arXiv subjects

G. Carraro

Publications and source records attributed to G. Carraro.

At least 37 records · Page 2Linked to original sources

arXiv2021

Gaia-ESO Survey: Detailed elemental abundances in red giants of the peculiar globular cluster NGC 1851

Context. NGC 1851 is one of several globular clusters for which multiple stellar populations of the subgiant branch have been clearly identified and a difference in metallicity detected. A crucial piece of information on the formation history of this cluster can be provided by the sum of A(C+N+O) abundances. However, these values have lacked a general consensus thus far. The separation of the subgiant branch can be based on age and/or A(C+N+O) abundance differences. Aims. Our main aim was to determine carbon, nitrogen, and oxygen abundances for evolved giants in the globular cluster NGC1851 in order to check whether or not the double populations of stars are coeval. Methods. High-resolution spectra, observed with the FLAMES-UVES spectrograph on the ESO VLT telescope, were analysed using a differential model atmosphere method. Results. We provide abundances of up to 29 chemical elements for a sample of 45 giants in NGC 1851. The investigated stars can be separated into two populations with a difference of 0.07 dex in the mean metallicity, 0.3 dex in the mean C/N, and 0.35 dex in the mean s-process dominated element-to-iron abundance ratios [s/Fe]. No significant difference was determined in the mean values of A(C+N+O) as well as in abundance to iron ratios of carbon, alpha- and iron-peak-elements, and of europium. Conclusions. As the averaged A(C+N+O) values between the two populations do not differ, additional evidence is given that NGC 1851 is composed of two clusters, the metal-rich cluster being by about 0.6 Gyr older than the metal-poor one. A global overview of NGC 1851 properties and the detailed abundances of chemical elements favour its formation in a dwarf spheroidal galaxy that was accreted by the Milky Way.

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arXiv2021

The Gaia-ESO Survey: Membership probabilities for stars in 63 open and 7 globular clusters from 3D kinematics

Spectroscopy from the final internal data release of the Gaia-ESO Survey (GES) has been combined with Gaia EDR3 to assign membership probabilities to targets observed towards 63 Galactic open clusters and 7 globular clusters. The membership probabilities are based chiefly on maximum likelihood modelling of the 3D kinematics of the targets, separating them into cluster and field populations. From 43211 observed targets, 13985 are identified as highly probable cluster members ($P>0.9$), with an average membership probability of 0.993. The addition of GES radial velocities successfully drives down the fraction of false positives and we achieve better levels of discrimination in most clusters over the use of astrometric data alone, especially those at larger distances. Since the membership selection is almost purely kinematic, the union of this catalogue with GES and Gaia is ideal for investigating the photometric and chemical properties of clusters as a function of stellar mass, age and Galactic position.

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arXiv2021

The Gaia-ESO Survey: A new approach to chemically characterising young open clusters II. Abundances of the neutron-capture elements Cu, Sr, Y, Zr, Ba, La, and Ce

Young open clusters (t<200 Myr) have been observed to exhibit several peculiarities in their chemical compositions, from a slightly sub-solar iron content, super-solar abundances of some atomic species (e.g. ionised chromium), and atypical enhancements of [Ba/Fe], with values up to +0.7 dex. Regarding the behaviour of the other $s$-process elements like yttrium, zirconium, lanthanum, and cerium, there is general disagreement in the literature. In this work we expand upon our previous analysis of a sample of five young open clusters (IC2391, IC2602, IC4665, NGC2516, and NGC2547) and one star-forming region (NGC2264), with the aim of determining abundances of different neutron-capture elements, mainly CuI, SrI, SrII, YII, ZrII, BaII, LaII, and CeII. We analysed high-resolution, high signal-to-noise spectra of 23 solar-type stars observed within the \textit{Gaia}-ESO survey. We find that our clusters have solar [Cu/Fe] within the uncertainties, while we confirm the super-solar [Ba/Fe] values (from +0.22 to +0.64 dex). Our analysis also points to mildly enhanced [Y/Fe] values (from 0 and +0.3 dex). For the other $s$-process elements we find that [X/Fe] ratios are solar at all ages. It is not possible to reconcile the anomalous behaviour of Ba and Y at young ages with standard stellar yields and Galactic chemical evolution model predictions. Thus, we explore different possible scenarios related to the behaviour of spectral lines, from the sensitivity to the presence of magnetic fields to the first ionisation potential effect. We also investigate the possibility that they may arise from alterations of the structure of the stellar photosphere due to higher levels of activity in such young stars. We are still unable to explain these enhancements, but we suggest that other elements (i.e. La) might be more reliable tracer of the $s$-process at young ages and encourage further observations.

