Examinando por Autor "Alfaro E.J."
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Ítem The Gaia -ESO Survey: Calibrating the lithium-age relation with open clusters and associations: I. Cluster age range and initial membership selections(EDP Sciences, 2020-11) Albarrán M.L.G.; Montes D.; Garrido M.G.; Tabernero H.M.; Hernández J.I.G.; Marfil E.; Frasca A.; Lanzafame A.C.; Klutsch A.; Franciosini E.; Randich S.; Smiljanic R.; Korn A.J.; Gilmore G.; Alfaro E.J.; Baratella M.; Bayo A.; Bensby T.; Bonito R.; Carraro G.; Delgado Mena E.; Feltzing S.; Gonneau A.; Heiter U.; Hourihane A.; Esteban F.J.; Jofre P.; Masseron T.; Monaco L.; Morbidelli L.; Prisinzano L.; Roccatagliata V.; Sousa S.; Van Der Swaelmen M.; Worley C.C.; Zaggia S.Context. 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. Aims. 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. Methods. 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 radial velocity 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. Results. 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. Conclusions. 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. © ESO 2020.Ítem The gaia -ESO survey: Calibration strategy(EDP Sciences, 2017-02) Pancino E.; Lardo C.; Altavilla G.; Marinoni S.; Ragaini S.; Cocozza G.; Bellazzini M.; Sabbi E.; Zoccali M.; Donati P.; Heiter U.; Koposov S.E.; Blomme R.; Morel T.; Símon-Díaz S.; Lobel A.; Soubiran C.; Montalban J.; Valentini M.; Casey A.R.; Blanco-Cuaresma S.; Jofré P.; Worley C.C.; Magrini L.; Hourihane A.; François P.; Feltzing S.; Gilmore G.; Randich S.; Asplund M.; Bonifacio P.; Drew J.E.; Jeffries R.D.; Micela G.; Vallenari A.; Alfaro E.J.; Allende Prieto C.; Babusiaux C.; Bensby T.; Bragaglia A.; Flaccomio E.; Hambly N.; Korn A.J.; Lanzafame A.C.; Smiljanic R.; Van Eck S.; Walton N.A.; Bayo A.; Carraro G.; Costado M.T.; Damiani F.; Edvardsson B.; Franciosini E.; Frasca A.; Lewis J.; Monaco L.; Morbidelli L.; Prisinzano L.; Sacco G.G.; Sbordone L.The Gaia-ESO survey (GES) is now in its fifth and last year of observations and has produced tens of thousands of high-quality spectra of stars in all Milky Way components. This paper presents the strategy behind the selection of astrophysical calibration targets, ensuring that all GES results on radial velocities, atmospheric parameters, and chemical abundance ratios will be both internally consistent and easily comparable with other literature results, especially from other large spectroscopic surveys and from Gaia. The calibration of GES is particularly delicate because of (i) the large space of parameters covered by its targets, ranging from dwarfs to giants, from O to M stars; these targets have a large wide of metallicities and also include fast rotators, emission line objects, and stars affected by veiling; (ii) the variety of observing setups, with different wavelength ranges and resolution; and (iii) the choice of analyzing the data with many different state-of-the-art methods, each stronger in a different region of the parameter space, which ensures a better understanding of systematic uncertainties. An overview of the GES calibration and homogenization strategy is also given, along with some examples of the usage and results of calibrators in GES iDR4, which is the fourth internal GES data release and will form the basis of the next GES public data release. The agreement between GES iDR4 recommended values and reference values for the calibrating objects are very satisfactory. The average offsets and spreads are generally compatible with the GES measurement errors, which in iDR4 data already meet the requirements set by the main GES scientific goals. © ESO, 2017.Ítem The Gaia -ESO Survey: Exploring the complex nature and origins of the Galactic bulge populations(EDP Sciences, 2017-05) Rojas-Arriagada A.; Recio-Blanco A.; De Laverny P.; Mikolaitis Š.; Matteucci F.; Spitoni E.; Schultheis M.; Hayden M.; Hill V.; Zoccali M.; Minniti D.; Gonzalez O.A.; Gilmore G.; Randich S.; Feltzing S.; Alfaro E.J.; Babusiaux C.; Bensby T.; Bragaglia A.; Flaccomio E.; Koposov S.E.; Pancino E.; Bayo A.; Carraro G.; Casey A.R.; Costado M.T.; Damiani F.; Donati P.; Franciosini E.; Hourihane A.; Jofré P.; Lardo C.; Lewis J.; Lind K.; Magrini L.; Morbidelli L.; Sacco G.G.; Worley C.C.; Zaggia S.Context. As observational evidence steadily accumulates, the nature of the Galactic bulge has proven to be rather complex: the structural, kinematic, and chemical analyses often lead to contradictory conclusions. The nature of the metal-rich bulge - and especially of