| 1. |
- Smiljanic, R., et al.
(author)
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The Gaia-ESO Survey: The analysis of high-resolution UVES spectra of FGK-type stars
- 2014
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In: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 570
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Journal article (peer-reviewed)abstract
- Context. The ongoing Gaia-ESO Public Spectroscopic Survey is using FLAMES at the VLT to obtain high-quality medium-resolution Giraffe spectra for about 10(5) stars and high-resolution UVES spectra for about 5000 stars. With UVES, the Survey has already observed 1447 FGK-type stars. Aims. These UVES spectra are analyzed in parallel by several state-of-the-art methodologies. Our aim is to present how these analyses were implemented, to discuss their results, and to describe how a final recommended parameter scale is defined. We also discuss the precision (method-to-method dispersion) and accuracy (biases with respect to the reference values) of the final parameters. These results are part of the Gaia-ESO second internal release and will be part of its first public release of advanced data products. Methods. The final parameter scale is tied to the scale defined by the Gaia benchmark stars, a set of stars with fundamental atmospheric parameters. In addition, a set of open and globular clusters is used to evaluate the physical soundness of the results. Each of the implemented methodologies is judged against the benchmark stars to define weights in three different regions of the parameter space. The final recommended results are the weighted medians of those from the individual methods. Results. The recommended results successfully reproduce the atmospheric parameters of the benchmark stars and the expected T-eff-log g relation of the calibrating clusters. Atmospheric parameters and abundances have been determined for 1301 FGK-type stars observed with UVES. The median of the method-to-method dispersion of the atmospheric parameters is 55K for T-eff, 0.13dex for log g and 0.07 dex for [Fe/H]. Systematic biases are estimated to be between 50-100 K for T-eff, 0.10-0.25 dex for log g and 0.05-0.10 dex for [Fe/H]. Abundances for 24 elements were derived: C, N, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ba, Nd, and Eu. The typical method-to-method dispersion of the abundances varies between 0.10 and 0.20 dex. Conclusions. The Gaia-ESO sample of high-resolution spectra of FGK-type stars will be among the largest of its kind analyzed in a homogeneous way. The extensive list of elemental abundances derived in these stars will enable significant advances in the areas of stellar evolution and Milky Way formation and evolution.
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| 2. |
- Cantat-Gaudin, T., et al.
(author)
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The Gaia-ESO Survey: Stellar content and elemental abundances in the massive cluster NGC 6705
- 2014
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In: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 569
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Journal article (peer-reviewed)abstract
- Context. Chemically inhomogeneous populations are observed in most globular clusters, but not in open clusters. Cluster mass seems to play a key role in the existence of multiple populations. Aims. Studying the chemical homogeneity of the most massive open clusters is needed to better understand the mechanism of their formation and determine the mass limit under which clusters cannot host multiple populations. Here we studied NGC 6705, which is a young and massive open cluster located towards the inner region of the Milky Way. This cluster is located inside the solar circle. This makes it an important tracer of the inner disk abundance gradient. Methods. This study makes use of BVI and ri photometry and comparisons with theoretical isochrones to derive the age of NGC 6705. We study the density profile of the cluster and the mass function to infer the cluster mass. Based on abundances of the chemical elements distributed in the first internal data release of the Gaia-ESO Survey, we study elemental ratios and the chemical homogeneity of the red clump stars. Radial velocities enable us to study the rotation and internal kinematics of the cluster. Results. The estimated ages range from 250 to 316 Myr, depending on the adopted stellar model. Luminosity profiles and mass functions show strong signs of mass segregation. We derive the mass of the cluster from its luminosity function and from the kinematics, finding values between 3700 M-circle dot and 11 000 M-circle dot. After selecting the cluster members from their radial velocities, we obtain a metallicity of [Fe/H] = 0.10 +/- 0.06 based on 21 candidate members. Moreover, NGC 6705 shows no sign of the typical correlations or anti-correlations between Al, Mg, Si, and Na, which are expected in multiple populations. This is consistent with our cluster mass estimate, which is lower than the required mass limit proposed in the literature to develop multiple populations.
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| 3. |
- Donati, P., et al.
