arXiv subjects

A. P. Milone

Publications and source records attributed to A. P. Milone.

At least 127 records · Page 7Linked to original sources

arXiv2015

The extended Main Sequence Turn Off cluster NGC1856: rotational evolution in a coeval stellar ensemble

Multiple or extended turnoffs in young clusters in the Magellanic Clouds have recently received large attention. A number of studies have shown that they may be interpreted as the result of a significant age spread (several 10^8yr in clusters aged 1--2 Gyr), while others attribute them to a spread in stellar rotation. We focus on the cluster NGC 1856, showing a splitting in the upper part of the main sequence, well visible in the color m_{F336W}-m_{F555W}$, and a very wide turnoff region. Using population synthesis available from the Geneva stellar models, we show that the cluster data can be interpreted as superposition of two main populations having the same age (~350Myr), composed for 2/3 of very rapidly rotating stars, defining the upper turnoff region and the redder main sequence, and for 1/3 of slowly/non-rotating stars. Since rapid rotation is a common property of the B-A type stars, the main question raised by this model concerns the origin of the slowly/non-rotating component. Binary synchronization is a possible process behind the slowly/non-rotating population; in this case, many slowly/non-rotating stars should still be part of binary systems with orbital periods in the range from 4 to 500 days. Such periods imply that Roche lobe overflow occurs, during the evolution of the primary off the main sequence, so most primaries may not be able to ignite core helium burning, consistently why the lack of a red clump progeny of the slowly rotating population.

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arXiv2015

The Hubble Space Telescope UV Legacy Survey of galactic globular clusters. VI. The internal kinematics of the multiple stellar populations in NGC 2808

Numerous observational studies have revealed the ubiquitous presence of multiple stellar populations in globular clusters and cast many hard challenges for the study of the formation and dynamical history of these stellar systems. In this Letter we present the results of a study of the kinematic properties of multiple populations in NGC 2808 based on high-precision Hubble Space Telescope proper-motion measurements. In a recent study, Milone et al. have identified five distinct populations (A, B, C, D, and E) in NGC 2808. Populations D and E coincide with the helium-enhanced populations in the middle and the blue main sequences (mMS and bMS) previously discovered by Piotto et al.; populations A, B, and C correspond to the redder main sequence (rMS) that in the Piotto et al. was associated with the primordial stellar population. Our analysis shows that, in the outermost regions probed (between about 1.5 and 2 times the cluster half-light radius), the velocity distribution of populations D and E is radially anisotropic (the deviation from an isotropic distribution is significant at the ~3.5-sigma level). Stars of populations D and E have a smaller tangential velocity dispersion than those of populations A, B, and C, while no significant differences are found in the radial-velocity dispersion. We present the results of a numerical simulation showing that the observed differences between the kinematics of these stellar populations are consistent with the expected kinematic fingerprint of the diffusion towards the cluster outer regions of stellar populations initially more centrally concentrated.

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arXiv2015

An HST/WFC3 view of stellar populations on the Horizontal Branch of NGC 2419

We use images acquired with the Hubble Space Telescope Wide Field Camera 3 and new models to probe the Horizontal Branch (HB) population of the We use images acquired with the Hubble Space Telescope Wide Field Camera 3 and new models to probe the horizontal branch (HB) population of the Galactic globular cluster (GC) NGC 2419. A detailed analysis of the composite HB highlights three populations:(1) the blue luminous HB, hosting standard helium stars (Y=0.25) with a very small spread of mass, (2) a small population of stars with intermediate helium content (0.26<Y<=0.29), and (3) the well-populated extreme HB. We can fit the last group with models having high helium abundance (Y \sim 0.36), half of which (the hottest part, 'blue hook' stars) are identified as possible 'late flash mixed stars'. The initial helium abundance of this extreme population is in nice agreement with the predicted helium abundance in the ejecta of massive asymptotic giant branch (AGB) stars of the same metallicity as NGC 2419. This result further supports the hypothesis that second-generation stars in GCs formed from the ashes of intermediate-mass AGB stars. We find that the distribution in magnitude of the blue hook stars is larger than that predicted by theoretical models. We discuss the possible uncertainties in the magnitude scales and different attempts to model this group of stars. Finally, we suggest that consistency can be better achieved if we assume core masses larger than predicted by our models. This may be possible if the progenitors were fast rotators on the main sequence. If further study confirms this interpretation, a fast initial rotation would be a strong signature of the peculiarity of extreme second-generation stars in GCs.