the metal-poor bulge - and their relation with other Galactic components, still need to be firmly defined on the basis of statistically significant high-quality data samples. Aims. We used the fourth internal data release of the Gaia-ESO survey to characterize the bulge metallicity distribution function (MDF), magnesium abundance, spatial distribution, and correlation of these properties with kinematics. Moreover, the homogeneous sampling of the different Galactic populations provided by the Gaia-ESO survey allowed us to perform a comparison between the bulge, thin disk, and thick disk sequences in the [Mg/Fe] vs. [Fe/H] plane in order to constrain the extent of their eventual chemical similarities. Methods. We obtained spectroscopic data for ∼2500 red clump stars in 11 bulge fields, sampling the area -10° ≥ l ≥ +8° and -10° ≥ b ≥ -4° from the fourth internal data release of the Gaia-ESO survey. A sample of ∼6300 disk stars was also selected for comparison. Spectrophotometric distances computed via isochrone fitting allowed us to define a sample of stars likely located in the bulge region. Results. From a Gaussian mixture models (GMM) analysis, the bulge MDF is confirmed to be bimodal across the whole sampled area. The relative ratio between the two modes of the MDF changes as a function of b, with metal-poor stars dominating at high latitudes. The metal-rich stars exhibit bar-like kinematics and display a bimodality in their magnitude distribution, a feature which is tightly associated with the X-shape bulge. They overlap with the metal-rich end of the thin disk sequence in the [Mg/Fe] vs. [Fe/H] plane. On the other hand, metal-poor bulge stars have a more isotropic hot kinematics and do not participate in the X-shape bulge. Their Mg enhancement level and general shape in the [Mg/Fe] vs. [Fe/H] plane is comparable to that of the thick disk sequence. The position at which [Mg/Fe] starts to decrease with [Fe/H], called the "knee", is observed in the metal-poor bulge at [Fe/H]knee = -0:37 ± 0:09, being 0.06 dex higher than that of the thick disk. Although this difference is inside the error bars, it suggest a higher star formation rate (SFR) for the bulge than for the thick disk. We estimate an upper limit for this difference of Δ[Fe/H]knee = 0:24 dex. Finally, we present a chemical evolution model that suitably fits the whole bulge sequence by assuming a fast (<1 Gyr) intense burst of stellar formation that takes place at early epochs. Conclusions.We associate metal-rich stars with the bar boxy/peanut bulge formed as the product of secular evolution of the early thin disk. On the other hand, the metal-poor subpopulation might be the product of an early prompt dissipative collapse dominated by massive stars. Nevertheless, our results do not allow us to firmly rule out the possibility that these stars come from the secular evolution of the early thick disk. This is the first time that an analysis of the bulge MDF and α-abundances has been performed in a large area on the basis of a homogeneous, fully spectroscopic analysis of high-resolution, high S/N data. © ESO 2017.Ítem The Gaia -ESO Survey: Lithium enrichment histories of the Galactic thick and thin disc(EDP Sciences, 2018-02) Fu X.; Romano D.; Bragaglia A.; Mucciarelli A.; Lind K.; Delgado Mena E.; Sousa S.G.; Randich S.; Bressan A.; Sbordone L.; Martell S.; Korn A.J.; Abia C.; Smiljanic R.; Jofré P.; Pancino E.; Tautvaišiene G.; Tang B.; Magrini L.; Lanzafame A.C.; Carraro G.; Bensby T.; Damiani F.; Alfaro E.J.; Flaccomio E.; Morbidelli L.; Zaggia S.; Lardo C.; Monaco L.; Frasca A.; Donati P.; Drazdauskas A.; Chorniy Y.; Bayo A.; Kordopatis G.Lithium abundance in most of the warm metal-poor main sequence stars shows a constarnt plateau (A(Li) ~ 2.2 dex) and then the upper envelope of the lithium vs. metallicity distribution increases as we approach solar metallicity. Meteorites, which carry information about the chemical composition of the interstellar medium (ISM) at the solar system formation time, show a lithium abundance A(Li) ~ 3.26 dex. This pattern reflects the Li enrichment history of the ISM during the Galaxy lifetime. After the initial Li production in big bang nucleosynthesis, the sources of the enrichment include asymptotic giant branch (AGB) stars, low-mass red giants, novae, type II supernovae, and Galactic cosmic rays. The total amount of enriched Li is sensitive to the relative contribution of these sources. Thus different Li enrichment histories are expected in the Galactic thick and thin disc. We investigate the main sequence stars observed with UVES in Gaia-ESO Survey iDR4 catalogue and find a Li-anticorrelation independent of [Fe/H], Teff, and log(g). Since in stellar evolution different α enhancements at the same metallicity do not lead to a measurable Li abundance change, the anticorrelation indicates that more Li is produced during the Galactic thin disc phase than during the Galactic thick disc phase. We also find a correlation between the abundance of Li and s-process elements Ba and Y, and they both decrease above the solar metallicity, which can be explained in the framework of the adopted Galactic chemical evolution models. © ESO 2018.