(author)
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The Gaia-ESO Survey : Reevaluation of the parameters of the open cluster Trumpler 20 using photometry and spectroscopy
- 2014
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 561, s. A94-
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Journal article (peer-reviewed)abstract
- Trumpler 20 is an old open cluster (OC) located toward the Galactic centre, at about 3 kpc from the Sun and similar to 7 kpc from the Galactic centre. Its position makes this cluster particularly interesting in the framework of the chemical properties of the Galactic disc because very few old OCs reside in the inner part of the disc. For this reason it has been selected as a cluster target of the Gaia-ESO Survey, and spectra of many stars in the main-sequence and red-clump phases are now available. Moreover, although it has been studied by several authors in the past, no consensus on the evolutionary status of Tr 20 has been reached. The heavy contamination of field stars (the line of sight of Tr 20 crosses the Carina spiral arm) complicates a correct interpretation. Another interesting aspect of the cluster is that it shows a broadened main-sequence turn-off and a prominent and extended red-clump, characteristics that are not easily explained by classical evolutionary models. Exploiting both spectroscopic information from the Gaia-ESO Survey (and the ESO archive) and literature photometry, we obtain a detailed and accurate analysis of the properties of the cluster. We make use of the first accurate metallicity measurement ever obtained from several spectra of red clump stars, and of cluster membership determination using radial velocities. According to the evolutionary models adopted, we find that Tr 20 has an age in the range 1.35-1.66 Gyr, an average reddening E(B - V) in the range 0.31-0.35 mag, and a distance modulus (m - M)(0) between 12.64 and 12.72 mag. The spectroscopic metallicity is [Fe/H] = +0.17 dex. We discuss the structural properties of the object and constrain possible hypotheses for its broadened upper main sequence by estimating the effect of differential reddening and its extended red clump.
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| 4. |
- Friel, E. D., et al.
(author)
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Gaia-ESO Survey : Properties of the intermediate age open cluster NGC 4815
- 2014
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 563, s. A117-
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Journal article (peer-reviewed)abstract
- Context. NGC 4815 is a populous similar to 500 Myr open cluster at R-gc similar to 7 kpc observed in the first six months of the Gaia-ESO Survey. Located in the inner Galactic disk, NGC 4815 is an important potential tracer of the abundance gradient, where relatively few intermediate age open clusters are found. Aims. The Gaia-ESO Survey data can provide an improved characterization of the cluster properties, such as age, distance, reddening, and abundance profile. Methods. We use the survey derived radial velocities, stellar atmospheric parameters, metallicity, and elemental abundances for stars targeted as potential members of this cluster to carry out an analysis of cluster properties. The radial velocity distribution of stars in the cluster field is used to define the cluster systemic velocity and derive likely cluster membership for stars observed by the Gaia-ESO Survey. We investigate the distributions of Fe and Fe-peak elements, alpha-elements, and the light elements Na and Al and characterize the cluster's internal chemical homogeneity comparing it to the properties of radial velocity non-member stars. Utilizing these cluster properties, the cluster color-magnitude diagram is analyzed and theoretical isochrones are fit to derive cluster reddening, distance, and age. Results. NGC 4815 is found to have a mean metallicity of [Fe/H] = +0.03 +/- 0.05 dex (s.d.). Elemental abundances of cluster members show typically very small internal variation, with internal dispersions of similar to 0.05 dex. The alpha-elements [Ca/Fe] and [Si/Fe] show solar ratios, but [Mg/Fe] is moderately enhanced, while [Ti/Fe] appears slightly deficient. As with many open clusters, the light elements [Na/Fe] and [Al/Fe] are enhanced, [Na/Fe] significantly so, although the role of internal mixing and the assumption of local thermodynamical equilibrium in the analysis remain to be investigated. From isochrone fits to color-magnitude diagrams, we find a cluster age of 0.5 to 0.63 Gyr, a reddening of E(B-V) = 0.59 to 0.65, and a distance modulus (m -M)(0) = 11.95 to 12.20, depending on the choice of theoretical models, leading to a Galactocentric distance of 6.9 kpc.
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| 5. |
- Jeffries, R. D., et al.
(author)
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The Gaia-ESO Survey: Kinematic structure in the Gamma Velorum cluster
- 2014
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In: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 563
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Journal article (peer-reviewed)abstract
- Context. A key science goal of the Gaia-ESO survey (GES) at the VLT is to use the kinematics of low-mass stars in young clusters and star forming regions to probe their dynamical histories and how they populate the field as they become unbound. The clustering of low-mass stars around the massive Wolf-Rayet binary system gamma(2) Velorum was one of the first GES targets. Aims. We empirically determine the radial velocity precision of GES data, construct a kinematically unbiased sample of cluster members and characterise their dynamical state. Methods. Targets were selected from colour-magnitude diagrams and intermediate resolution spectroscopy was used to derive radial velocities and assess membership from the strength of the Li I 6708 angstrom line. The radial velocity distribution was analysed using a maximum likelihood technique that accounts for unresolved binaries. Results. The GES radial velocity precision is about 0.25 km s(-1) and sufficient to resolve velocity structure in the low-mass population around gamma(2) Vel. The structure is well fitted by two kinematic components with roughly equal numbers of stars; the first has an intrinsic dispersion of 0.34 +/- 0.16 km s(-1), consistent with virial equilibrium. The second has a broader dispersion of 1.60 +/- 0.37 km s(-1) and is offset from the first by congruent to 2 kms(-1). The first population is older by 1-2 Myr based on a greater level of Li depletion seen among its M-type stars and is probably more centrally concentrated around gamma(2) Vel. Conclusions. We consider several formation scenarios, concluding that the two kinematic components are a bound remnant of the original, denser cluster that formed gamma(2) Vel, and a dispersed population from the wider Vela OB2 association, of which gamma(2) Vel is the most massive member. The apparent youth of gamma(2) Vel compared to the older (>= 10 Myr) low-mass population surrounding it suggests a scenario in which the massive binary formed in a clustered environment after the formation of the bulk of the low-mass stars.