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arXiv2015

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. III. A quintuple stellar population in NGC2808

In this study we present first results from multi-wavelength Hubble Space Telescope (HST) observations of the Galactic globular cluster GC NGC2808 as an extension of the Hubble Space Telescope UV Legacy Survey of Galactic GCs (GO-13297 and previous proprietary and HST archive data). Our analysis allowed us to disclose a multiple-stellar-population phenomenon in NGC2808 even more complex than previously thought. We have separated at least five different populations along the main sequence and the red giant branch (RGB), that we name A, B, C, D and E (though an even finer subdivision may be suggested by the data). We identified the RGB bump in four out of the five RGBs. To explore the origin of this complex CMD, we have combined our multi-wavelength HST photometry with synthetic spectra, generated by assuming different chemical compositions. The comparison of observed colors with synthetic spectra suggests that the five stellar populations have different contents of light elements and helium. Specifically, if we assume that NGC2808 is homogeneous in [Fe/H] (as suggested by spectroscopy for Populations B, C, D, E, but lacking for Population A) and that population A has a primordial helium abundance, we find that populations B, C, D, E are enhanced in helium by Delta Y~0.03, 0.03, 0.08, 0.13, respectively. We obtain similar results by comparing the magnitude of the RGB bumps with models. Planned spectroscopic observations will test whether also Population A has the same metallicity, or whether its photometric differences with Population B can be ascribed to small [Fe/H] and [O/H] differences rather than to helium.

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arXiv2015

UV Insights into the Complex Populations of M 87 Globular Clusters

We have imaged with HST's WFC3/UVIS the central 2.7$\times$2.7 arcmin$^2$ region of the giant elliptical galaxy M 87, using the ultraviolet filter F275W. In combination with archival ACS/WFC data taken through the F606W and F814W filters, covering the same field, we have constructed integrated-light UV-optical colors and magnitudes for 1460 objects, most of which are believed to be globular clusters belonging to M 87. The purpose was to ascertain whether the multiple-populations syndrome, ubiquitous among Galactic globular clusters (GCs), exists also among the M 87 family of clusters. To achieve this goal, we sought those GCs with exceptionally blue UV-to-optical colors, because helium-enriched sub-populations produce a horizontal-branch morphology that is well populated at high effective temperature. For comparison, integrated, synthetic UV$-$optical and purely optical colors and magnitudes have been constructed for 45 Galactic GCs, starting from individual-star photometry obtained with the same instruments and the same filters. We identify a small group of M 87 clusters exhibiting a radial UV$-$optical color gradient, representing our best candidate GCs hosting multiple populations with extreme helium content. We also find that the central spatial distribution of the bluer GCs is flattened in a direction parallel to the jet, while the distribution of redder GCs is more spherical. We release to the astronomical community our photometric catalog in F275W, F606W and F814W bands and the high-quality image stacks in the same bands.

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arXiv2015

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. IV. Helium content and relative age of multiple stellar populations within NGC 6352

In this paper we combine WFC3/UVIS F275W, F336W, and F438W data from the "UV Legacy Survey of Galactic Globular Clusters: Shedding Light on Their Populations and Formation" (GO-13297) HST Treasury program with F606W, F625W, F658N, and F814W ACS archive data for a multi-wavelength study of the globular cluster NGC 6352. In the color-magnitude and two-color diagrams obtained with appropriate combination of the photometry in the different bands we separate two distinct stellar populations and trace them from the main sequence to the subgiant, red giant, horizontal and asymptotic giant branches. We infer that the two populations differ in He by Delta Y=0.029+/-0.006. With a new method, we also estimate the age difference between the two sequences. Assuming no difference in [Fe/H] and [alpha/Fe], and the uncertainties on Delta Y, we found a difference in age between the two populations of 10+/-120 Myr. If we assume [Fe/H] and [alpha/Fe] differences of 0.02 dex (well within the uncertainties of spectroscopic measurements), the total uncertainty in the relative age rises to ~300 Myr.

