arXiv subjects

Eugenio Carretta

Publications and source records attributed to Eugenio Carretta.

At least 55 records · Page 3Linked to original sources

arXiv2012

Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters

Recent progress in studies of globular clusters has shown that they are not simple stellar populations, being rather made of multiple generations. Evidence stems both from photometry and spectroscopy. A new paradigm is then arising for the formation of massive star clusters, which includes several episodes of star formation. While this provides an explanation for several features of globular clusters, including the second parameter problem, it also opens new perspectives about the relation between globular clusters and the halo of our Galaxy, and by extension of all populations with a high specific frequency of globular clusters, such as, e.g., giant elliptical galaxies. We review progress in this area, focusing on the most recent studies. Several points remain to be properly understood, in particular those concerning the nature of the polluters producing the abundance pattern in the clusters and the typical timescale, the range of cluster masses where this phenomenon is active, and the relation between globular clusters and other satellites of our Galaxy.

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arXiv2011

A Stromgren view of the multiple populations in globular clusters

We discuss a variety of photometric indices assembled from the uvby Stromgren system. Our aim is to examine the pros and cons of the various indices to find the most suitable one(s) to study the properties of multiple populations in globular clusters (GCs) discovered by spectroscopy. We explore in particular the capabilities of indices like m_1 and c_y at different metallicities. We define a new index delta_4=(u-v)-(b-y) to separate first and second stellar generations in GCs of any metal abundance, since it keeps the sensitivity to multiple stellar populations over all the metallicity range and at the same time minimizes the sensitivity to photometric errors. We detecte clear differences in the red giant branches of the GCs examined, like skewness or bi/multi-modality in color distribution. We connect the photometric information with the spectroscopic results on O, Na abundances we obtained in our survey of GCs. Finally, we compute the effects of different chemical composition on the Stromgren filters and indices using synthetic spectra.

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arXiv2011

Chemical enrichment mechanisms in Omega Centauri: clues from neutron-capture elements

In the complex picture of multiple stellar populations in globular clusters (GCs), a special role is played by NGC 5139 Omega Centauri. At variance with the majority of GCs, Omega Cen exhibits significant star-to-star variations in metallicity and in relative neutron-capture element abundance ratios with respect to Fe, along with split evolutionary sequences as revealed from colour-magnitude diagrams. Combining information from photometry and spectroscopy, several studies suggested that an age spread of several Gyr has to be invoked to explain (at least partially) some of the observed features. However, a comprehensive understanding of the formation, evolution and chemical enrichment processes is still not at hand. Relatively metal-rich Omega cen stars display neutron-capture abundance distributions dominated by contributions from the s-process, but it is not clear what roles have been played by the so-called main and weak s-process components in generating these abundances. To gain better insight into this question we derived lead (Pb) abundances for several cluster members, because this element can only be produced by the main s-process. We analysed high-resolution UVES@VLT spectra of a sample of twelve red-giant branch stars, deriving abundances of Pb and also of Y, Zr, La, Ce, Eu, and the C+N+O sum. Spectral synthesis was applied to all features, taking into account isotopic shifts and/or hyperfine structure as needed. We measured for the first time the Pb content in Omega Cen, discovering a clear hint for a Pb production occurring at [Fe/H]>-1.7 dex. Our data suggest that the role of the weak component in the production of s-process elements is negligible. Moreover, evidence gathered from the abundances of other elements indicates that the main component occurring in this GC is peculiar and shifted towards higher mass polluters than the standard one.