Ítem The Gaia -ESO Survey: Spectroscopic-asteroseismic analysis of K2 stars in Gaia -ESO(EDP Sciences, 2020-11) Worley C.C.; Jofré P.; Rendle B.; Miglio A.; Magrini L.; Feuillet D.; Gavel A.; Smiljanic R.; Lind K.; Korn A.; Gilmore G.; Randich S.; Hourihane A.; Gonneau A.; Francois P.; Lewis J.; Sacco G.; Bragaglia A.; Heiter U.; Feltzing S.; Bensby T.; Irwin M.; Gonzalez Solares E.; Murphy D.; Bayo A.; Sbordone L.; Zwitter T.; Lanzafame A.C.; Walton N.; Zaggia S.; Alfaro E.J.; Morbidelli L.; Sousa S.; Monaco L.; Carraro G.; Lardo C.Context. 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. Aims. 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. Methods. 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 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 with a parallax-based log and to investigate instances of variations in the velocity and possible binaries within the dataset. Results. This analysis produced a high-quality catalogue of stellar parameters for 90 red giant stars from K2@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. © ESO 2020.Ítem The Gaia -ESO Survey: The origin and evolution of s -process elements(EDP Sciences, 2018-09) Magrini L.; Spina L.; Randich S.; Friel E.; Kordopatis G.; Worley C.; Pancino E.; Bragaglia A.; Donati P.; Tautvaišienė G.; Bagdonas V.; Delgado-Mena E.; Adibekyan V.; Sousa S.G.; Jiménez-Esteban F.M.; Sanna N.; Roccatagliata V.; Bonito R.; Sbordone L.; Duffau S.; Gilmore G.; Feltzing S.; Jeffries R.D.; Vallenari A.; Alfaro E.J.; Bensby T.; Francois P.; Koposov S.; Korn A.J.; Recio-Blanco A.; Smiljanic R.; Bayo A.; Carraro G.; Casey A.R.; Costado M.T.; Damiani F.; Franciosini E.; Frasca A.; Hourihane A.; Jofré P.; De Laverny P.; Lewis J.; Masseron T.; Monaco L.; Morbidelli L.; Prisinzano L.; Sacco G.; Zaggia S.Context. Several works have found an increase of the abundances of the s-process neutron-capture elements in the youngest Galactic stellar populations. These trends provide important constraints on stellar and Galactic evolution and they need to be confirmed with large and statistically significant samples of stars spanning wide age and distance intervals. Aims. We aim to trace the abundance patterns and the time evolution of five s-process elements - two belonging to the first peak, Y and Zr, and three belonging to the second peak, Ba, La, and Ce - using the Gaia-ESO IDR5 results for open clusters and disc stars. Methods. From the UVES spectra of cluster member stars, we determined the average composition of clusters with ages >0.1 Gyr. We derived statistical ages and distances of field stars, and we separated them into thin and thick disc populations. We studied the time-evolution and dependence on metallicity of abundance ratios using open clusters and field stars whose parameters and abundances were derived in a homogeneous way. Results. Using our large and homogeneous sample of open clusters, thin and thick disc stars, spanning an age range larger than 10 Gyr, we confirm an increase towards young ages of s-process abundances in the solar neighbourhood. These trends are well defined for open clusters and stars located nearby the solar position and they may be explained by a late enrichment due to significant contribution to the production of these elements from long-living low-mass stars. At the same time, we find a strong dependence of the s-process abundance ratios on the Galactocentric distance and on the metallicity of the clusters and field stars. Conclusions. Our results, derived from the largest and most homogeneous sample of s-process abundances in the literature, confirm the growth with decreasing stellar ages of the s-process abundances in both field and open cluster stars. At the same time, taking advantage of the abundances of open clusters located in a wide Galactocentric range, these results offer a new perspective on the dependence of the s-process evolution on the metallicity and star formation history, pointing to different behaviours at various Galactocentric distances. © 2018 ESO.