arXiv subjects

Giovanni Carraro

Publications and source records attributed to Giovanni Carraro.

At least 91 records · Page 5Linked to original sources

arXiv2010

An analysis of the blue straggler population in the Sgr dSph globular cluster Arp 2

We present and discuss new BVI CCD photometry in the field of the globular cluster Arp~2, which is considered a member of the Sagittarius Dwarf Spheroidal Galaxy. The main goal of this investigation is to study of the statistics and spatial distribution of blue straggler stars in the cluster. Blue stragglers are stars observed to be hotter and bluer than other stars with the same luminosity in their environment. As such, they appear to be much younger than the rest of the stellar population. Two main channels have been suggested to produce such stars: (1) collisions between stars in clusters or (2) mass transfer between, or merger of, the components of primordial short-period binaries. The spatial distribution of these stars inside a star cluster, compared with the distribution of stars in different evolutionary stages, can cast light on the most efficient production mechanism at work. In the case of Arp~2, we found that blue straggler stars are significantly more concentrated than main sequence stars, while they show the same degree of concentration as evolved stars (either red giants or horizontal branch stars). Since Arp~2 is not a very concentrated cluster, we suggest that this high central concentration is an indication that blue stragglers are mostly primordial binary stars.

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arXiv2010

Open clusters towards the Galactic center: chemistry and dynamics. A VLT spectroscopic study of NGC6192, NGC6404, NGC6583

In the framework of the study of the Galactic metallicity gradient and its time evolution, we present new high-resolution spectroscopic observations obtained with FLAMES and the fiber link to UVES at VLT of three open clusters (OCs) located within $\sim$7~kpc from the Galactic Center (GC): NGC~6192, NGC~6404, NGC~6583. We also present new orbit determination for all OCs with Galactocentric distances (R$_{\rm{GC}}) \leq$8~kpc and metallicity from high-resolution spectroscopy. We aim to investigate the slope of the inner disk metallicity gradient as traced by OCs and at discussing its implication on the chemical evolution of our Galaxy. We have derived memberships of a group of evolved stars for each clusters, obtaining a sample of 4, 4, and 2 member stars in NGC~6192, NGC~6404, and NGC~6583, respectively. Using standard LTE analysis we derived stellar parameters and abundance ratios for the iron-peak elements Fe, Ni, Cr, and for the $α$-elements Al, Mg, Si, Ti, Ca. We calculated the orbits of the OCs currently located within 8~kpc from the GC, and discuss their implication on the present-time radial location. {The average metallicities of the three clusters are all oversolar: [Fe/H]= $+0.12\pm0.04$ (NGC~6192), $+0.11\pm0.04$ (NGC 6404), $+0.37\pm0.03$ (NGC 6583). They are in qualitative agreement with their Galactocentric distances, being all internal OCs, and thus expected to be metal richer than the solar neighborhood. The abundance ratios of the other elements over iron [X/Fe] are consistent with solar values. The clusters we have analysed, together with other OC and Cepheid data, confirm a steep gradient in the inner disk, a signature of an evolutionary rate different than in the outer disk.

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arXiv2010

Photometric characterization of the Galactic star cluster Trumpler 20

We present deep UBVI photometry for Trumpler 20, a rich, intermediate-age open cluster located at l=301.47, b=+2.22 (RA=12:39:34, DEC=-60:37:00, J2000.0) in the fourth Galactic quadrant. In spite of its interesting properties, this cluster has received little attenti on, probably because the line of sight to it crosses twice the Carina spiral arm, which causes a significant contamination of its color-magnitude diagram (CMD) by field stars, therefore complicating seriously its interpretation. We provide more robust estimates of the fundamental parameters of Trumpler 20, and investigate the most prominent features of its CMD: a rich He-burning star clump, and a vertical sequence of stars above the turnoff, which can be either blue stragglers or field stars. Our precise photometry has allowed us to derive updated values of the age and heliocentric distance of Trumpler 20, which we estimate to be 1.4 $\pm$ 0.2 Gyr and 3.0 $\pm$ 0.3 kpc, respectively. As predicted by models, at this age the clump has a tail towards fainter magnitudes and bluer colors, thus providing further confirmation of the evolutionary status of stars in this particular phase. The derived heliocentric distance places the cluster in the inter-arm region between the Carina and Scutum arms, which naturally explains the presence of the vertical sequence of stars (which was originally interpreted as the cluster itself) observed in the upper part of the CMD.Most of these stars would therefore belong to the general galactic field, while only a few of themwould be bona fide cluster blue stragglers. Our data suggest that the cluster metallicity is solar, and that its reddening is \textit{E(B-V)} = 0.35 $\pm$ 0.04.