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arXiv2011

Multiple stellar populations in the globular cluster NGC 1851

[Abridged] Detailed chemical tagging of individual stellar populations in the Galactic globular cluster (GC) NGC 1851 is presented. Abundances are derived from FLAMES spectra for the largest sample of giants (124) ever analysed in this peculiar GC using O, Na, Mg, Al, Si, Ca, Ti, Sc, V, Mn, Co, Ni, Cu, Y, Zr, Ba, La, Ce, Nd, Eu, Dy. We confirm a [Fe/H] spread in this GC, but too small to clearly separate different sub-populations. We instead propose a classification using a combination of Fe and Ba (much more abundant in the more metal-rich group) through a cluster analysis: we separated stars into a metal-rich (MR) and a metal-poor (MP) populations, each of them showing a Na-O anticorrelation, signature of genuine GC, although with different ratios of primordial to polluted stars. The ratio [alpha/H] tracks iron and is higher in the MR population, suggesting an additional contribution by core-collapse supernovae. The MR population shows a larger enrichment in s-process elements compared to the MP one. This is probably due to the contribution by intermediate-low mass stars, because we find that the level of heavy s-process elements is larger than the level of light s-process nuclei in the MR stars; however, a large contribution by low mass stars is unlikely, because it would likely cancel the O-Na anticorrelation. We also confirm the presence of correlations between p-capture and s-process elements previously found by other studies, at least for the MR population. This finding apparently requires a quite long delay for the second generation of the MR component. A merger of two distinct GCs in a parent dwarf galaxy, each one with a different Ba level and an age difference of ~1 Gyr, might explain (i) the double SGB, (ii) a possible difference in C content between the two original GCs, and (iii) the Stromgren photometry of this peculiar GC.

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arXiv2011

Chemical composition of evolved stars in the open cluster NGC 2506

In this study we present abundances of 12^C, 13^C, N, O and up to 26 other chemical elements in two first ascent giants and two core-helium-burning 'clump' stars of the open cluster NGC 2506. Abundances of carbon were derived using the C_2 Swan (0,1) band head at 5635.5 A. The wavelength interval 7940-8130 A, with strong CN features, was analysed in order to determine nitrogen abundances and carbon isotope ratios. The oxygen abundances were determined from the [O I] line at 6300 A. NGC 2506 was found to have a mean [Fe/H]=-0.24 +- 0.05 (standard deviation). Compared with the Sun and other dwarf stars of the Galactic disc, mean abundances in the investigated clump stars suggest that carbon is depleted by about 0.2 dex, nitrogen is overabundant by about 0.3 dex and other chemical elements have abundance ratios close to solar. The C/N and 12^C/13^C ratios are lowered to 1.25 +- 0.27 and 11 +- 3, respectively.

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arXiv2011

Chemical composition of evolved stars in the open cluster IC 4651

We present an analysis of high-resolution spectra of three core-helium-burning "clump" stars and two first ascent giants in the open cluster IC 4651. Atmospheric parameters (T_eff, log_g, v_t, and [Fe/H]) were determined in our previous study by Carretta et al. (2004). In this study we present abundances of C, N, O and up to 24 other chemical elements. Abundances of carbon were derived using the C_2 Swan (0,1) band head at 5635.5 A. The wavelength interval 7980-8130 A, with strong CN features, was analysed in order to determine nitrogen abundances and 12^C/13^C isotope ratios. The oxygen abundances were determined from the [O i] line at 6300 A. Compared with the Sun and other dwarf stars of the Galactic disk, mean abundances in the investigated clump stars suggest that carbon is depleted by about 0.3 dex, nitrogen is overabundant by about 0.2 dex and oxygen is close to solar. This has the effect of lowering the mean C/N ratio to 1.36 +- 0.11. The mean 12^C/13^C ratios are lowered to 16 +- 2. Other investigated chemical elements have abundance ratios close to the solar ones.

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arXiv2010

Diluting the material forming the second generation stars in Globular Clusters: the contribution by unevolved stars

In this short communication we consider the possibility that stars less evolved than the polluters are the source of the dilution needed to explain the observed composition of second-generation globular cluster (GC) stars and the Na-O and Mg-Al anticorrelations. If these stars can lose 0.5-1% of their mass during the relevant epochs, there is enough diluting material to produce the observed anticorrelations. In this case, the original mass of proto-GCs was several tens times higher than the current mass of GCs. While not strictly impossible, this is a stringent hypothesis that needs more support. Should this scenario be found true, then the link between the primordial (first-generation) population in GC and the field population would be very strong.

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arXiv2010

Ba stars and other binaries in first and second generation stars in globular clusters

The determination of the Ba abundance in globular cluster (GC) stars is a very powerful test to address several issues in the framework of multiple population scenarios. We measured the Ba content for a sample of more than 1200 stars in 15 Galactic GCs, using high-resolution FLAMES/Giraffe spectra. We found no variation in [Ba/Fe] ratios for different stellar populations within each cluster; this means that low-mass asymptotic giant branch stars do not significantly contribute to the intra-cluster pollution. Very interestingly, we obtained that the fraction of Ba-stars in first generation (FG) stars is close to the values derived for field stars ($\sim$2\%); on the other hand, second generation (SG) stars present a significant lower fraction. An independent and successful test, based on radial velocity variations among giant stars in NGC~6121, confirms our finding: the binary fraction among FG stars is about $\sim$12\% to be compared with $\sim$1\% of SG stars. This is an evidence that SG stars formed in a denser environment, where infant mortality of binary systems was particularly efficient.