Ítem The Gaia-ESO Survey: Evidence of atomic diffusion in M67?(Oxford University Press, 2018-07) Bertelli Motta C.; Pasquali A.; Richer J.; Michaud G.; Salaris M.; Bragaglia A.; Magrini L.; Randich S.; Grebel E.K.; Adibekyan V.; Blanco-Cuaresma S.; Drazdauskas A.; Fu X.; Martell S.; Tautvaišiene G.; Gilmore G.; Alfaro E.J.; Bensby T.; Flaccomio E.; Koposov S.E.; Korn A.J.; Lanzafame A.C.; Smiljanic R.; Bayo A.; Carraro G.; Casey A.R.; Costado M.T.; Damiani F.; Franciosini E.; Heiter U.; Hourihane A.; Jofré P.; Lardo C.; Lewis J.; Monaco L.; Morbidelli L.; Sousa S.G.; Worley C.C.; Zaggia S.; Sacco, G.G.Investigating the chemical homogeneity of stars born from the same molecular cloud at virtually the same time is very important for our understanding of the chemical enrichment of the interstellar medium and with it the chemical evolution of the Galaxy. One major cause of inhomogeneities in the abundances of open clusters is stellar evolution of the cluster members. In this work, we investigate variations in the surface chemical composition of member stars of the old open clusterM67 as a possible consequence of atomic diffusion effects taking place during the main-sequence phase. The abundances used are obtained from high-resolution UVES/FLAMES spectra within the framework of the Gaia-ESO Survey. We find that the surface abundances of stars on the main sequence decrease with increasing mass reaching a minimum at the turn-off. After deepening of the convective envelope in subgiant branch stars, the initial surface abundances are restored.We found themeasured abundances to be consistent with the predictions of stellar evolutionary models for a cluster with the age and metallicity of M67. Our findings indicate that atomic diffusion poses a non-negligible constraint on the achievable precision of chemical tagging methods. © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.Ítem The Gaia-ESO Survey: Lithium depletion in the Gamma Velorum cluster and inflated radii in low-mass pre-main-sequence stars(Oxford University Press, 2017-01) Jeffries R.D.; Jackson R.J.; Franciosini E.; Randich S.; Barrado D.; Frasca A.; Klutsch A.; Lanzafame A.C.; Prisinzano L.; Sacco G.G.; Gilmore G.; Vallenari A.; Alfaro E.J.; Koposov S.E.; Pancino E.; Bayo A.; Casey A.R.; Costado M.T.; Damiani F.; Hourihane A.; Lewis J.; Jofre P.; Magrini L.; Monaco L.; Morbidelli L.; Worley C.C.; Zaggia S.; Zwitter T.We show that non-magnetic models for the evolution of pre-main-sequence (PMS) stars cannot simultaneously describe the colour-magnitude diagram (CMD) and the pattern of lithium depletion seen in the cluster of young, low-mass stars surrounding γ 2 Velorum. The age of 7.5 ± 1 Myr inferred from the CMD is much younger than that implied by the strong Li depletion seen in the cluster M-dwarfs, and the Li depletion occurs at much redder colours than predicted. The epoch at which a star of a given mass depletes its Li and the surface temperature of that star are both dependent on its radius. We demonstrate that if the low-mass stars have radii ~10 per cent larger at a given mass and age, then both the CMD and the Li-depletion pattern of the Gamma Velorum cluster are explained at a common age of ≃ 18- 21 Myr. This radius inflation could be produced by some combination of magnetic suppression of convection and extensive cool starspots. Models that incorporate radius inflation suggest that PMS stars, similar to those in the Gamma Velorum cluster, in the range 0.2 < M/M ⊙ < 0.7, are at least a factor of 2 older and ~7 per cent cooler than previously thought and that their masses are much larger (by > 30 per cent) than inferred from conventional, non-magnetic models in the Hertzsprung-Russell diagram. Systematic changes of this size may be of great importance in understanding the evolution of young stars, disc lifetimes and the formation of planetary systems. © 2016 The Authors.