arXiv subjects

Giovanni Carraro

Publications and source records attributed to Giovanni Carraro.

At least 37 records · Page 2Linked to original sources

arXiv2022

Very metal-poor stars in the solar vicinity: age determination

The ages of the oldest and most metal-poor stars in the Milky Way bear important information on the age of the Universe and its standard model. We analyze a sample of 28 extremely metal-poor field stars in the solar vicinity culled from the literature and carefully determine their ages. To this aim, we critically make use of Gaia data to derive their distances and associated uncertainties. Particular attention has been paid to the estimate of the reddening and its effect on the derivation of stellar ages. We employed different reddenings and super-impose isochrones from different sources in the stars color-magnitude diagram built up with different photometric systems. We highlight subtle metallicity effects when using the Johnson photometry for low metallicity stars and finally adopt Gaia photometry. An automatic fitting method is devised to assign ages to each individual star taking into account the uncertainties in the input parameters. The mean age of the sample turns out to be $13.9 \pm 0.5$ Gyr using Padova isochrones, and $13.7 \pm 0.4$ Gyr using BASTI isochrones. We found also a group of very metal-poor stars ($\left[\frac{Fe}{H}\right]$: -2.7 -2.0 dex) with relatively young ages, in the range 8 --10 Gyr.

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arXiv2022

The impact of binaries on the dynamical mass estimate of dwarf galaxies

Binary stars are recognized to be important in driving the dynamical evolution of stellar systems and also in determining some of their observational features. In this study, we explore the role that binary stars have in modulating the estimates of the velocity dispersion of stellar systems. To this aim, we developed a tool which allows to investigate the dependence of synthetic velocity dispersion on a number of crucial quantities characterizing the binary content: binary fraction and the distributions of their mass ratio, eccentricity and semi-major axis. As an application, we evaluate the impact that binary stars have on the estimation of the dynamical mass of dwarf spheroidal and ultra-faint dwarf galaxies, finding that it can be particularly relevant, especially for low mass and low density systems. These results bear profound implications for the interpretation of the measured velocity dispersion in such systems, since it weakens or relieves the claim for the need of large amounts of dark matter.

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arXiv2022

Milky Way thin and thick disk kinematics with GAIA EDR3 and RAVE DR5

We present a detailed analysis of kinematics of the Milky Way disk in solar neighborhood using GAIA DR3 catalog. To determine the local kinematics of the stellar disks of the Milky Way galaxy we use a complete sample of 278,228 red giant branch (RGB) stars distributed in a cylinder, centered at the Sun with a 1 kpc radius and half-height of 0.5 kpc. We determine separately the kinematical properties of RGB stars for each Galactic hemisphere in search for possible asymmetries. The kinematical properties of the RGB stars reveal the existence of two kinematically distinct components: the thin disk with mean velocities ${V_R}, {V_ϕ}, {V_Z}$ of about -1, -239, 0 km s$^{-1}$ correspondingly and velocity dispersions $σ_R, σ_ϕ, σ_Z$ of 31, 20 and 11 km s$^{-1}$, and the Thick disk with mean velocities components of about +1, -225, 0 km s$^{-1}$, and velocity dispersions of 49, 35, and 22 km s$^{-1}$. We find that up to 500 pc height above/below the galactic plane, Thick disk stars comprise about half the stars of the disk. Such high amount of RGB stars with Thick disk kinematics points at the secular evolution scenario origin for the Thick disk of the Milky Way galaxy.

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arXiv2021

Updated parameters of 1743 open clusters based on Gaia DR2

In this study we follow up our recent paper (Monteiro et al. 2020) and present a homogeneous sample of fundamental parameters of open clusters in our Galaxy, entirely based on Gaia DR2 data. We used published membership probability of the stars derived from Gaia DR2 data and applied our isochrone fitting code, updated as in Monteiro et al. (2020), to GB and GR Gaia DR2 data for member stars. In doing this we take into account the nominal errors in the data and derive distance, age, and extinction of each cluster. This work therefore provides parameters for 1743 open clusters and, as a byproduct, a list of likely not physical or dubious open clusters is provided as well for future investigations. Furthermore, it was possible to estimate the mean radial velocity of 831 clusters (198 of which are new and unpublished so far) using stellar radial velocities from Gaia DR2 catalog. By comparing the open cluster distances obtained from isochrone fitting with those obtained from a maximum likelihood estimate of individual member parallaxes, we found a systematic offset of $(-0.05\pm0.04)$mas.