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arXiv2021

The Gaia-ESO Survey: Galactic evolution of lithium from iDR6

We exploit the unique characteristics of a sample of open clusters (OCs) and field stars for which high-precision 7Li abundances and stellar parameters are homogeneously derived by the Gaia-ESO Survey (GES). We derive possibly undepleted 7Li abundances for 26 OCs and star forming regions with ages from young to old spanning a large range of Galactocentric distances, which allows us to reconstruct the local late Galactic evolution of lithium as well as its current abundance gradient along the disc. Field stars are added to look further back in time and to constrain 7Li evolution in other Galactic components. The data are then compared to theoretical tracks from chemical evolution models that implement different 7Li forges. We find that the upper envelope of the 7Li abundances measured in field stars of nearly solar metallicities traces very well the level of lithium enrichment attained by the ISM as inferred from observations of cluster stars. We confirm previous findings that the abundance of 7Li in the solar neighbourhood does not decrease at supersolar metallicity. The comparison of the data with the chemical evolution model predictions favours a scenario in which the majority of the 7Li abundance in meteorites comes from novae. Current data also seem to suggest that the nova rate flattens out at later times. This requirement might have implications for the masses of the white dwarf nova progenitors and deserves further investigation. Neutrino-induced reactions taking place in core-collapse supernovae also produce some fresh lithium. This likely makes a negligible contribution to the meteoritic abundance, but could be responsible for a mild increase of the 7Li abundance in the ISM of low-metallicity systems that would counterbalance the astration processes.

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arXiv2021

Infrared photometry and CaT spectroscopy of globular cluster M 28 (NGC 6626)

Recent studies show that the inner Galactic regions host genuine bulge globular clusters, but also halo intruders, complex remnants of primordial building blocks, and objects likely accreted during major merging events. In this study we focus on the properties of M 28, a very old and massive cluster currently located in the Galactic bulge. We analysed wide-field infrared photometry collected by the VVV survey, VVV proper motions, and intermediate-resolution spectra in the calcium triplet range for 113 targets in the cluster area. Our results in general confirm previous estimates of the cluster properties available in the literature. We find no evidence of differences in metallicity between cluster stars, setting an upper limit of Delta[Fe/H]<0.08 dex to any internal inhomogeneity. We confirm that M 28 is one of the oldest objects in the Galactic bulge (13-14 Gyr). From this result and the literature data, we find evidence of a weak age-metallicity relation among bulge globular clusters that suggests formation and chemical enrichment. In addition, wide-field density maps show that M 28 is tidally stressed and that it is losing mass into the general bulge field. Our study indicates that M 28 is a genuine bulge globular cluster, but its very old age and its mass loss suggest that this cluster could be the remnant of a larger structure, possibly a primeval bulge building block.

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arXiv2021

A new, Gaia based, catalogue of blue straggler stars in open clusters

Blue straggler stars are exotic objects present in all stellar environments whose nature and formation channels are still partially unclear. They seem to be particularly abundant in open clusters (OCs), thus offering a unique chance to tackle these problems statistically.We aim to build up a new and homogeneous catalogue of blue straggler stars (BSS) in Galactic OCs using Gaia to provide a more solid assessment of the membership of these stars. We also aim to explore possible relationships of the straggler abundance with the parent cluster's structural and dynamical parameters. As a by-product, we also search for possible yellow straggler stars (YSS), which are believed to be stragglers in a more advanced evolution stage. We employed photometry, proper motions, and parallaxes extracted from Gaia DR2 for 408 Galactic star clusters and searched for stragglers within them after performing a careful membership analysis. The number of BBS emerging from our more stringent, selection criteria turns out to be significantly smaller than in previous versions of this catalogue. OCs are therefore not a preferable environment for these kinds of stars anymore. In addition, we found that BSS start to appear in clusters with ages larger than log(t) ~ 8.7 and are therefore absent in very young star clusters.The present catalogue supersedes the previous ones in several ways: membership assessment, number of stragglers found, and so forth. The new list includes 897 BSS and 77 YSS candidates in 408 OCs. We expect this catalogue to be the basis for a new round of studies of BSS and YSS.