Ítem The Gaia-ESO survey: Matching chemodynamical simulations to observations of the Milky Way(Oxford University Press, 2018-01) Thompson B.B.; Few C.G.; Bergemann M.; Gibson B.K.; MacFarlane B.A.; Serenelli A.; Gilmore G.; Randich S.; Vallenari A.; Alfaro E.J.; Bensby T.I.; Francois P.; Korn A.J.; Bayo A.; Carraro G.; Casey A.R.; Costado M.T.; Donati P.; Franciosini E.; Frasca A.; Hourihane A.; Jofrè P.; Hill V.; Heiter U.; Koposov S.E.; Lanzafame A.; Lardo C.; de Laverny P.; Lewis J.; Magrini L.; Marconi G.; Masseron T.; Monaco L.; Morbidelli L.; Pancino E.; Prisinzano L.; Recio-Blanco A.; Sacco G.; Sousa S.G.; Tautvaišiene G.; Worley C.C.; Zaggia S.The typical methodology for comparing simulated galaxies with observational surveys is usually to apply a spatial selection to the simulation to mimic the region of interest covered by a comparable observational survey sample. In this work, we compare this approach with a more sophisticated post-processing in which the observational uncertainties and selection effects (photometric, surface gravity and effective temperature) are taken into account. We compare a 'solar neighbourhood analogue' region in a model MilkyWay-like galaxy simulated with RAMSES-CH with fourth release Gaia-ESO survey data. We find that a simple spatial cut alone is insufficient and that the observational uncertainties must be accounted for in the comparison. This is particularly true when the scale of uncertainty is large compared to the dynamic range of the data, e.g. in our comparison, the [Mg/Fe] distribution is affected much more than the more accurately determined [Fe/H] distribution. Despite clear differences in the underlying distributions of elemental abundances between simulation and observation, incorporating scatter to our simulation results to mimic observational uncertainty produces reasonable agreement. The quite complete nature of the Gaia-ESO survey means that the selection function has minimal impact on the distribution of observed age and metal abundances but this would become increasingly more important for surveys with narrower selection functions. © 2017 The Author(s).Ítem The Gaia-ESO Survey: Structural and dynamical properties of the young cluster Chamaeleon i(EDP Sciences, 2017-05) Sacco G.G.; Spina L.; Randich S.; Palla F.; Parker R.J.; Jeffries R.D.; Jackson R.; Meyer M.R.; Mapelli M.; Lanzafame A.C.; Bonito R.; Damiani F.; Franciosini E.; Frasca A.; Klutsch A.; Prisinzano L.; Tognelli E.; Degl'Innocenti S.; Prada Moroni P.G.; Alfaro E.J.; Micela G.; Prusti T.; Barrado D.; Biazzo K.; Bouy H.; Bravi L.; Lopez-Santiago J.; Wright N.J.; Bayo A.; Gilmore G.; Bragaglia A.; Flaccomio E.; Koposov S.E.; Pancino E.; Casey A.R.; Costado M.T.; Donati P.; Hourihane A.; Jofré P.; Lardo C.; Lewis J.; Magrini L.; Monaco L.; Morbidelli L.; Sousa S.G.; Worley C.C.; Zaggia S.Investigating the physical mechanisms driving the dynamical evolution of young star clusters is fundamental to our understanding of the star formation process and the properties of the Galactic field stars. The young (~2 Myr) and partially embedded cluster Chamaeleon I is one of the closest laboratories for the study of the early stages of star cluster dynamics in a low-density environment. The aim of this work is to study the structural and kinematical properties of this cluster combining parameters from the high-resolution spectroscopic observations of the Gaia-ESO Survey with data from the literature. Our main result is the evidence of a large discrepancy between the velocity dispersion (σstars = 1.14 ± 0.35 km s-1) of the stellar population and the dispersion of the pre-stellar cores (~0.3 km s-1) derived from submillimeter observations. The origin of this discrepancy, which has been observed in other young star clusters, is not clear. It has been suggested that it may be due to either the effect of the magnetic field on the protostars and the filaments or to the dynamical evolution of stars driven by two-body interactions. Furthermore, the analysis of the kinematic properties of the stellar population puts in evidence a significant velocity shift (~1 km s-1) between the two subclusters located around the north and south main clouds of the cluster. This result further supports a scenario where clusters form from the evolution of multiple substructures rather than from a monolithic collapse. Using three independent spectroscopic indicators (the gravity indicator γ, the equivalent width of the Li line at 6708 Å, and the Hα 10% width), we performed a new membership selection. We found six new cluster members all located in the outer region of the cluster, proving that Chamaeleon I is probably more extended than previously thought. Starting from the positions and masses of the cluster members, we derived the level of substructure Q, the surface density Σ, and the level of mass segregation ΛMSR of the cluster. The comparison between these structural properties and the results of N-body simulations suggests that the cluster formed in a low-density environment, in virial equilibrium or a supervirial state, and highly substructured. © 2017 ESO.