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arXiv2015

Multiple stellar populations in Magellanic Cloud clusters. III. The first evidence of an extended main sequence turn-off in a young cluster: NGC1856

Recent studies have shown that the extended main-sequence turn off (eMSTO) is a common feature of intermediate-age star clusters in the Magellanic Clouds (MCs). The most simple explanation is that these stellar systems harbor multiple generations of stars with an age difference of a few hundred Myrs. However, while an eMSTO has been detected in a large number of clusters with ages between ~1-2 Gyrs, several studies of young clusters in both MCs and in nearby galaxies do not find any evidence for a prolonged star-formation history, i.e. for multiple stellar generations. These results have suggested alternative interpretation of the eMSTOs observed in intermediate-age star clusters. The eMSTO could be due to stellar rotation mimicking an age spread or to interacting binaries. In these scenarios, intermediate-age MC clusters would be simple stellar populations, in close analogy with younger clusters. Here we provide the first evidence for an eMSTO in a young stellar cluster. We exploit multi-band Hubble Space Telescope photometry to study the ~300-Myr old star cluster NGC1856 in the Large Magellanic Cloud and detected a broadened MSTO that is consistent with a prolonged star-formation which had a duration of about 150 Myrs. Below the turn-off, the MS of NGC1856 is split into a red and blue component, hosting 33+/-5% and 67+/-5% of the total number of MS stars, respectively. We discuss these findings in the context of multiple-stellar-generation, stellar-rotation, and interacting-binary hypotheses.

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arXiv2015

Iron and s-elements abundance variations in NGC5286: comparison with anomalous globular clusters and Milky Way satellites

We present a high resolution spectroscopic analysis of 62 red giants in the Milky Way globular cluster NGC5286. We have determined abundances of representative light proton-capture, alpha, Fe-peak and neutron-capture element groups, and combined them with photometry of multiple sequences observed along the colour-magnitude diagram. Our principal results are: (i) a broad, bimodal distribution in s-process element abundance ratios, with two main groups, the s-poor and s-rich groups; (ii) substantial star-to-star Fe variations, with the s-rich stars having higher Fe, e.g. <[Fe/H]>_s-rich - <[Fe/H]>_s-poor ~ 0.2~dex; and (iii) the presence of O-Na-Al (anti-)correlations in both stellar groups. We have defined a new photometric index, c_{BVI}=(B-V)-(V-I), to maximise the separation in the colour-magnitude diagram between the two stellar groups with different Fe and s-element content, and this index is not significantly affected by variations in light elements (such as the O-Na anticorrelation). The variations in the overall metallicity present in NGC5286 add this object to the class of "anomalous" GCs. Furthermore, the chemical abundance pattern of NGC5286 resembles that observed in some of the anomalous GCs, e.g. M22, NGC1851, M2, and the more extreme Omega Centauri, that also show internal variations in s-elements, and in light elements within stars with different Fe and s-elements content. In view of the common variations in s-elements, we propose the term s-Fe-anomalous GCs to describe this sub-class of objects. The similarities in chemical abundance ratios between these objects strongly suggest similar formation and evolution histories, possibly associated with an origin in tidally disrupted dwarf satellites.

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arXiv2015

Hubble Space Telescope observations of the Kepler-field cluster NGC 6819. I. The bottom of the white dwarf cooling sequence

We use Hubble Space Telescope (HST) to reach the end of the white dwarf (WD) cooling sequence (CS) in the solar-metallicity open cluster NGC 6819. Our photometry and completeness tests show a sharp drop in the number of WDs along the CS at magnitudes fainter than mF606W = 26.050+/- 0.075. This implies an age of 2.25+/-0.20 Gyr, consistent with the age of 2.25+/-0.30 Gyr obtained from fits to the main-sequence turn-off. The use of different WD cooling models and initial-final-mass relations have a minor impact the WD age estimate, at the level of ~0.1 Gyr. As an important by-product of this investigation we also release, in electronic format, both the catalogue of all the detected sources and the atlases of the region (in two filters). Indeed, this patch of sky studied by HST (of size ~70 arcmin sq.) is entirely within the main Kepler-mission field, so the high-resolution images and deep catalogues will be particularly useful.