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arXiv2020

The blue straggler population of the open clusters Trumpler 5, Trumpler 20, and NGC 2477

We present a study, based on Gaia DR2, of the population of blue straggler stars (BSS) in the open clusters Trumpler 5, Trumpler 20, and NGC 2477. All candidates were selected according to their position in the color-magnitude diagram, to their proper motion components, and to their parallax. We also looked for yellow stragglers, i.e., possible evolved blue stragglers. We found that Trumpler 5 hosts a large BSS population, which allowed us to analyze their radial distribution as a probe of the cluster's dynamical status. The BSS distribution was compared with that of red giant branch stars (RGB) to evaluate mass segregation. Our results indicate that blue straggler stars are not more centrally concentrated than RGB stars in any of the clusters. The radial distribution of BSS in Trumpler 5 is flat. Additionally, using a multi-epoch radial velocity survey conducted with the high-resolution spectrograph FLAMES/GIRAFFE at VLT, we measured the radial velocities of a sample of stragglers, for the sake of comparison with the mean radial velocity and the velocity dispersion of the clusters. Based on the radial velocity variations for different epochs, we roughly classified these stars as possible close-or long-period binaries.

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arXiv2020

The Gaia-ESO Survey: Calibrating the lithium-age relation with open clusters and associations. I. Cluster age range and initial membership selections

Previous studies of open clusters have shown that lithium depletion is not only strongly age dependent but also shows a complex pattern with other parameters that is not yet understood. For pre- and main-sequence late-type stars, these parameters include metallicity, mixing mechanisms, convection structure, rotation, and magnetic activity. We perform a thorough membership analysis for a large number of stars observed within the Gaia-ESO survey (GES) in the field of 20 open clusters, ranging in age from young clusters and associations, to intermediate-age and old open clusters. Based on the parameters derived from the GES spectroscopic observations, we obtained lists of candidate members for each of the clusters in the sample by deriving RV distributions and studying the position of the kinematic selections in the EW(Li) versus Teff plane to obtain lithium members. We used gravity indicators to discard field contaminants and studied [Fe/H] metallicity to further confirm the membership of the candidates. We also made use of studies using recent data from the Gaia DR1 and DR2 releases to assess our member selections. We identified likely member candidates for the sample of 20 clusters observed in GES (iDR4) with UVES and GIRAFFE, and conducted a comparative study that allowed us to characterize the properties of these members, as well as identify field contaminant stars, both lithium-rich giants and non-giant outliers. This work is the first step towards the calibration of the lithium-age relation and its dependence on other GES parameters. During this project we aim to use this relation to infer the ages of GES field stars, and identify their potential membership to young associations and stellar kinematic groups of different ages.

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arXiv2020

The Gaia-ESO Survey: Spectroscopic-asteroseismic analysis of K2 stars in Gaia-ESO

The extensive stellar spectroscopic datasets that are available for studies in Galactic Archeaology thanks to, for example, the Gaia-ESO Survey, now benefit from having a significant number of targets that overlap with asteroseismology projects such as Kepler, K2 and CoRoT. Combining the measurements from spectroscopy and asteroseismology allows us to attain greater accuracy with regard to the stellar parameters needed to characterise the stellar populations of the Milky Way. The aim of this Gaia-ESO Survey special project is to produce a catalogue of self-consistent stellar parameters by combining measurements from high-resolution spectroscopy and precision asteroseismology. We carried out an iterative analysis of 90 K2@Gaia-ESO red giants. The spectroscopic values of Teff were used as input in the seismic analysis to obtain log(g) values. The seismic estimates of log(g) were then used to re-determine the spectroscopic values of Teff and [Fe/H]. Only one iteration was required to obtain parameters that are in good agreement for both methods and thus, to obtain the final stellar parameters. A detailed analysis of outliers was carried out to ensure a robust determination of the parameters. The results were then combined with Gaia DR2 data to compare the seismic log(g) with a parallax-based log(g) and to investigate instances of variations in the velocity and possible binaries within the dataset. This analysis produced a high-quality catalogue of stellar parameters for 90 red giant stars observed by both K2 and Gaia-ESO that were determined through iterations between spectroscopy and asteroseismology. We compared the seismic gravities with those based on Gaia parallaxes to find an offset which is similar to other studies that have used asteroseismology. Our catalogue also includes spectroscopic chemical abundances and radial velocities, as well as indicators for possible binary detections.

