arXiv subjects

Sara Lucatello

Publications and source records attributed to Sara Lucatello.

At least 73 records · Page 4Linked to original sources

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

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

A Puzzle Involving Galactic Bulge Microlensing Events

We study a sample of 16 microlensed Galactic bulge main sequence turnoff region stars for which high dispersion spectra have been obtained with detailed abundance analyses. We demonstrate that there is a very strong and highly statistically significant correlation between the maximum magnification of the microlensed bulge star and the value of the [Fe/H] deduced from the high resolution spectrum of each object. Physics demands that this correlation, assuming it to be real, be the result of some sample bias. We suggest several possible explanations, but are forced to reject them all,and are left puzzled. To obtain a reliable metallicity distribution in the Galactic bulge based on microlensed dwarf stars it will be necessary to resolve this issue through the course of additional observations.

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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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arXiv2008

Detailed Abundances for 28 Metal-poor Stars: Stellar Relics in the Milky Way

We present the results of an abundance analysis for a sample of stars with $-4<$[Fe/H]$<-2$. The data were obtained with the HIRES spectrograph at Keck Observatory. The set includes 28 stars, with effective temperature ranging from 4800 to 6600 K. For 13 stars with [Fe/H]$<-2.6$, including nine with [Fe/H]$<-3.0$ and one with [Fe/H]$=-4.0$, these are the first reported detailed abundances. For the most metal-poor star in our sample, CS 30336-049, we measure an abundance pattern that is very similar to stars in the range [Fe/H]$\sim-3.5$, including a normal C+N abundance. We also find that it has very low but measurable Sr and Ba, indicating some neutron-capture activity even at this low of a metallicity. We explore this issue further by examining other very neutron-capture-deficient stars, and find that at the lowest levels, [Ba/Sr] exhibits the ratio of the main r-process. We also report on a new r-process-enhanced star, CS 31078-018. This star has [Fe/H]$=-2.85$, [Eu/Fe]$=1.23$, and [Ba/Eu]$=-0.51$. CS 31078-018 exhibits an ``actinide boost'', i.e. much higher [Th/Eu] than expected and at a similar level to CS 31082-001. Our spectra allow us to further constrain the abundance scatter at low metallicities, which we then use to fit to the zero-metallicity Type II supernova yields of Heger & Woosley (2008). We find that supernovae with progenitor masses between 10 and 20 M$_{\odot}$ provide the best matches to our abundances.

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arXiv2007

A Search for Binary Stars at Low Metallicity

We present initial results measuring the companion fraction of metal-poor stars ([Fe/H]$<-$2.0). We are employing the Lick Observatory planet-finding system to make high-precision Doppler observations of these objects. The binary fraction of metal-poor stars provides important constraints on star formation in the early Galaxy (Carney et al. 2003). Although it has been shown that a majority of solar metallicity stars are in binaries, it is not clear if this is the case for metal-poor stars. Is there a metallicity floor below which binary systems do not form or become rare? To test this we are determining binary fractions at metallicities below [Fe/H]$=-2.0$. Our measurments are not as precise as the planet finders', but we are still finding errors of only 50 to 300 m/s, depending on the signal-to-noise of a spectrum and stellar atmosphere of the star. At this precision we can be much more complete than previous studies in our search for stellar companions.

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arXiv2007

Carbon and Strontium Abundances of Metal-Poor Stars

We present carbon and strontium abundances for 100 metal-poor stars measured from R$\sim $7000 spectra obtained with the Echellette Spectrograph and Imager at the Keck Observatory. Using spectral synthesis of the G-band region, we have derived carbon abundances for stars ranging from [Fe/H]$=-1.3$ to [Fe/H]$=-3.8$. The formal errors are $\sim 0.2$ dex in [C/Fe]. The strontium abundance in these stars was measured using spectral synthesis of the resonance line at 4215 Å. Using these two abundance measurments along with the barium abundances from our previous study of these stars, we show it is possible to identify neutron-capture-rich stars with our spectra. We find, as in other studies, a large scatter in [C/Fe] below [Fe/H]$ = -2$. Of the stars with [Fe/H]$<-2$, 9$\pm$4% can be classified as carbon-rich metal-poor stars. The Sr and Ba abundances show that three of the carbon-rich stars are neutron-capture-rich, while two have normal Ba and Sr. This fraction of carbon enhanced stars is consistent with other studies that include this metallicity range.

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arXiv2007

Na-O Anticorrelation and HB. II. The Na-O anticorrelation in the globular cluster NGC 6752

We are studying the Na-O anticorrelation in several globular clusters of different Horizontal Branch (HB) morphology in order to derive a possible relation between (primordial) chemical inhomogeneities and morphological parameters of the cluster population. We used the multifiber spectrograph FLAMES on the ESO Very Large Telescope UT2 and derived atmospheric parameters and elemental abundances of Fe, O and Na for about 150 red giant stars in the Galactic globular cluster NGC 6752. The average metallicity we derive is [Fe/H]=-1.56, in agreement with other results from red giants, but lower than obtained for dwarfs or early subgiants. In NGC 6752 there is not much space for an intrinsic spread in metallicity: on average, the rms scatter in [Fe/H] is 0.037+/-0.003 dex, while the scatter expected on the basis of the major error sources is 0.039+/-0.003 dex. The distribution of stars along the Na-O anticorrelation is different to what was found in the first paper of this series for the globular cluster NGC 2808: in NGC 6752 it is skewed toward more Na-poor stars, and it resembles more the one in M 13. Detailed modeling is required to clarify whether this difference may explain the very different distributions of stars along the HB.