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arXiv2014

Variable stars in two open clusters within the Kepler 2-Campaign-0 field: M 35 and NGC 2158

We present a multi-year survey aimed at collecting (1) high-precision (~5 milli-mag), (2) fast-cadence (~3 min), and (3) relatively long duration (~10 days) multi-band photometric series. The goal of the survey is to discover and characterize efficiently variable objects and exoplanetary transits in four fields containing five nearby open clusters spanning a broad range of ages. More in detail, our project will (1) constitute a preparatory survey for HARPS-N@TNG, which will be used for spectroscopic follow-up of any target of interest that this survey discovers or characterizes, (2) measure rotational periods and estimate the activity level of targets we are already monitoring with HARPS and HARPS-N for exoplanet transit search, and (3) long term characterization of selected targets of interest in open clusters within the planned K2 fields. In this first paper we give an overview of the project, and report on the variability of objects within the first of our selected fields, which contains two open clusters: M 35 and NGC 2158. We detect 519 variable objects, 273 of which are new discoveries, while the periods of most of the previously known variables are considerably improved.

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arXiv2014

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. II. The seven stellar populations of NGC7089 (M2)

We present high-precision multi-band photometry for the globular cluster (GC) M2. We combine the analysis of the photometric data obtained from the Hubble Space Telescope UV Legacy Survey of Galactic GCs GO-13297, with chemical abundances by Yong et al.(2014), and compare the photometry with models in order to analyze the multiple stellar sequences we identified in the color-magnitude diagram (CMD). We find three main stellar components, composed of metal-poor, metal-intermediate, and metal-rich stars (hereafter referred to as population A, B, and C, respectively). The components A and B include stars with different $s$-process element abundances. They host six sub-populations with different light-element abundances, and exhibit an internal variation in helium up to Delta Y~0.07 dex. In contrast with M22, another cluster characterized by the presence of populations with different metallicities, M2 contains a third stellar component, C, which shows neither evidence for sub-populations nor an internal spread in light-elements. Population C does not exhibit the typical photometric signatures that are associated with abundance variations of light elements produced by hydrogen burning at hot temperatures. We compare M2 with other GCs with intrinsic heavy-element variations and conclude that M2 resembles M22, but it includes an additional stellar component that makes it more similar to the central region of the Sagittarius galaxy, which hosts a GC (M54) and the nucleus of the Sagittarius galaxy itself.

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arXiv2014

Observing multiple stellar populations with FORS2@VLT - Main sequence photometry in outer regions of NGC 6752, NGC 6397, and NGC 6121 (M 4)

We present the photometric analysis of the external regions of three Galactic Globular Clusters: NGC 6121, NGC 6397 and NGC 6752. The main goal is the characterization of the multiple stellar populations along the main sequence (MS) and the study of the radial trend of the different populations hosted by the target clusters. The data have been collected using FORS2 mounted at the ESO/VLT@UT1 telescope in UBVI filters. From these data sets we extracted high-accuracy photometry and constructed color-magnitude diagrams. We exploit appropriate combination of colors and magnitudes which are powerful tools to identify multiple stellar populations, like B versus U-B and V versus c_{U,B,I}=(U-B)-(B-I) CMDs. We confirm previous findings of a split MS in NGC 6752 and NGC 6121. Apart from the extreme case of omega Centauri, this is the first detection of multiple MS from ground-based photometry. For NGC 6752 and NGC 6121 we compare the number ratio of the blue MS to the red MS in the cluster outskirts with the fraction of first and second generation stars measured in the central regions. There is no evidence for significant radial trend. The MS of NGC 6397 is consistent with a simple stellar population. We propose that the lack of multiple sequences is due both to observational errors and to the limited sensitivity of U,B,V,I photometry to multiple stellar populations in metal-poor GCs. Finally, we compute the helium abundance for the stellar populations hosted by NGC 6121 and NGC 6752, finding a mild (Delta Y ~ 0.02) difference between stars in the two sequences.