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arXiv2010

The edge of the young Galactic disc

In this work we report and discuss the detection of two distant diffuse stellar groups in the third Galactic quadrant. They are composed of young stars, with spectral types ranging from late O to late B, and lie at galactocentric distances between 15 and 20 kpc. These groups are located in the area of two cataloged open clusters (VdB-Hagen~04 and Ruprecht~30), projected towards the Vela-Puppis constellations, and within the core of the Canis Major over-density. Their reddening and distance has been estimated analyzing their color-color and color-magnitude diagrams, derived from deep $UBV$ photometry. The existence of young star aggregates at such extreme distances from the Galactic center challenges the commonly accepted scenario in which the Galactic disc has a sharp cut-off at about 14 kpc from the Galactic center, and indicates that it extends to much greater distances (as also supported by recent detection of CO molecular complexes well beyond this distance). While the groups we find in the area of Ruprecht~30 are compatible with the Orion and Norma-Cygnus spiral arms, respectively, the distant group we identify in the region of VdB-Hagen~4 lies in the external regions of the Norma-Cygnus arm, at a galactocentric distance ($\sim$20 kpc) where no young stars had been detected so far in the optical.

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arXiv2010

Optical photometry and basic parameters of 10 unstudied open clusters

We present BVI CCD photometry of 10 northern open clusters, Berkeley 43, Berkeley 45, Berkeley 47, NGC 6846, Berkeley 49, Berkeley 51, Berkeley 89, Berkeley 91, Tombaugh 4 and Berkeley 9, and estimate their fundamental parameters. Eight of the clusters are located in the first galactic quadrant and 2 are in the second. This is the first optical photometry for 8 clusters. All of them are embedded in rich galactic fields and have large reddening towards them (E(B-V) = 1.0 - 2.3 mag). There is a possibility that some of these difficult-to-study clusters may be asterisms rather than physical systems, but assuming they are physical clusters, we find that 8 of them are located beyond 2 kpc, and 6 clusters (60% of the sample) are located well above or below the Galactic plane. Seven clusters have ages 500 Myr or less and the other 3 are 1 Gyr or more in age. This sample of clusters has increased the optical photometry of clusters in the second half of the first galactic quadrant, beyond 2 kpc, from 10 to 15. NGC 6846 is found to be one of the most distant clusters in this region of the Galaxy.

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arXiv2009

Abundance analysis of a sample of evolved stars in the outskirts of Omega Centauri

The globular cluster $ω$ Centauri (NGC 5139) is a puzzling stellar system harboring several distinct stellar populations whose origin still represents a unique astrophysical challenge. Current scenarios range from primordial chemical inhomogeneities in the mother cloud to merging of different sub-units and/or subsequent generations of enriched stars - with a variety of different pollution sources- within the same potential well. In this paper we study the chemical abundance pattern in the outskirts of Omega Centauri, half-way to the tidal radius (covering the range of 20-30 arcmin from the cluster center), and compare it with chemical trends in the inner cluster regions, in an attempt to explore whether the same population mix and chemical compositions trends routinely found in the more central regions is also present in the cluster periphery.We extract abundances of many elements from FLAMES/UVES spectra of 48 RGB stars using the equivalent width method and then analyze the metallicity distribution function and abundance ratios of the observed stars. We find, within the uncertainties of small number statistics and slightly different evolutionary phases, that the population mix in the outer regions cannot be distinguished from the more central regions, although it is clear that more data are necessary to obtain a firmer description of the situation. From the abundance analysis, we did not find obvious radial gradients in any of the measured elements.