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arXiv2021

Unresolved multiple stars and Galactic clusters' mass estimates

If not properly accounted for, unresolved binary stars can induce a bias in the photometric determination of star cluster masses inferred from star counts and the luminosity function. A correction factor close to 1.15 (for a binary fraction of 0.35) was found in \citep{Boro19}, which needs to be applied to blind photometric mass estimates. This value for the correction factor was found to be smaller than literature values. In an attempt to lift this discrepancy, in this work the focus is on higher order multiple stars with the goal of investigating the effect of triple and quadruple systems adopting the same methodology and data-set as in the quoted work. Then the result is found that when triple and quadruple together with binary systems are properly accounted for, the actual cluster mass (computed as all stars were single) should be incremented by a factor of 1.18$-$1.27, depending on the cluster and when the binary fraction $α$ is 0.35. Fitting formulae are provided to derive the increment factor for different binary star percentages.

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arXiv2020

The Gaia-ESO Survey: Oxygen abundance in the Galactic thin and thick disks

We analyze the oxygen abundances of a stellar sample representative of the two major Galactic populations: the thin and thick disks. The aim is to investigate the differences between members of the Galactic disks and to contribute to the understanding on the origin of oxygen chemical enrichment in the Galaxy. The analysis is based on the [O\,{\sc i}]=6300.30\,Å~ oxygen line in HR spectra ($R\sim$52,500) obtained from the GES Survey. By comparing the observed spectra with a theoretical dataset, computed in LTE with the SPECTRUM synthesis and ATLAS12 codes, we derive the oxygen abundances of 516 FGK dwarfs for which we have previously measured carbon abundances. Based on kinematic, chemical and dynamical considerations we identify 20 thin and 365 thick disk members. We study potential trends of both subsamples in terms of their chemistry ([O/H], [O/Fe], [O/Mg], and [C/O] versus [Fe/H] and [Mg/H]), age, and position in the Galaxy. Main results are: (a) [O/H] and [O/Fe] ratios versus [Fe/H] show systematic differences between thin and thick disk stars with enhanced O abundance of thick disk stars with respect to thin disk members and a monotonic decrement of [O/Fe] with increasing metallicity, even at metal-rich regime; (b) a smooth correlation of [O/Mg] with age in both populations, suggesting that this abundance ratio can be a good proxy of stellar ages within the Milky Way; (c) thin disk members with [Fe/H]$\simeq0$ display a [C/O] ratio smaller than the solar value, suggesting a possibly outward migration of the Sun from lower Galactocentric radii.

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arXiv2020

The Gaia-ESO survey: 3D NLTE abundances in the open cluster NGC 2420 suggest atomic diffusion and turbulent mixing at the origin of chemical abundance variations

Atomic diffusion and mixing processes in stellar interiors influence the structure and the surface composition of stars. Some of these processes cannot yet be modelled from the first principles. This limits their applicability in stellar models used for studies of stellar populations and Galactic evolution. Our main goal is to put constrains on the stellar structure and evolution models using new refined measurements of chemical composition in stars of Galactic open cluster. We use medium-resolution, 19 200 <= R <= 21 500, optical spectra of the stars in the open cluster NGC 2420 obtained within the Gaia-ESO survey. The sample covers all evolutionary stages from the main-sequence to red giant branch. Stellar parameters are derived using a combined Bayesian analysis of spectra, 2MASS photometry, and astrometric data from Gaia DR2. The abundances of Mg, Ca, Fe, and Li are determined from non-local thermodynamic equilibrium (NLTE) synthetic spectra, computed using one-dimensional (1D) and averaged three-dimensional (3D) model atmospheres. We compare our results with a grid of Code d'Evolution Stellaire Adaptatif et Modulaire (CESTAM) stellar evolution models, which include atomic diffusion, turbulent and rotational mixing. We find prominent evolutionary trends in the abundances of Fe, Ca, Mg, and Li with the mass of the stars in the cluster. Fe, Mg, and Ca show a depletion at the cluster turn-off, but the abundances gradually increase and flatten near the base of the RGB. The abundance trend for Li displays a signature of rotational mixing on the main-sequence and abrupt depletion on the subgiant branch, which is caused by advection of Li-poor material to the surface. The analysis of abundances combined with the CESTAM model predictions allows us to place limits on the parameter space of the models and to constrain the zone in the stellar interior where turbulent mixing takes place.