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arXiv2010

On the radial distribution of stars of different stellar generations in the globular cluster NGC 3201

We study the radial distribution of stars of different stellar generations in the globular cluster NGC 3201. From recently published multicolour photometry, a radial dependence of the location of stars on the giant branch was found. We coupled the photometric information to our sample of 100 red giants with Na, O abundances and known classification as first or second-generation stars. We find that giants stars of the second generation in NGC 3201 show a tendency to be more centrally concentrated than stars of the first generation, supporting less robust results from our spectroscopic analysis.

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arXiv2010

Lithium and proton-capture elements in Globular Cluster dwarfs: the case of 47 Tuc

Previous surveys in a few metal-poor globular clusters (GCs) showed that the determination of abundances for Li and proton-capture elements offers a key tool to address the intracluster pollution scenario. In this Letter, we present Na, O, and Li abundances in a large sample of dwarf stars in the metal-rich GC 47 Tucanae. We found a clear Na-O anticorrelation, in good agreement with what obtained for giant members by Carretta et al. While lithium and oxygen abundances appear to be positively correlated with each other, there is a large scatter, well exceeding observational errors, and no anticorrelation with sodium. These findings suggest that Li depletion, due to mechanisms internal to the stars (which are cooler and more metal-rich than those on the Spite plateau), combines with the usual pollution scenario responsible for the Na-O anticorrelation.

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arXiv2010

Chemical composition of clump stars in the open cluster NGC 6134

We present an analysis of high-resolution spectra of six core-helium-burning "clump" stars in the open cluster NGC 6134. Atmospheric parameters T_eff, log_g, v_t, and [Fe/H] were determined in our previous study by Carretta et al. (2004). In this study we present abundances of C, N, O and up to 24 other chemical elements. Abundances of carbon were derived using the C_2 Swan (0,1) band head at 5635.5 A (FEROS spectra) and the C_2 Swan (1,0) band head at 4737 A (UVES spectra). The wavelength interval 7980-8130 A, with strong CN features, was analysed in order to determine nitrogen abundances and 12^C/13^C isotope ratios. The oxygen abundances were determined from the [O I] line at 6300 A. Compared with the Sun and other dwarf stars of the Galactic disk, mean abundances in the investigated clump stars suggest that carbon is depleted by about 0.2 dex, nitrogen is overabundant by about 0.3 dex and oxygen is underabundant by about 0.1 dex. This has the effect of lowering the mean C/N ratio to 1.2 +- 0.2. The mean 12^C/13^C ratios are lowered to 9 +-2.5. Concerning other chemical elements, the analysis of sodium and magnesium lines (in NLTE), lines of other alpha-elements, iron-group and heavier chemical elements gave abundance ratios close to the solar ones.

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arXiv2010

Helium in first and second-generation stars in Globular Clusters from spectroscopy of red giants

(abridged) Recent spectroscopic and photometric observations show the existence of various generations of stars in GCs, differing in the abundances of products of H-burning at high temperatures (the main final product being He). It is important to study the connections between stars properties and He content. We consider here the about 1400 stars on the Red Giant Branch (RGB) observed with FLAMES@VLT in 19 Galactic GCs, part of out Na-O anticorrelation projet. Stars with different He are expected to have different temperatures (i.e. colours), slightly different [Fe/H], and different luminosity levels of the RGB bump. All these differences are small, but our study has the necessary precision, good statistics, and homogeneity to detect them. We also computed suitable sets of stellar models (BaSTI) for various assumptions about the initial helium content. Differences in observable quantities that can be attributed to variations in He content are generally detectable between stars of the Primordial (P, first-generation) and Extreme (E, second-generation) populations, but not between the Primordial and Intermediate ones (I). The only exception (differences are significant also between P and I populations) is NGC2808, where three populations are clearly separated also on the Main Sequence and the Horizontal Branch. The average enhancement in the He mass fraction Y between P and E stars is about 0.05-0.11, depending on the assumptions. The differences in Y, for NGC2808 alone, are about 0.11-0.14 between P and I stars, and about 0.15-0.19 between P and E stars, again depending on the assumptions. The RGB bump luminosity of first and second-generation stars has different levels; the implied Y difference is more difficult to quantify, but is in agreement with the other determinations.