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arXiv2009

Not an open cluster after all: the NGC 6863 asterism in Aquila

Shortly after birth, open clusters start dissolving; gradually losing stars into the surrounding star field. The time scale for complete disintegration depends both on their initial membership and location within the Galaxy. Open clusters undergoing the terminal phase of cluster disruption or open cluster remnants (OCRs) are notoriously difficult to identify. From an observational point, a combination of low number statistics and minimal contrast against the general stellar field conspire to turn them into very challenging objects. To make the situation even worst, random samples of field stars often display features that may induce to classify them erroneously as extremely evolved open clusters. In this paper, we provide a detailed study of the stellar content and kinematics of NGC 6863, a compact group of a few stars located in Aquila and described by the POSS as a non existent cluster. Nonetheless, this object has been recently classified as OCR. The aim of the present work is to either confirm or disprove its OCR status by a detailed star-by-star analysis. The analysis is performed using wide-field photometry in the UBVI pass-band, proper motions from the UCAC3 catalogue, and high resolution spectroscopy as well as results from extensive $N$-body calculations. Our results show that the four brightest stars commonly associated to NGC 6863 form an asterism, a group of non-physically associated stars projected together, leading to the conclusion that NGC 6863 is not a real open cluster.

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arXiv2009

The Metallicity of the Open Cluster Tombaugh 2

We investigate the nature of the chemical composition of the outer disc open cluster Tombaugh 2, that a recent study by Frinchaboy et al. (2008) suggested to possess an intrinsic metal abundance dispersion. We aim to investigate such claims by high resolution spectra obtained for a number of stars in the Tombaugh 2 field, together with independent UBVIc photometry. The spectra, together with input atmospheric parameters and model atmospheres, are used to determine detailed chemical abundances for a variety of elements in 13 members having good spectra. We find the mean metallicity to be [Fe/H]=-0.31+-0.02 with no evidence for an intrinsic abundance dispersion, in contrary to the recent results of Frinchaboy et al. (2008). We find Ca and Ba to be slightly enhanced while Ni and Sc are solar. The r-process element Eu was found to be enhanced, giving an average [Eu/Ba]=+0.17. The Li abundance decreases with Teff on the upper giant branch and maintains a low level for red clump stars. The mean metallicity we derive is in good agreement with that expected from the radial abundance gradient in the disc for a cluster at its Galactocentric distance. The surprising result found by Frinchaboy et al. (2008), that is the presence of 2 distinct abundance groups within the cluster, implying either a completely unique open cluster with an intrinsic metallicity spread, or a very unlikely superposition of a cold stellar stream and a very distant open cluster, is not supported by our new result.

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arXiv2009

Model-Independent Diagnostics of Highly Reddened Milky Way Star Clusters: Age Calibration

The next generation near- and mid-infrared Galactic surveys will yield a large number of new highly obscured star clusters. Detailed characterization of these new objects with spectroscopy is time-consuming. Diagnostic tools that will be able to characterize clusters based only on the available photometry will be needed to study large samples of the newly found objects. The brightness difference between the red clump and the main-sequence turn-off point have been used as a model-independent age calibrator for clusters with ages from a few 10$^8$ to 10$^{10}$ yr in the optical. Here we apply for the first time the method in the near-infrared. We calibrated this difference in $K$-band, which is likely to be available for obscured clusters, and we apply it to a number of test clusters with photometry comparable to the one that will be yielded by the current or near-future surveys. The new calibration yields reliable ages over the range of ages for which the red clump is present in clusters. The slope of the relation is smoother than that of the corresponding $V$-band relation, reducing the uncertainty in the age determinations with respect to the optical ones.

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arXiv2009

A spectroscopic study of the Open Cluster NGC 6475 (M 7) Chemical Abundances from stars in the range T = 4500-10000 K