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arXiv2020

On the origin of stellar associations

In this review I discuss different theories of the formation of OB associations in the Milky Way, and provide the observational evidences in support of them. In fact, the second release of Gaia astrometric data (April 2018) is revolutionising the field, because it allows us to unravel the 3D structure and kinematics of stellar associations with unprecedented details by providing precise distances and a solid membership assessment. As an illustration, I summarise some recent studies on three OB associations: Cygnus OB2, Vela OB2, and Scorpius OB1, focussing in more detail to Sco OB1. A multi-wavelength study, in tandem with astrometric and kinematic data from Gaia DR2, seems to lend support, at least in this case, to a scenario in which star formation is not monolithic. As a matter of fact, besides one conspicuous star cluster, NGC 6231, and the very sparse star cluster Trumpler 24, there are several smaller groups of young OB and pre-main sequence stars across the association, indicating that star formation is highly structured and with no preferred scale. A new revolution is expected with the incoming much awaited third release of Gaia data.

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arXiv2020

The relation of the Alpha Persei star cluster with the nearby stellar stream

A map of 100 degrees on a side extracted from Gaia DR2 and centred on Alpha Persei reveals two distinct structures | the Alpha Persei star cluster and a conspicuous stellar stream, as widely documented in recent literature. In this work we employ DBSCAN to assess individual stars' membership and attempt at separating stars belonging to the cluster and to the stream from the general field. In turn, we characterize the stream and investigate its relation with the cluster. The stream population turned out to be significantly older (5+/-1Gyr) than the cluster, and to be positioned ~90 pc away from the cluster, in its background. The stream exhibits a sizeable thickness of ~180 pc in the direction of the line of view. Finally, the stream harbours a prominent population of white dwarf stars. We estimated an upper limit of the stream mass of ~6000 solar masses?. The stream would therefore be the leftover of a relatively massive old cluster. The surface density map of the Alpha Persei evidences the presence of tidal tails. While it is tempting to ascribe them to the interaction with the disrupting old star cluster, we prefer to believe, conservatively, they are of Galactic origin.

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arXiv2020

Effect of binarity in star cluster dynamical mass determination

In this paper we explore the effects that the presence of a fraction of binary stars has in the determination of a star cluster mass via the virial theorem. To reach this aim in an accurate and consistent way, we run a set of simulations using the direct summation, high precision, code NBODY7. By means of this suite of simulations we are able to quantify the overestimate of open-star-cluster-like models' dynamical masses when making a straight application of the virial theorem using available position and radial velocity measurements. The mass inflation caused by the binary 'heating' contribution to the measured velocity dispersion depends, of course, on the initial binary fraction, $f_{b0}$ and its following dynamical evolution. For an $f_{b}$ (evolved up to 1.5 Gyr) in the range $8\% - 42\%$ the overestimate of the mass done using experimentally sounding estimates for the velocity dispersion can be up to a factor 45. We provide a useful fitting formula to correct the dynamical mass determination for the presence of binaries, and underline how neglecting the role of binaries in stellar systems might lead to erroneous conclusions about their total mass budget. If this trend remains valid for larger systems like dwarf spheroidal galaxies, which are still far out of reach for high precision dynamical simulations taking account their binaries, it would imply an incorrect over-estimation of their dark matter content, as inferred by means of available velocity dispersion measurements.

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arXiv2020

On the Origin of the Globular Cluster FSR 1758

Globular clusters in the Milky Way are thought to have either an {\it in situ} origin, or to have been deposited in the Galaxy by past accretion events, like the spectacular Sagittarius dwarf galaxy merger. We aim to probe the origin of the recently discovered globular cluster FSR 1758, often associated with some past merger event, and which happens to be projected toward the Galactic bulge, by a detailed study of its Galactic orbit, and to assign it to the most suitable Galactic component. We employ three different analytical time-independent potential models to calculate the orbit of the cluster by using the Gauss Radau spacings integration method. In addition, a time-dependent bar potential model is added to account for the influence of the Galactic bar. We run a large suite of simulations to account for the uncertainties in the initial conditions, in a Montecarlo fashion. We confirm previous indications that the globular cluster FSR 1758 possesses a retrograde orbits with high eccentricity. The comparative analysis of the orbital parameters of star clusters in the Milky Way, in tandem with recent metallicity estimates, allows us to conclude that FSR1758 is indeed a Galactic bulge intruder. The cluster can therefore be considered an old metal poor halo globular cluster formed {\it in situ} and which is passing right now in the bulge region. Its properties, however, can be roughly accounted for also assuming that the cluster is part of some stream of extra-Galactic origin. We conclude that assessing the origin, either Galactic or extra-galactic, of globular clusters is surely a tantalising task. In any case, by using an {\it Occam's razor} argument, we tend to prefer an {\it in situ} origin for FSR 1758.