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arXiv2020

The Gaia-ESO Survey: Galactic evolution of lithium at high metallicity

Reconstructing the Galactic evolution of lithium (Li) is the main tool used to constrain the source(s) of Li enrichment in the Galaxy. Recent results have suggested a decline in Li at supersolar metallicities, which may indicate reduced production. We exploit the unique characteristics of the Gaia-ESO Survey open star cluster sample to further investigate this issue and to better constrain the evolution of Li at high metallicity. We trace the the upper envelope of Li abundance versus metallicity evolution using 18 clusters and considering members that should not have suffered any Li depletion. At variance with previous claims, we do not find any evidence of a Li decrease at high metallicity. The most metal-rich clusters in the sample ([Fe/H] about 0.3) actually show the highest Li abundances, with A(Li) > 3.4. Our results clearly show that previous findings, which were based on field stars, were affected by selection effects. The metal-rich population in the solar neighbourhood is composed of relatively old and cool stars that have already undergone some Li depletion; hence, their measured Li does not represent the initial interstellar medium abundance, but a lower limit to it.

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arXiv2020

The Gaia-ESO Survey: an extremely Li-rich giant in the globular cluster NGC 1261

Lithium rich stars in globular clusters are rare. In fact, only 14 have been found so far, in different evolutionary phases from dwarfs to giants. Different mechanisms have been proposed to explain this enhancement, but it is still an open problem. Using spectra collected within the Gaia-ESO Survey, obtained with the GIRAFFE spectrograph at the ESO Very Large Telescope, we present the discovery of the first Li-rich star in the cluster NGC 1261, the second star known in the red giant branch bump phase. The star shows an extreme Li overabundance of A(Li)_LTE=3.92\pm0.14, corresponding to A(Li)_NLTE=3.40 dex. We propose that the Li enhancement is caused by fresh Li production through an extra mixing process (sometimes referred to as {\em cool bottom burning}) or could be a pre-existing Li overabundance resulting from binary mass transfer, likely from a red giant branch star, because of the low barium abundance. To unambiguously explain the Li enhancement in globular cluster stars, however, a reliable determination of the abundance of key species like Be, 6Li, 12C/13C, and several s-process elements is required, as well as detailed modeling of chromospheric activity indicators.

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arXiv2020

A new look at Sco OB1 association with Gaia DR2

We present and discuss photometric optical data in the area of the OB association Sco OB1 covering about 1 squared degree. UBVI photometry is employed in tandem with Gaia DR2 data to investigate the 3 dimensional structure and the star formation history of the region. By combining parallaxes and proper motions we identify 7 physical groups located between the young open cluster NGC 6231 and the bright nebula IC4628. The most prominent group coincides with the sparse open cluster Trumpler 24. We confirm the presence of the intermediate age star cluster VdB-Hagen 202, which is unexpected in this environment, and provide for the first time estimates of its fundamental parameters. After assessing individual groups membership, we derive mean proper motion components, distances, and ages. The seven groups belong to two different families. To the younger family (family I) belong several pre-Main Sequence stars as well. These are evenly spread across the field, and also in front of VdB-Hagen 202. VdB-Hagen 202 and two smaller, slightly detached, groups of similar properties form family II, which do not belong to the association, but are caught in the act of passing through it. As for the younger population, this forms an arc-like structure from the bright nebula IC 4628 down to NGC 6231, as previously found. Moreover, the pre-Main Sequence stars density seems to increase from NGC 6231 northward to Trumpler 24.