Clusters of stars are key objects for the study of the dynamical and chemical evolution of the Galaxy and its neighbors. In particular chemical composition is obtained from different kinds of stars (hot main-sequence stars, cool main-sequence stars, horizontal-branch stars, RGB stars) using different methodologies. Our aim is to apply these methodologies to the stars of the Open Cluster NGC 6475. Obtaining a census of the most important elements we will be able to test their consistence. We finally want to establish more robust fundamental parameters for this cluster.We selected high S/N high resolution spectra of 7 stars of the Open Cluster NGC 6475 from the ESO database covering the T$_{\rm eff}$ range 4500-10000 K and of luminosity class V (dwarf) and III (giants). We determined the chemical abundances of several elements. For hot stars (T$_{\rm eff}$$>$9000 K) we applied the Balmer Lines fitting method to obtained atmospheric parameters. For cool stars (T$_{\rm eff}$$<$6500 K) we used the FeI/II abundance equilibrium method. For the two groups of stars the use of different line-lists was mandatory. LTE approximation was used. For elements affected by NLTE deviation (C,N,O,Na,Mg) corrections were applied. The abundance of many elements were obtained from the measurement of the equivalent width of spectral lines. For those elements for which only blended lines were available (O, He) comparison of real spectrum with synthetic ones was used. Hyperfine structure was taken in account for V and Ba.

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arXiv2009

Old open clusters in the Sagittarius dSph tidal stream -- kith or kin?

A widely supported formation scenario for the Galactic disc is that it formed inside-out from material accumulated via accretion events. The Sagittarius dwarf spheroidal galaxy (Sgr dSph) is the best example of a such accretion, and its ongoing disruption has resulted in that its stars are being deposited in the Milky Way halo and outer disc. It is therefore appealing to search for possible signatures of the Sgr dSph contribution to the build-up of the Galactic disc. Interestingly, models of the Sgr dSph stream indicate clearly that the trailing tail passes through the outer Galactic disc, at the same galactocentric distance as some anti-centre old open star clusters. We investigate in this Letter the possibility that the two outermost old open clusters, Berkeley~29 and Saurer~1, could have formed inside the Sgr dSph and then left behind in the outer Galactic disc as a result of tidal interaction with the Milky Way. The actual location of these two star clusters, inside the Sgr dSph trailing tail, is compatible with this scenario, and their chemical and kinematical properties, together with our present understanding of the age-metallicity relationship in the Sgr dSph, lends further support to this possible association. Hence, we find it likely that the old open star clusters Berkeley~29 and Saurer~1 have extra-galactic origins.

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arXiv2009

Homogeneous photometry and star counts in the field of 9 Galactic star clusters

We present homogeneous $V,I$ CCD photometry of nine stellar fields in the two inner quadrants of the Galactic plane. The lines-of-view to most of these fields aim in the direction of the very inner Galaxy, where the Galactic field is very dense, and extinction is high and patchy. Our nine fields are, according to several catalogs, centred on Galactic star clusters, namely Trumpler 13, Trumpler 20, Lynga 4, Hogg 19, Lynga 12, Trumpler 25, Trumpler 26, Ruprecht 128, and Trumpler 34. Apart from their coordinates, and in some cases additional basic data (mainly from the 2MASS archive), their properties are poorly known. By means of star count techniques and field star decontaminated Color-Magnitude diagrams, the nature and size of these visual over-densities has been established; and, when possible, new cluster fundamental parameters have been derived. To strengthen our findings, we complement our data-set with JHK$_{s}$ photometry from the 2MASS archive, that we analyze using a suitably defined Q-parameter. Most clusters are projected towards the Carina-Sagittarium spiral arm. Because of that, we detect in the Color Magnitude Diagrams of most of the other fields several distinctive sequences produced by young population within the arm. All the clusters are of intermediate or old age. The most interesting cases detected by our study are, perhaps, that of Trumpler 20, which seems to be much older than previously believed, as indicated by its prominent -and double- red clump; and that of Hogg 19, a previously overlooked old open cluster, whose existence in such regions of the Milky Way is puzzling.

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arXiv2009

Extragalactic chemical abundances: do HII regions and young stars tell the same story? The case of the spiral galaxy NGC 300

