed with primordial kinematical differences.

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arXiv2019

Light element variations within the different age-metallicity populations in the nucleus of the Sagittarius dwarf

The cluster M54 lies at the centre of the Sagittarius dwarf spheroidal galaxy, and therefore may be the closest example of a nuclear star cluster. Either in-situ star formation, inspiralling globular clusters, or a combination have been invoked to explain the wide variety of stellar sub-populations in nuclear star clusters. Globular clusters are known to exhibit light element variations, which can be identified using the photometric construct called a chromosome map. In this letter, we create chromosome maps for three distinct age-metallicity sub-populations in the vicinity of M54. We find that the old, metal-poor population shows the signature of light element variations, while the young and intermediate-age metal rich populations do not. We conclude that the nucleus of Sagittarius formed through a combination of in-situ star formation and globular cluster accretion. This letter demonstrates that properly constructed chromosome maps of iron-complex globular clusters can provide insight into the formation locations of the different stellar populations.

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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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arXiv2018

Spatial mixing of binary stars in multiple-population globular clusters

We present the results of a study aimed at investigating the effects of dynamical evolution on the spatial distribution and mixing of primordial binary stars in multiple-population globular clusters. Multiple stellar population formation models predict that second-generation (SG) stars form segregated in the inner regions of a more extended first-generation (FG) cluster. Our study, based on the results of a survey of N-body simulations, shows that the spatial mixing process for binary stars is more complex than that of single stars since additional processes such as binary ionization, recoil and ejection following binary-single and binary-binary interactions play a key role in determining the spatial distribution of the population of surviving binaries. The efficiency and relative importance of these additional effects depends on the binary binding energy and determines the timescale of the spatial mixing of FG and SG binaries. Our simulations illustrate the role of ionization, recoil and ejection combined with the effects of mass segregation driven by two-body relaxation and show that the complex interplay of all these processes results in a significant extension of the time needed for the complete spatial mixing of FG and SG binaries compared to that of single stars. Clusters in which FG and SG single stars have already reached complete spatial mixing might be characterized by a significant radial gradient in the ratio of the FG-to-SG binary fraction. The implications of the delayed mixing of FG and SG binaries for the differences between the kinematics of the two populations are discussed.

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arXiv2018

Chemical Inhomogeneities Amongst First Population Stars in Globular Clusters: Evidence for He variations

Spreads in light element abundances among stars (a.k.a. multiple populations) are observed in nearly all globular clusters. One way to map such chemical variations using high-precision photometry is to employ a suitable combination of stellar magnitudes in the F275W, F336W, F438W, and F814W filters (the so called "chromosome map"), to maximise the separation between the different multiple populations. For each individual cluster its chromosome map separates the so-called first population -with metal abundance patterns typical of field halo stars- from the second population, that displays distinctive abundance variations among a specific group of light-elements. Surprisingly, the distribution of first population stars in chromosome maps of several -but not all- clusters has been found to be more extended than expected from purely observational errors, suggesting a chemically inhomogeneous origin. We consider here three clusters with similar metallicity ([Fe/H]~-1.3) and different chromosome maps, namely NGC 288, M 3 and NGC 2808, and argue that the first population extended distribution (as observed in two of these clusters) is due to spreads of the initial helium abundance and possibly a small range of nitrogen abundances as well. The presence of a range of initial He and N abundances amongst stars traditionally thought to have homogeneous composition, plus the fact that these spreads appear only in some clusters, challenge the scenarios put forward so far to explain the multiple population phenomenon.

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arXiv2018

COCOA Code for Creating Mock Observations of Star Cluster Models

We introduce and present results from the COCOA (Cluster simulatiOn Comparison with ObservAtions) code that has been developed to create idealized mock photometric observations using results from numerical simulations of star cluster evolution. COCOA is able to present the output of realistic numerical simulations of star clusters carried out using Monte Carlo or \textit{N}-body codes in a way that is useful for direct comparison with photometric observations. In this paper, we describe the COCOA code and demonstrate its different applications by utilizing globular cluster (GC) models simulated with the MOCCA (MOnte Carlo Cluster simulAtor) code. COCOA is used to synthetically observe these different GC models with optical telescopes, perform PSF photometry and subsequently produce observed colour magnitude diagrams. We also use COCOA to compare the results from synthetic observations of a cluster model that has the same age and metallicity as the Galactic GC NGC 2808 with observations of the same cluster carried out with a 2.2 meter optical telescope. We find that COCOA can effectively simulate realistic observations and recover photometric data. COCOA has numerous scientific applications that maybe be helpful for both theoreticians and observers that work on star clusters. Plans for further improving and developing the code are also discussed in this paper.

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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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arXiv2016

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. VII. Implications from the Nearly Universal Nature of Horizontal Branch Discontinuities

The UV-initiative Hubble Space Telescope Treasury survey of Galactic globular clusters provides a new window into the phenomena that shape the morphological features of the horizontal branch (HB). Using this large and homogeneous catalog of UV and blue photometry, we demonstrate that the HB exhibits discontinuities that are remarkably consistent in color (effective temperature). This consistency is apparent even among some of the most massive clusters hosting multiple distinct sub-populations (such as NGC 2808, omega Cen, and NGC 6715), demonstrating that these phenomena are primarily driven by atmospheric physics that is independent of the underlying population properties. However, inconsistencies arise in the metal-rich clusters NGC 6388 and NGC 6441, where the discontinuity within the blue HB (BHB) distribution shifts ~1,000 K to 2,000 K hotter. We demonstrate that this shift is likely due to a large helium enhancement in the BHB stars of these clusters, which in turn affects the surface convection and evolution of such stars. Our survey also increases the number of Galactic globular clusters known to host blue-hook stars (also known as late hot flashers) from 6 to 23 clusters. These clusters are biased toward the bright end of the globular cluster luminosity function, confirming that blue-hook stars tend to form in the most massive clusters with significant self-enrichment.

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arXiv2016

Multiple stellar populations in the globular cluster M3 (NGC 5272): a Strömgren perspective

We present Strömgren photometry of the Galactic Globular Cluster M3 to study its multiple generations phenomenon. The use of different colour-magnitude diagrams and especially of the notoriously efficient c_y index allowed us to detect a double Red Giant Branch in the cluster CMD. After decontamination from fore- and background sources, the two sequences turned out to be equally populated. The two components also show a bimodal radial distribution well corresponding to that predicted by numerical simulations for clusters living in an intermediate dynamical evolutive state and with a population with modified chemical composition that was born more centrally concentrated than the primordial. The analysis of high-resolution spectra quantitatively demonstrates that the two detected sequences correspond to the first (Na-poor) generation and the second (Na-rich) generation, thus confirming the importance of synergy between photometry and spectroscopy.

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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

Evidences of tidal distortion and mass loss from the old open cluster NGC 6791

We present the first evidence of clear signatures of tidal distortions in the density distribution of the fascinating open cluster NGC~6791. We find that the 2D density map shows a clear elongation and an irregular distribution starting from $\sim 300^{\prime\prime}$ from the cluster center and two tails extending in opposite directions beyond the tidal radius. These features are aligned to both the absolute proper motion and to the Galactic centre directions. Accordingly we find that both the surface brightness and star count density profiles reveal a departure from a King model starting from $\sim600^{\prime\prime}$. These observational evidences suggest that NGC~6791 is currently undergoing mass-loss likely due to gravitational shocking and interactions with the tidal field of the Milky Way. We derive the expected mass-loss due to stellar evolution and tidal interactions and we estimate the initial cluster mass to be $M_{ini} = (1.5-4.0 ) \times 10^5 M_{\odot}$.

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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

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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