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arXiv2020

Milky Way Subsystems from Globular Clusters Kinematics Using Gaia DR2 and HST Data

We employ Gaia DR2 proper motions for 151 Milky Way globular clusters from Vasiliev (2019) in tandem with distances and line-of-sight velocities to derive their kinematical properties. To assign clusters to the Milky Way thick disk, bulge, and halo we follow the approach of Posti et al. (2018) who distinguished among different Galactic stellar components using stars's orbits. In particular, we use the ratio $L_{z}/e$, the $Z$ projection of the angular momentum to the eccentricity, as population tracer, which we complement with chemical abundances extracted from the literature and Monte-Carlo simulations. We find that 20 globular clusters belong to the bar/bulge of the Milky Way, 35 exhibit disk properties, and 96 are members of the halo. Moreover, we find that halo globular clusters have close to zero rotational velocity with average value $<Θ>$ =1$\pm$ 4 km s$^{-1}$. On the other hand, the sample of clusters that belong to the thick disk possesses a significant rotation with average rotational velocity 179 $\pm$ 6 km s$^{-1}$. The twenty globular clusters orbiting within the bar/bulge region of the Milky Way galaxy have average rotational velocity of 49 $\pm$ 11 km s$^{-1}$.

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arXiv2020

Sky Quality Meter and satellite correlation for the night cloud cover analysis at astronomical sites

The analysis of the night cloud cover is very important for astronomical observation in real time, considering a typical observation time of about 15 minutes, and to have a statistics of the night cloud cover. In this paper we use the SQM (Sky Quality Meter) for high resolution temporal analysis of the La Silla and Asiago (Ekar observatory) sky: 3 and 5 minutes respectively. We investigate the annual temporal evolution of the natural contributions of the sky in a site not influenced by artificial light at night (ALAN) and one highly influenced respectively. We also make a correlation between GOES and AQUA satellites data and ground-based SQM data to confirm a relationship between the SQM data and cloud cover. We develop an algorithm that allows the use of the SQM for night cloud detection and we reach a correlation of 97.2\% at La Silla and 94.6\% at Asiago with the nighttime cloud cover detected by the GOES and AQUA satellites. Our algorithm also classifies the photometric (PN) and spectroscopic nights (SN). We measure 59.1\% PN and 21.7\% SN for a total percentage of clear nights of 80.8\% at La Silla in 2018. The respective Ekar observatory values are 31.1\% PN, 24.0\% SN and 55.1\% of total clear nights time. Application to the SQM network would involve the development of long-term statistics and big data forecasting models, for site testing and real-time astronomical observation.

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arXiv2020

Sixteen overlooked open clusters in the fourth Galactic quadrant. A combined analysis of UBVI photometry and Gaia DR2 with ASteCA

Aims: This paper has two main objectives: (1) To determine the intrinsic properties of 16 faint and mostly unstudied open clusters in the poorly known sector of the Galaxy at 270$^\circ-$300$^\circ$, to probe the Milky Way structure in future investigations. (2) To address previously reported systematics in Gaia DR2 parallaxes by comparing the cluster distances derived from photometry with those derived from parallaxes. Methods: Deep UBVI photometry of 16 open clusters was carried out. Observations were reduced and analyzed in an automaticway using the ASteCA package to get individual distances, reddening, masses, ages and metallicities. Photometric distances were compared to those obtained from a Bayesian analysis of Gaia DR2 parallaxes. Results: Ten out of the 16 clusters are true or highly probable open clusters. Two of them are quite young and follow the trace of the Carina Arm and the already detected warp. The rest of the clusters are placed in the interarm zone between the Perseus and Carina Arms as expected for older objects. We found that the cluster van den Berg-Hagen 85 is 7.5$\times$10$^9$ yrs old becoming then one of the oldest open cluster detected in our Galaxy so far. The relationship of these ten clusters with the Galaxy structure in the solar neighborhood is discussed. The comparison of distances from photometry and parallaxes data, in turn, reveals a variable level of disagreement. Conclusions: Various zero point corrections for Gaia DR2 parallax data recently reported were considered for a comparison between photometric and parallax based distances. The results tend to improve with some of these corrections. Photometric distance analysis suggest an average correction of $\sim$+0.026 mas (to be added to the parallaxes). The correction may have a more intricate distance dependency, but addressing that level of detail will require a larger cluster sample.