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arXiv2006

The Frequency of Carbon-Enhanced Metal-Poor Stars in the Galaxy from the HERES sample

We estimate the frequency of Carbon-Enhanced Metal-Poor (CEMP) stars among very metal-poor stars, based on an analysis of 349 stars with available high-resolution spectra observed as part of the Hamburg/ESO R-process Enhanced Star (HERES) survey. We obtain that {\it a lower limit of} 21 $\pm$ 2% of stars with [Fe/H] $\leq -2.0$ exhibit [C/Fe] $\geq +1.0$. These fractions are higher than have been reported by recent examinations of this question, based on substantially smaller samples of stars. We discuss the source of this difference and suggest that in order to take into account effects that result in a decrease of surface carbon abundance with advancing evolution, a definition of CEMP stars based on a [C/Fe] cutoff that varies as a function of luminosity is more appropriate. We discuss the likely occurrence of dilution and mixing for many CEMP stars, which, if properly accounted for, would increase this fraction still further.

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arXiv2005

Na-O Anticorrelation And HB I. The Na-O anticorrelation in NGC 2808

We derived atmospheric parameters and elemental abundances of Fe, O and Na for about 120 red giant stars in the Galactic globular cluster NGC 2808. Our results are based on the analysis of medium-high resolution (R=22000-24000) GIRAFFE spectra acquired with the FLAMES spectrograph at VLT-UT2 as a part of a project aimed at studying the Na-O anticorrelation as a function of physical parameters in globular clusters. We present here the anticorrelation of Na and O abundances in NGC 2808, and we discuss the distribution function of stars along this relation. Besides a bulk of O-normal stars, with composition typical of field halo stars, NGC 2808 seems to host two other groups of O-poor and super O-poor stars. In this regard, NGC 2808 is similar to M 13, the template cluster for the Na-O anticorrelation. However, at variance with M 13, most stars in NGC 2808 are O-rich. This might be related to the horizontal branch morphologies which are very different in these two clusters. The average metallicity we found for NGC 2808 is [Fe/H]=-1.10 (rms=0.065 dex, from 123 stars). We also found some evidence of a small intrinsic spread in metallicity, but more definitive conclusions are hampered by the presence of a small differential reddening.

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arXiv2004

The Binary Frequency Among Carbon-Enhanced, s-Process Rich, Metal-Poor Stars

We discuss radial velocities for a sample of carbon-enhanced, s-process rich, very metal-poor stars (CEMP-s hereafter), analyzed with high-resolution spectroscopy obtained over multiple epochs. We find that ~68% of the stars in the sample show evidence of radial velocity variations. The expected detection fraction for these stars, adopting the measured binary fraction in the field (~60%), and assuming that they share the same period and eccentricity distribution, is ~22%. Even if one assumes that the true binary fraction of these stars is 100%, the expected detection percentage is ~36%. These values indicate that the binary fraction among CEMP-s stars is higher than the field binary fraction, suggesting that all of these objects are in double (or multiple) systems. The fact that the observed frequency of velocity variation exceeds the expected detection fraction in the case of an assumed binary fraction of 100% is likely due to a more restricted distribution of orbital periods for these objects, as compared to normal field binaries. Our results indicate that CEMP-s stars are the metal-poor analogues of classical CH-stars.

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arXiv2004

Observational evidence for a different IMF in the early Galaxy

The unexpected high incidence of carbon-enhanced, s-process enriched unevolved stars amongst extremely metal-poor stars in the halo provides a significant constraint on the Initial Mass Function (IMF) in the early Galaxy. We argue that these objects are evidence for the past existence of a large population of intermediate-mass stars, and conclude that the IMF in the early Galaxy was different from the present, and shifted toward higher masses.

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arXiv2004

Metallicities on the Double Main Sequence of omega Centauri Imply Large Helium Enhancement

Having shown in a recent paper that the main sequence of omega Centauri is split into two distinct branches, we now present spectroscopic results showing that the bluer sequence is_less_ metal-poor. We have carefully combined GIRAFFE@VLT spectra of 17 stars on each side of the split into a single spectrum for each branch, with adequate S/N to show clearly that the stars of the blue main sequence are less metal poor by 0.3 dex than those of the dominant red one. From an analysis of the individual spectra, we could not detect any abundance spread among the blue main sequence stars, while the red main sequence stars show a 0.2 dex spread in metallicity. We use stellar-structure models to show that only greatly enhanced helium can explain the color difference between the two main sequences, and we discuss ways in which this enhancement could have arisen.

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