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arXiv2014

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. I. Overview of the Project and Detection of Multiple Stellar Populations

In this paper we describe a new UV-initiative HST project (GO-13297) that will complement the existing F606W and F814W database of the ACS Globular Cluster (GC) Treasury by imaging most of its clusters through UV/blue WFC3/UVIS filters F275W, F336W and F438W. This "magic trio" of filters has shown an uncanny ability to disentangle and characterize multiple-population (MP) patterns in GCs in a way that is exquisitely sensitive to C, N, and O abundance variations. Combination of these passbands with those in the optical also gives the best leverage for measuring helium enrichment. The dozen clusters that had previously been observed in these bands exhibit a bewildering variety of MP patterns, and the new survey will map the full variance of the phenomenon. The ubiquity of multiple stellar generations in GCs has made the formation of these cornerstone objects more intriguing than ever; GC formation and the origin of their MPs have now become one and the same problem. In the present paper we will describe the data base and our data reduction strategy, as well as the uses we intend to make of the final photometry, astrometry, and proper motions. We will also present preliminary color-magnitude diagrams from the data so far collected. These diagrams also draw on data from GO-12605 and GO-12311, which served as a pilot project for the present GO-13297.

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arXiv2014

Stellar Models of Multiple Populations in Globular Clusters. I. The Main Sequence of NGC 6752

We present stellar atmosphere and evolution models of main sequence stars in two stellar populations of the Galactic globular cluster NGC 6752. These populations represent the two extremes of light-element abundance variations in the cluster. NGC 6752 is a benchmark cluster in the study of multiple stellar populations because of the rich array of spectroscopic abundances and panchromatic Hubble Space Telescope photometry. The spectroscopic abundances are used to compute stellar atmosphere and evolution models. The synthetic spectra for the two populations show significant differences in the ultraviolet and, for the coolest temperatures, in the near-infrared. The stellar evolution models exhibit insignificant differences in the H-R diagram except on the lower main sequence. The appearance of multiple sequences in the colour-magnitude diagrams (CMDs) of NGC 6752 is almost exclusively due to spectral effects caused by the abundance variations. The models reproduce the observed splitting and/or broadening of sequences in a range of CMDs. The ultraviolet CMDs are sensitive to variations in carbon, nitrogen, and oxygen but the models are not reliable enough to directly estimate abundance variations from photometry. On the other hand, the widening of the lower main sequence in the near-infrared CMD, driven by oxygen-variation via the water molecule, is well-described by the models and can be used to estimate the range of oxygen present in a cluster from photometry. We confirm that it is possible to use multiband photometry to estimate helium variations among the different populations, with the caveat that the estimated amount of helium-enhancement is model-dependent.

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arXiv2014

The Carina Project VII: Towards the breaking of the age-metallicity degeneracy of red giant branch stars using the c_UBI index

We present an analysis of photometric and spectroscopic data of the Carina dSph galaxy, testing a new approach similar to that used to disentangle multiple populations in Galactic globular clusters (GCs). We show that a proper colour combination is able to separate a significant fraction of the red giant branch (RGB) of the two main Carina populations (the old one, \sim 12 Gyr, and the intermediate-age one, 4-8 Gyr). In particular, the c_UBI=(U-B)-(B-I) pseudo-colour allows us to follow the RGB of both populations along a relevant portion of the RGB. We find that the oldest stars have more negative c_UBI pseudo-colour than intermediate-age ones. We correlate the pseudo-colour of RGB stars with their chemical properties, finding a significant trend between the iron content and the c_UBI. Stars belonging to the old population are systematically more metal poor ([Fe/H]=-2.32\pm0.08 dex) than the intermediate-age ones ([Fe/H]=-1.82\pm0.03 dex). This gives solid evidence on the chemical evolution history of this galaxy, and we have a new diagnostic that can allow us to break the age-metallicity degeneracy of H-burning advanced evolutionary phases. We compared the distribution of stars in the c_UBI plane with theoretical isochrones, finding that no satisfactory agreement can be reached with models developed in a theoretical framework based on standard heavy element distributions. Finally, we discuss possible systematic differences when compared with multiple populations in GCs.