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| 6. |
- Bergemann, M., et al.
(author)
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The Gaia-ESO Survey : radial metallicity gradients and age-metallicity relation of stars in the Milky Way disk
- 2014
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 565, s. A89-
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Journal article (peer-reviewed)abstract
- We study the relationship between age, metallicity, and alpha-enhancement of FGK stars in the Galactic disk. The results are based upon the analysis of high-resolution UVES spectra from the Gaia-ESO large stellar survey. We explore the limitations of the observed dataset, i.e. the accuracy of stellar parameters and the selection effects that are caused by the photometric target preselection. We find that the colour and magnitude cuts in the survey suppress old metal-rich stars and young metal-poor stars. This suppression may be as high as 97% in some regions of the age-metallicity relationship. The dataset consists of 144 stars with a wide range of ages from 0.5 Gyr to 13.5 Gyr, Galactocentric distances from 6 kpc to 9.5 kpc, and vertical distances from the plane 0 < vertical bar Z vertical bar < 1.5 kpc. On this basis, we find that i) the observed age-metallicity relation is nearly flat in the range of ages between 0 Gyr and 8 Gyr; ii) at ages older than 9 Gyr, we see a decrease in [Fe/H] and a clear absence of metal-rich stars; this cannot be explained by the survey selection functions; iii) there is a significant scatter of [Fe/H] at any age; and iv) [Mg/Fe] increases with age, but the dispersion of [Mg/Fe] at ages > 9 Gyr is not as small as advocated by some other studies. In agreement with earlier work, we find that radial abundance gradients change as a function of vertical distance from the plane. The [Mg/Fe] gradient steepens and becomes negative. In addition, we show that the inner disk is not only more alpha-rich compared to the outer disk, but also older, as traced independently by the ages and Mg abundances of stars.
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| 7. |
- Damiani, F., et al.
(author)
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Gaia-ESO Survey: Empirical classification of VLT/Giraffe stellar spectra in the wavelength range 6440-6810 angstrom in the gamma Velorum cluster, and calibration of spectral indices
- 2014
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In: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 566
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Journal article (peer-reviewed)abstract
- We present a study of spectral diagnostics available from optical spectra with R = 17 000 obtained with the VLT/Giraffe HR15n setup, using observations from the Gaia-ESO Survey, on the gamma Vel young cluster, with the purpose of classifying these stars and finding their fundamental parameters. We define several spectroscopic indices, sampling the amplitude of TiO bands, the H alpha line core and wings, and temperature- and gravity-sensitive sets of lines, each useful as a T-eff or log g indicator over a limited range of stellar spectral types. H alpha line indices are also useful as chromospheric activity or accretion indicators. Furthermore, we use all indices to define additional global T-eff - and logg-sensitive indices tau and gamma, valid for the entire range of types in the observed sample. We find a clear difference between gravity indices of main-sequence and pre-main-sequence stars, as well as a much larger difference between these and giant stars. The potentially great usefulness of the (gamma, t) diagram as a distance-independent age measurement tool for young clusters is discussed. We discuss the effect on the defined indices of classical T Tauri star veiling, which is however detected in only a few stars in the present sample. Then, we present tests and calibrations of these indices, on the basis of both photometry and literature reference spectra, from the UVES Paranal Observatory Project and the ELODIE 3.1 Library. The known properties of these stars, spanning a wide range of stellar parameters, enable us to obtain a good understanding of the performances of our new spectral indices. For non-peculiar stars with known temperature, gravity, and metallicity, we are able to calibrate quantitatively our indices, and derive stellar parameters for a wide range of stellar types. To this aim, a new composite index is defined, providing a good metallicity indicator. The ability of our indices to select peculiar, or otherwise rare classes of stars is also established. For pre-main-sequence stars outside the parameter range of the ELODIE dataset, index calibration relies on model isochrones. We check our calibrations against current Gaia-ESO UVES results, plus a number of Survey benchmark stars, and also against Gaia-ESO observations of young clusters, which contribute to establishing the good performance of our method across a wide range of stellar parameters. Our gravity determination for late-type PMS stars is found to be accurate enough to let us obtain gravity-based age estimates for PMS clusters. Finally, our gravity determinations support the existence of an older pre-main-sequence population in the gamma Vel sky region, in agreement with evidence obtained from the lithium depletion pattern of the same stars.