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arXiv2010

The second and third parameters of the Horizontal Branch in Globular Clusters

The Horizontal Branch (HB) second parameter of Globular Clusters (GCs) is a major open issue in stellar evolution. Large photometric and spectroscopic databases allow a re-examination of this issue. We derive median and extreme (90% of the distribution) colours and magnitudes of stars along the HB for about a hundred GCs. We transform these into median and extreme masses of stars on the HB taking into account evolutionary effects, and compare these masses with those expected at the tip of the Red Giant Branch to derive the total mass lost by the stars. A simple linear dependence on metallicity of this total mass lost explains well the median colours of HB stars. Adopting this mass loss law as universal, we find that age is the main second parameter. However, at least a third parameter is clearly required. The most likely candidate is the He abundance, which might be different in GCs stars belonging to the different stellar generations whose presence was previously derived from the Na-O and Mg-Al anticorrelations. Variations in the median He abundance allow explaining the extremely blue HB of some GCs; such variations are correlated with the R-parameter. Suitable He abundances allow deriving ages from the HB which are consistent with those obtained from the Main Sequence. Small corrections to these latter ages are then proposed, producing a tight age-metallicity relation for disk and bulge GCs. Star-to-star variations in the He content explain the extension of the HB. There is a strong correlation between this extension and the interquartile of the Na-O anticorrelation. The main driver for the variations in the He-content within GCs seems the total cluster mass. 47 Tuc and M3 exhibit exceptional behaviours; however, they can be accommodated in a scenario for the formation of GCs that relates their origin to cooling flows generated after very large episodes of star formation.

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arXiv2010

Properties of stellar generations in Globular Clusters and relations with global parameters

ABRIDGED) We revise the formation of Galactic GCs by adding the detailed chemical composition of their different stellar generations (from 1200 giants in 19 GCs) to their global parameters. We propose to identify as GCs those showing the Na-O anticorrelation, and we classify the GCs according to kinematics and location in the Galaxy in disk/bulge, inner, and outer halo. We find that the LF of GCs is fairly independent of their population, suggesting that it is imprinted by the formation mechanism, and only marginally affected by the ensuing evolution. We show that a large fraction of the primordial population should have been lost by the proto-GCs. The extremely low Al abundances found for the primordial population of massive GCs indicate a very fast enrichment process before the formation of the primordial population. We suggest a scenario for the formation of GCs including at least 3 main phases: i) the formation of a precursor population (likely due to the interaction of cosmological structures similar to those leading to dwarf spheroidals, but residing at smaller Rgc, with the early Galaxy or with other structures), ii) which triggers a large episode of star formation (the primordial population), and iii) the formation of the current GC, mainly within a cooling flow formed by the slow winds of a fraction of the primordial population. The precursor population is very effective in raising the metal content in massive and/or metal poor (mainly halo) clusters, while its role is minor in small and/or metal rich (mainly disk) ones. Finally, we use PCA and multivariate relations to study the phase of metal-enrichment from 1st to 2nd generation. Most of the chemical signatures of GCs may be ascribed to a few parameters, the most important being [Fe/H], mass, and age of the cluster, with the location within the Galaxy also playing some role.

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arXiv2010

Calcium and light-elements abundance variations from high resolution spectroscopy in globular clusters

We use abundances of Ca, O, Na, Al from high resolution UVES spectra of 200 red giants in 17 globular clusters (GCs) to investigate the correlation found by Lee et al. (2009) between chemical enrichment from SN II and star-to-star variations in light elements in GC stars. We find that (i) the [Ca/H] variations between first and second generation stars are tiny in most GCs (~0.02-0.03 dex, comparable with typical observational errors). In addition, (ii) using a large sample of red giants in M 4 with abundances from UVES spectra from Marino et al. (2008), we find that Ca and Fe abundances in the two populations of Na-poor and Na-rich stars are identical. These facts suggest that the separation seen in color-magnitude diagrams using the U band or hk index (as observed in NGC 1851 by Han et al. 2009) are not due to Ca variations. Small differences in [Ca/H] as associated to hk variations might be due to a small systematic effect in abundance analysis, because most O-poor/Na-rich (He-rich) stars have slightly larger [Fe/H] (by 0.027 dex on average, due to decreased H in the ratio) than first generation stars and are then located at redder positions in the V,hk plane. While a few GCs (M 54, omega Cen, M 22, maybe even NGC 1851) do actually show various degree of metallicity spread, our findings eliminate the need of a close link between the enrichment by core-collapse SNe with the mechanism responsible for the Na-O anticorrelation.