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arXiv2014

Helium and Multiple Populations in the Massive Globular Cluster NGC6266 (M62)

Recent studies suggest that the helium content of multiple stellar populations in globular clusters (GCs) is not uniform. The range of helium varies from cluster to cluster with more massive GCs having, preferentially, large helium spread. GCs with large helium variations also show extended-blue horizontal branch (HB). I exploit Hubble Space Telescope photometry to investigate multiple stellar populations in NGC6266 and infer their relative helium abundance. This cluster is an ideal target to investigate the possible connection between helium, cluster mass, and HB morphology, as it exhibits an extended HB and is among the ten more luminous GCs in the Milky Way. The analysis of color-magnitude diagrams from multi-wavelength photometry reveals that also NGC6266, similarly to other massive GCs, hosts a double main sequence (MS), with the red and the blue component made up of the 79+-1% and the 21+-1% of stars, respectively. The red MS is consistent with a stellar population with primordial helium while the blue MS is highly helium-enhanced by Delta Y=0.08+-0.01. Furthermore, the red MS exhibits an intrinsic broadening that can not be attributed to photometric errors only and is consistent with a spread in helium of ~0.025 dex. The comparison between NGC6266 and other GCs hosting helium-enriched stellar populations supports the presence of a correlation among helium variations, cluster mass, and HB extension.

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arXiv2014

The VVV Templates Project. Towards an Automated Classification of VVV Light-Curves. I. Building a database of stellar variability in the near-infrared

Context. The Vista Variables in the Vía Láctea (VVV) ESO Public Survey is a variability survey of the Milky Way bulge and an adjacent section of the disk carried out from 2010 on ESO Visible and Infrared Survey Telescope for Astronomy (VISTA). VVV will eventually deliver a deep near-IR atlas with photometry and positions in five passbands (ZYJHK_S) and a catalogue of 1-10 million variable point sources - mostly unknown - which require classifications. Aims. The main goal of the VVV Templates Project, that we introduce in this work, is to develop and test the machine-learning algorithms for the automated classification of the VVV light-curves. As VVV is the first massive, multi-epoch survey of stellar variability in the near-infrared, the template light-curves that are required for training the classification algorithms are not available. In the first paper of the series we describe the construction of this comprehensive database of infrared stellar variability. Methods. First we performed a systematic search in the literature and public data archives, second, we coordinated a worldwide observational campaign, and third we exploited the VVV variability database itself on (optically) well-known stars to gather high-quality infrared light-curves of several hundreds of variable stars. Results. We have now collected a significant (and still increasing) number of infrared template light-curves. This database will be used as a training-set for the machine-learning algorithms that will automatically classify the light-curves produced by VVV. The results of such an automated classification will be covered in forthcoming papers of the series.

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arXiv2014

The halo+cluster system of the Galactic globular cluster NGC1851

NGC1851 is surrounded by a stellar component that extends more than ten times beyond the tidal radius. Although the nature of this stellar structure is not known, it has been suggested to be a sparse halo of stars or associated with a stellar stream. We analyse the nature of this intriguing stellar component surrounding NGC1851 by investigating its radial velocities and chemical composition, in particular in comparison with those of the central cluster analysed in a homogeneous manner. In total we observed 23 stars in the halo with radial velocities consistent with NGC1851, and for 15 of them we infer [Fe/H] abundances. Our results show that: (i) stars dynamically linked to NGC1851 are present at least up to ~2.5 tidal radii, supporting the presence of a halo of stars surrounding the cluster; (ii) apart from the NGC1851 radial velocity-like stars, our observed velocity distribution agrees with that expected from Galactic models, suggesting that no other sub-structure (such as a stream) at different radial velocities is present in our field; (iii) the chemical abundances for the s-process elements Sr and Ba are consistent with the s-normal stars observed in NGC1851; (iv) all halo stars have metallicities, and abundances for the other studied elements Ca, Mg and Cr, consistent with those exhibited by the cluster. The complexity of the whole NGC1851 cluster+halo system may agree with the scenario of a tidally-disrupted dwarf galaxy in which NGC1851 was originally embedded.

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