(Abridged) We have obtained new spectrophotometric data for 28 HII regions in the spiral galaxy NGC 300, a member of the nearby Sculptor Group. The detection of auroral lines, including [OIII]4363, [SIII]6312 and [NII]5755, has allowed us to measure electron temperatures and direct chemical abundances for the whole sample. We determine for the first time in this galaxy a radial gas-phase oxygen abundance gradient based solely on auroral lines, and obtain the following least-square solution: 12+log(O/H)=8.57-0.41 R/R25, where the galactocentric distance is expressed in terms of the isophotal radius R25. The gradient corresponds to -0.077 dex/kpc, and agrees very well with the galactocentric trend in metallicity obtained for 29 B and A supergiants in the same galaxy. The intercept of the regression for the nebular data virtually coincides with the intercept obtained from the stellar data. This allows little room for depletion of nebular oxygen onto dust grains, although in this kind of comparison we are somewhat limited by systematic uncertainties, such as those related to the atomic parameters used to derive the chemical compositions. We discuss the implications of our result with regard to strong-line abundance indicators commonly used to estimate the chemical compositions of star-forming galaxies, such as R23. By applying a few popular calibrations of these indices based on grids of photoionization models on the NGC 300 HII region fluxes we find metallicities that are higher by 0.3 dex (a factor of two) or more relative to our nebular (Te-based) and stellar ones. We confirm a metallicity dependence of the `softness' parameter eta=(O+/O++)/(S+/S++), in the sense that softer stellar continua are found at high metallicity.

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arXiv2009

The globular cluster AM 4: yet another young globular associated with the Sgr Dwarf Spheroidal galaxy?

The complete census of globular clusters formerly belonging to the Sgr dSph and now deposited into the Galactic halo is an important contribution to our comprehension of the evolution and disruption of this dwarf galaxy. We investigate in this study the possibility that the poorly known "old" globular AM 4 might be associated with the Sagittarius dwarf galaxy, and at the same time provide more solid estimate of its basic parameters. New high quality BVI photometry is presented, from which an improved Color Ma gnitude Diagram is constructed, and estimates of age and distance are then derived. The distance and Galactic position are finally investigated in details. AM~4 is found to be a low luminosity (M$_V$=-1.82) cluster undergoing strong tidal stress by the Milky Way and on the verge to be dissolved. Besides, and at odds with previous suggestions, we provide evidences that AM 4 is indeed young, with an age around 9 Gyrs (as Terzan~7), but somewhat more metal poor ([Fe/H=-0.97]). AM~4 is located at 33$_{-4}^{+3}$ kpc from the Sun, in a direction and at distance not totally incompatible with the Sgr dSph stream. Although we significantly improved our knowledge of AM 4, further studies are encouraged to obtain radial velocity and metallicity to d emonstrate more firmly (or deny) the association to Sgr

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arXiv2008

Searching for spiral features in the outer Galactic disk. The field towards WR38 and WR38a

The detailed spiral structure in the outer Galactic disk is still poorly known, and for several Galactic directions we rely on model extrapolations. One of these regions is the fourth Galactic quadrant, in the sector comprised between Vela and Carina (270 <l< 300) where no spiral arms have been detected so far in the optical beyond 270. By means of deep UBVI photometry, we search for spiral features in known low absorption windows.U photometry, although demanding, constitutes a powerful tool to detect and characterize distant aggregates, and allows to derive firmer distance estimates. We studied a direction close to the tangent (l=290) to the Carina arm, in an attempt to detect optical spiral tracers beyond the Carina branch, where radio observations and models predictions indicate the presence of the extension of the Perseus and Norma-Cygnus spiral arms in the fourth quadrant.Along this line of sight, we detect three distinct groups of young stars. Two of them, at 2.5 and 6.0 kpc, belong to the Carina spiral arm (which is crossed twice in this particular direction).The latter is here detected for the first time. The third group, at a distance of 12.7 kpc, is part of the Perseus arm which lies beyond the Carina arm, and constitutes the first optical detection of this arm in the fourth Galactic quadrant. The position of this feature is compatible with HI observations and model predictions. We furthermore present evidence that this extremely distant group, formerly thought to be a star cluster (Shorlin 1), is in fact a diffuse young population. In addition, our data-set does not support the possible presence of the Monoceros Ring toward this direction. This study highlights how multicolor optical studies can be effective to probe the spiral structure in the outer Galactic disk.