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arXiv2019

The Gaia-ESO Survey: Carbon abundance in the Galactic thin and thick disks

This paper focuses on carbon that is one of the most abundant elements in the Universe and is of high importance in the field of nucleosynthesis and galactic and stellar evolution. Even nowadays, the origin of carbon and the relative importance of massive and low- to intermediate-mass stars in producing it is still a matter of debate. In this paper we aim at better understanding the origin of carbon by studying the trends of [C/H], [C/Fe],and [C/Mg] versus [Fe/H], and [Mg/H] for 2133 FGK dwarf stars from the fifth Gaia-ESO Survey internal data release (GES iDR5). The availability of accurate parallaxes and proper motions from Gaia DR2 and radial velocities from GES iDR5 allows us to compute Galactic velocities, orbits and absolute magnitudes and, for 1751 stars, ages via a Bayesian approach. Three different selection methodologies have been adopted to discriminate between thin and thick disk stars. In all the cases, the two stellar groups show different abundance ratios, [C/H], [C/Fe], and [C/Mg], and span different age intervals, with the thick disk stars being, on average, older than those in the thin disk. The behaviours of [C/H], [C/Fe], and [C/Mg] versus [Fe/H], [Mg/H], and age all suggest that C is primarily produced in massive stars like Mg. The increase of [C/Mg] for young thin disk stars indicates a contribution from low-mass stars or the increased C production from massive stars at high metallicities due to the enhanced mass loss. The analysis of the orbital parameters Rmed and |Zmax| support an "inside-out" and "upside-down" formation scenario for the disks of Milky Way.

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arXiv2019

Unresolved binaries and Galactic clusters' mass estimates

Binary stars are present in all stellar systems, yet their role is far from being fully understood. We investigate the effect of unresolved binaries in the derivation of open clusters' mass by star counts. We start from the luminosity functions of five open clusters: IC 2714, NGC 1912, NGC 2099, NGC 6834 and NGC 7142. Luminosity functions are obtained via star counts extracted from the 2MASS database. The fraction of binaries is considered to be independent on stellar magnitude. We take into account different assumptions for the binary mass ratio distribution and assign binary masses using the so-called {\it luminosity-limited pairing} method and Monte-Carlo simulations. We show that cluster masses increase when binary stars are appropriately taken into account.

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arXiv2019

Ruprecht 147: a paradigm of dissolving star cluster

We employed recent Gaia/DR2 data to investigate the dynamical status of the nearby (300 pc), old (2.5 Gyr) open cluster Ruprecht~147. We found prominent leading and trailing tails of stars along the cluster orbit, which demonstrates that Ruprecht~147 is losing stars at fast pace. Star counts indicate the cluster has a core radius of 33.3 arcmin, and a tidal radius of 137.5 arcmin. The cluster also possesses an extended corona, which cannot be reproduced by a simple King model. We computed the present-day cluster mass using its luminosity and mass function, and derived an estimate of 234$\pm$52 $M_{\odot}$. We also estimated the cluster original mass using available recipes extracted from N-body simulations obtaining a mass at birth of 50000$\pm$6500 $M_{\odot}$. Therefore dynamical mass loss, mostly caused by tidal interaction with the Milky Way, reduced the cluster mass by about 99\%. We then conclude that Ruprecht~147 is rapidly dissolving into the general Galactic disc.

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arXiv2018

NGC 6791: a probable bulge cluster without multiple populations

NGC 6791 is a unique stellar cluster, key to our understanding of both the multiple stellar population phenomenon and the evolution and assembly of the Galaxy. However, despite many investigations, its nature is still very controversial. Geisler et al. (2012) found evidence suggesting it was the first open cluster to possess multiple populations but several subsequent studies did not corroborate this. It has also been considered a member of the thin or thick disk or even the bulge, and both as an open or globular cluster or even the remnant of a dwarf galaxy. Here, we present and discuss detailed abundances derived from high resolution spectra obtained with UVES at VLT and HIRES at Keck of 17 evolved stars of this cluster. We obtained a mean [Fe/H]=+0.313+-0.005, in good agreement with recent estimates, and with no indication of star-to-star metallicity variation, as expected. We also did not find any variation in Na, in spite of having selected the very same stars as in Geisler et al. (2012), where a Na variation was claimed. This points to the presence of probable systematics in the lower resolution spectra of this very high metallicity cluster analysed in that work. In fact, we find no evidence for an intrinsic spread in any element, corroborating recent independent APOGEE data. The derived abundances indicate that NGC 6791 very likely formed in the Galactic Bulge and that the proposed association with the Thick Disk is unlikely, despite its present Galactic location. We confirm the most recent hypothesis suggesting that the cluster could have formed in the Bulge and radially migrated to its current location, which appears the best explanation for this intriguing object.