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arXiv2009

Intrinsic iron spread and a new metallicity scale for Globular Clusters

We have collected spectra of about 2000 red giant branch (RGB) stars in 19 Galactic globular clusters (GC) using FLAMES@VLT (about 100 star with GIRAFFE and about 10 with UVES, respectively, in each GC). These observations provide an unprecedented, precise, and homogeneous data-set of Fe abundances in GCs. We use it to study the cosmic scatter of iron and find that, as far as Fe is concerned, most GCs can still be considered mono-metallic, since the upper limit to the scatter in iron is less than 0.05 dex, meaning that the degree of homogeneity is better than 12%. The scatter in Fe we find seems to have a dependence on luminosity, possibly due to the well-known inadequacies of stellar atmospheres for upper-RGB stars and/or to intrinsic variability. It also seems to be correlated with cluster properties, like the mass, indicating a larger scatter in more massive GCs which is likely a (small) true intrinsic scatter. The 19 GCs, covering the metallicity range of the bulk of Galactic GCs, define an accurate and updated metallicity scale. We provide transformation equations for a few existing scales. We also provide new values of [Fe/H], on our scale, for all GCs in the Harris' catalogue.

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arXiv2009

Na-O Anticorrelation and HB. VIII. Proton-capture elements and metallicities in 17 globular clusters from UVES spectra

We present homogeneous abundances for Fe and some of the elements involved in the proton-capture reactions (O, Na, Mg, Al, and Si) for 202 red giants in 17 Galactic globular clusters (GCs) from the analysis of high resolution UVES spectra obtained with FLAMES@ESO-VLT2. Our programme clusters span almost the whole range in metallicity of GCs and were selected to sample the widest range of global parameters (HB morphology, masses, concentration, etc). Here we focus on the discussion of the Na-O and Mg-Al anticorrelations and related issues. Our study finds clear Na and O star-to-star abundance variations in all GCs. Variations in Al are present in all but a few GCs. Finally, a spread in abundances of Mg and Si are also present in a few clusters. Mg is slightly less overabundant and Si slightly more overabundant in the most Al-rich stars. The correlation between Si and Al abundances is a signature of production of 28Si leaking from the Mg-Al cycle in a few clusters. The cross sections required for the proper reactions to take over in the cycle point to temperatures in excess of about 65 MK for the favoured site of production. We used a dilution model to infer the total range of Al abundances starting from the Al abundances in the UVES spectra, and the Na abundance distributions found from analysis of the much larger set of stars for which GIRAFFE spectra were available. We found that the maximum amount of additional Al produced by first generation polluters contributing to the composition of the second generation stars in each cluster is closely correlated with the same combination of metallicity and cluster luminosity that reproduced the minimum O abundances found from GIRAFFE spectra. We then suggest that the high temperatures required for the Mg-Al cycle are only reached in the most massive and most metal-poor polluters.

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arXiv2008

Properties of second generation stars in Globular Clusters

We present the first results from the analysis of GIRAFFE spectra of more than 1200 red giants stars in 19 Galactic Globular Clusters (GCs), to study the chemical composition of second generation stars and their link with global cluster parameters. We confirm that the extension of the Na-O anticorrelation (the most striking signature of polluted, second generation populations) is strictly related to the very blue (and hot) extreme of the Horizontal Branch (HB). Long anticorrelations seem to require large mass and large-sized, eccentric orbits, taking the GCs far away from the central regions of the Galaxy. We can separate three populations in each cluster (primordial, intermediate and extreme) based on the chemical composition. In all GCs we observe a population of primordial composition, similar to field stars of similar metallicity. We find that in all GCs the bulk (from 50 to 70%) of stars belong to the intermediate component. Finally, the extreme, very oxygen-poor component is observed preferentially in massive clusters, but is not present in all massive GCs.

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