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arXiv2008

The old open cluster NGC 2112: updated estimates of fundamental parameters based on a membership analysis

We report on a new, wide field ($20^{\prime} \times 20^{\prime}$), multicolor ($UBVI$), photometric campaign in the area of the nearby old open cluster NGC 2112. At the same time, we provide medium-resolution spectroscopy of 35 (and high-resolution of additional 5) Red Giant and Turn Off stars. This material is analyzed with the aim to update the fundamental parameters of this traditionally difficult cluster, which is very sparse and suffers from heavy field star contamination. Among the 40 stars with spectra, we identified 21 {\it bona fide} radial velocity members which allow us to put more solid constraints on the cluster's metal abundance, long suggested to be as low as the metallicity of globulars. As indicated earlier by us on a purely photometric basis (Carraro et al. 2002), the cluster [Fe/H] abundance is slightly super-solar ([Fe/H] =0.16$\pm$0.03) and close to the Hyades value, as inferred from a detailed abundance analysis of 3 of the 5 stars with higher resolution spectra. Abundance ratios are also marginally super solar. Based on this result, we revise the properties of NGC 2112 using stellar models from the Padova and Yale-Yonsei groups. For this metal abundance, we find the cluster's age, reddening, and distance values are 1.8 Gyr, 0.60 mag, and 940 pc, respectively. Both the Yale-Yonsei and Padova models predict the same values for the fundamental parameters within the errors. Overall, NGC 2112 is a typical solar neighborhood, thin disk star cluster, sharing the same chemical properties of F-G stars and open clusters close to the Sun. This investigation outlines the importance of a detailed membership analysis in the study of disk star clusters.

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arXiv2008

Stellar populations in the Canis Major over-density

We performed a photometric multicolor survey of the core of the Canis Major over-density at l 244, -8.0, reaching V = 22 and covering 0.3 x 1 degree. The main aim is to unravel the complex mixture of stellar populations toward this Galactic direction, where in the recent past important signatures of an accretion event have been claimed to be detected. While our previous investigations were based on disjointed pointings aimed at revealing the large scale structure of the third Galactic Quadrant, we now focus on a complete coverage of a smaller field centered on the Canis Major over-density. A large wave-length baseline, in the UBVRI bands, allows us to build up a suite of colour colour and colour magnitude diagrams, providing a much better diagnostic tool to disentangle the stellar populations of the region. In fact, the simple use of one colour magnitude diagram, widely employed in all the previous studies defending the existence of the Canis Major galaxy, does not allow one to separate the effects of the different parameters (reddening, age, metallicity, and distance) involved in the interpretation of data, forcing to rely on heavy modeling. In agreement with our previous studies in the same general region of the Milky Way, we recognize a young stellar population compatible with the expected structure and extension of the Local (Orion) and Outer (Norma-Cygnus) spiral arms in the Third Galactic Quadrant. Moreover we interpret the conspicuous intermediate-age metal poor population as belonging to the Galactic thick disk, distorted by the effect of strong disk warping at this latitude, and to the Galactic halo.

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arXiv2008

Constraints to Uranus' Great Collision. IV. The Origin of Prospero

It is widely accepted that the large obliquity of Uranus is the result of a great tangential collision (GC) with an Earth size proto-planet at the end of the accretion. We attempt to constraint the GC scenario as the cause of Uranus' obliquity as well as on the mechanisms able to give origin to the Uranian irregulars. Different capture mechanisms for irregulars operate at different stages on the giant planets formation process. The mechanisms able to capture the uranian irregulars before and after the GC are analysed. Assuming that they were captured before the GC, we calculate the orbital transfer of the nine irregulars by the impulse imparted by the GC. If their orbital transfer results dynamically implausible, they should have originated after the GC. We investigate and discuss the dissipative mechanisms able to operate later. In particular Prospero could not exist at the time of the GC. Different capture mechanisms for Prospero after the GC are investigated. Gas drag by Uranus'envelope and pull-down capture are not plausible mechanisms. Capture of Prospero through a collisionless interaction seems to be difficult. The GC itself provides a mechanism of permanent capture. However, the capture of Prospero by the GC is a low probable event. Catastrophic collisions could be a possible mechanism for the birth of Prospero and the other irregulars after the GC. Orbital and physical clusterings should then be expected. Either Prospero had to originate after the GC or the GC did not occur. In the former case, the mechanism for the origin of Prospero after the GC remains an open question. In the latter case, another theory to account for Uranus' obliquity and the formation of the Uranian regular satellites on the equatorial plane of the planet would be needed.

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