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arXiv2018

Radial velocity and chemical composition of evolved stars in the open clusters NGC 6940 and Tombaugh 5

We present and discuss medium resolution (R $\sim$ 13000), high signal-to-noise ($\mathrm{\frac{S}{N}} \sim 100$), spectroscopic observations in the field of the open clusters NGC\,6940 and Tombaugh\,5. Spectra were recorded for seven candidate red giant stars in both clusters. For the latter we present the very first chemical abundance analysis. We derive radial velocities for all the stars in NGC\,6940, confirming membership to the cluster for all of them, while on the same ground we exclude two stars in To\,5. We perform a chemical abundance analysis of different atomic species, in particular FeI, SiI, CaI, TiI and NiI. The mean metallicity of NGC\,6940 is [Fe/H]=+0.09$\pm$0.06\,dex, in good agreement with previous works, while for To\,5 is [Fe/H]=+0.06$\pm$0.11\,dex. Therefore, both clusters exhibit a chemical composition close to the solar value, and do not deviate from the [Fe/H] Galactic radial abundance gradient. With these new values we estimate the fundamental cluster parameters, after having derived clusters' distances from the \textit{Gaia} DR2 database. By adopting these distances, we derive updated estimated for the clusters ages: 1.0$\pm$0.1\,Gyr of NGC\,6940 and 0.25$\pm$0.05 Gyr for Tombaugh\,5.

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arXiv2018

A sextet of clusters in the Vela OB2 region revealed by Gaia

Using Gaia DR2 data, combined with OmegaCAM ground-based optical photometry from the AD-HOC survey, and detailed Radial Velocity measurements from ESO-Gaia, we analyse in detail a 10x5 deg region around the Wolf-Rayet star $γ^2$ Vel, including the previously known clusters Gamma Vel and NGC2547. Using clustering analysis that considers positions, proper motions and parallax, we discover 6 clusters or associations -- 4 of which appear new. Analysis of the colour-magnitude diagram for these clusters show that 4 of them formed coevally from the same molecular clouds 10 Myr ago, while NGC 2547 formed together with a newly discovered cluster 30 Myr ago. This study shows the incredible wealth of data provided by Gaia for the study of young stellar clusters.

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arXiv2018

Gaia-ESO Survey: INTRIGOSS - A new library of High Resolution Synthetic Spectra

We present a high resolution synthetic spectral library, INTRIGOSS, designed for studying FGK stars. The library is based on atmosphere models computed with specified individual element abundances via ATLAS12 code. Normalized SPectra (NSP) and surface Flux SPectra (FSP), in the 4830-5400 A, wavelength range, were computed with the SPECTRUM code. INTRIGOSS uses the solar composition by Grevesse et al. 2007 and four [alpha/Fe] abundance ratios and consists of 15,232 spectra. The synthetic spectra are computed with astrophysical gf-values derived by comparing synthetic predictions with a very high SNR solar spectrum and the UVES-U580 spectra of five cool giants. The validity of the NSPs is assessed by using the UVES-U580 spectra of 2212 stars observed in the framework of the Gaia-ESO Survey and characterized by homogeneous and accurate atmospheric parameter values and by detailed chemical compositions. The greater accuracy of NSPs with respect to spectra from the AMBRE, GES_Grid, PHOENIX, C14, and B17 synthetic spectral libraries is demonstrated by evaluating the consistency of the predictions of the different libraries for the UVES-U580 sample stars. The validity of the FSPs is checked by comparing their prediction with both observed spectral energy distribution and spectral indices. The comparison of FSPs with SEDs derived from ELODIE, INDO--U.S., and MILES libraries indicates that the former reproduce the observed flux distributions within a few percent and without any systematic trend. The good agreement between observational and synthetic Lick/SDSS indices shows that the predicted blanketing of FSPs well reproduces the observed one, thus confirming the reliability of INTRIGOSS FSPs.

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