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arXiv2020

The Gaia-ESO Survey: a new approach to chemically characterising young open clusters

Open clusters (OCs) are recognised as excellent tracers of Galactic thin-disc properties. At variance with intermediate-age and old OCs, for which a significant number of studies is now available, clusters younger than 150 Myr have been mostly overlooked in terms of their chemical composition, with few exceptions. On the other hand, previous investigations seem to indicate an anomalous behaviour of young clusters, which includes slightly sub-solar iron (Fe) abundances and extreme, unexpectedly high barium (Ba) enhancements. In a series of papers, we plan to expand our understanding of this topic and investigate whether these chemical peculiarities are instead related to abundance analysis techniques. We present a new determination of the atmospheric parameters for 23 dwarf stars observed by the Gaia-ESO survey in five young OCs (younger than 150 Myr) and one star-forming region (NGC 2264). We exploit a new method based on titanium (Ti) lines to derive the spectroscopic surface gravity, and most importantly, the microturbulence parameter. A combination of Ti I and Fe I lines is used to obtain effective temperatures. We also infer the abundances of Fe II, Ti II, Na I, Mg I, Al I, Si I, Ca I, Cr I and Ni I. Our findings are in fair agreement with Gaia-ESO iDR5 results for effective temperatures and surface gravities, but suggest that for very young stars, the microturbulence parameter is over-estimated when Fe lines are employed. This affects the derived chemical composition and causes the metal content of very young clusters to be under-estimated. Our clusters display a metallicity [Fe/H] between +0.04 and +0.12; they are not more metal poor than the Sun. Although based on a relatively small sample size, our explorative study suggests that we may not need to call for ad hoc explanations to reconcile the chemical composition of young OCs with Galactic chemical evolution models.

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arXiv2019

A study of the blue straggler population of the old open cluster Collinder 261

Blue Stragglers are stars located in an unexpected region of the color-magnitude diagram of a stellar population, as they appear bluer and more luminous than the stars in the turnoff region. They are ubiquitous, since they have been found among Milky Way field stars, in open and globular clusters, and also in other galaxies of the Local Group. Here we present a study on the blue straggler population of the old and metal-rich open cluster Collinder 261, based on Gaia DR2 data and on a multi-epoch radial velocity survey conducted with FLAMES@VLT. We also analyze the radial distribution of the blue straggler population to probe the dynamical status of the cluster. Blue straggler candidates were identified first with Gaia DR2, according to their position on the CMD, proper motions, and parallaxes. Their radial distribution was compared with those of the main sequence, red giant, and red clump stars, to evaluate mass segregation. Additionally, their radial velocities (and the associated uncertainties) were compared with the mean radial velocity and the velocity dispersion of the cluster. When possible, close binaries and long-period binaries were also identified, based on the radial velocity variations for the different epochs. We also looked for yellow stragglers, i.e., possible evolved blue stragglers. We found 53 blue stragglers members of Collinder 261, six of them already identified in previous catalogs. Among the blue straggler candidates with radial velocity measurements, we found one long-period binary, five close-binary systems, three non-variable stars; we also identified one yellow straggler.

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arXiv2019

The Gaia-ESO survey: Calibrating a relationship between Age and the [C/N] abundance ratio with open clusters

In the era of large high-resolution spectroscopic surveys, high-quality spectra can contribute to our understanding of the Galactic chemical evolution, providing chemical abundances belonging to the different nucleosynthesis channels, and also providing constraints to stellar age. Some abundance ratios have been proven to be excellent indicators of stellar ages. We aim at providing an empirical relationship between stellar ages and [C/N] using, as calibrators, open star clusters observed by both the Gaia-ESO and APOGEE surveys. We use stellar parameters and abundances from the Gaia-ESO and APOGEE of the Galactic field and open cluster stars. Ages of star clusters are retrieved from the literature sources and validated using a common set of isochrones. We use the same isochrones to determine, for each age and metallicity, the surface gravity at which the first dredge-up and red giant branch bump occur. We study the effect of extra-mixing processes in our sample of giant stars, and we derive the mean [C/N] in evolved stars, including only stars without evidence of extra-mixing. Combining the Gaia-ESO and APOGEE samples of open clusters, we derive a linear relationship between [C/N] and logarithmic cluster ages. We apply our relationship to selected giant field stars in both Gaia-ESO and APOGEE. We find an age separation between thin and thick disc stars and age trends within their populations, with an increasing age towards lower metallicity populations. With such empirical relationship, we are able to provide an age estimate for giant stars in which C and N abundances are measured. Isochrone fitting is less sensitive for giant than dwarf stars at the turn off. The present method can be thus considered as an additional tool to give an independent estimate of the age of giant stars, with uncertainties in their ages comparable to those obtained using isochrone fitting for dwarf stars.

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