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| 8. |
- Magrini, L., et al.
(author)
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The Gaia-ESO Survey: Abundance ratios in the inner-disk open clusters Trumpler 20, NGC 4815, NGC 6705
- 2014
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In: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 563
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Journal article (peer-reviewed)abstract
- Context. Open clusters are key tools to study the spatial distribution of abundances in the disk and their evolution with time. Aims. Using the first release of stellar parameters and abundances of the Gaia-ESO Survey, we analyse the chemical properties of stars in three old/intermediate-age open clusters, namely NGC 6705, NGC 4815, and Trumpler 20, which are all located in the inner part of the Galactic disk at Galactocentric radius R-GC similar to 7 kpc. We aim to prove their homogeneity and to compare them with the field population. Methods. We study the abundance ratios of elements belonging to two different nucleosynthetic channels: alpha-elements and iron-peak elements. For each element, we analyse the internal chemical homogeneity of cluster members, and we compare the cumulative distributions of cluster abundance ratios with those of solar neighbourhood turn-off stars and of inner-disk/bulge giants. We compare the abundance ratios of field and cluster stars with two chemical evolution models that predict different alpha-enhancement dependences on the Galactocentric distance due to different assumptions on the infall and star-formation rates. Results. The main results can be summarised as follows: i) cluster members are chemically homogeneous within 3 sigma in all analysed elements; ii) the three clusters have comparable [El/Fe] patterns within similar to 1 sigma, but they differ in their global metal content [El/H] with NGC 4815 having the lowest metallicity; their [El/Fe] ratios show differences and analogies with those of the field population, in both the solar neighbourhood and the bulge/inner disk; iii) comparing the abundance ratios with the results of two chemical evolution models and with field star abundance distributions, we find that the abundance ratios of Mg, Ni, and Ca in NGC 6705 might require an inner birthplace, implying a subsequent variation in its R-GC during its lifetime, which is consistent with previous orbit determination. Conclusions. Using the results of the first internal data release, we show the potential of the Gaia-ESO Survey through a homogeneous and detailed analysis of the cluster versus field populations to reveal the chemical structure of our Galaxy using a completely uniform analysis of different populations. We verify that the Gaia-ESO Survey data are able to identify the unique chemical properties of each cluster by pinpointing the composition of the interstellar medium at the epoch and place of formation. The full dataset of the Gaia-ESO Survey is a superlative tool to constrain the chemical evolution of our Galaxy by disentangling different formation and evolution scenarios.
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| 9. |
- Recio-Blanco, A., et al.
(author)
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The Gaia-ESO Survey: the Galactic thick to thin disc transition
- 2014
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In: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 567
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Journal article (peer-reviewed)abstract
- Aims. The nature of the thick disc and its relation to the thin disc is presently an important subject of debate. In fact, the structural and chemo-dynamical transition between disc populations can be used as a test of the proposed models of Galactic disc formation and evolution. Methods. We used the atmospheric parameters, [alpha/Fe] abundances, and radial velocities, which were determined from the Gaia-ESO Survey GIRAFFE spectra of FGK-type stars (first nine months of observations) to provide a chemo-kinematical characterisation of the disc stellar populations. We focussed on a subsample of 1016 stars with high-quality parameters, covering the volume vertical bar Z vertical bar < 4.5 kpc and R in the range 2-13 kpc. Results. We have identified a thin to thick disc separation in the [alpha/Fe] vs. [M/H] plane, thanks to the presence of a low-density region in the number density distribution. The thick disc stars seem to lie in progressively thinner layers above the Galactic plane, as metallicity increases and [alpha/Fe] decreases. In contrast, the thin disc population presents a constant value of the mean distance to the Galactic plane at all metallicities. In addition, our data confirm the already known correlations between V-phi and [M/H] for the two discs. For the thick disc sequence, a study of the possible contamination by thin disc stars suggests a gradient up to 64 +/- 9 km s(-1) dex(-1). The distributions of azimuthal velocity, vertical velocity, and orbital parameters are also analysed for the chemically separated samples. Concerning the gradients with galactocentric radius, we find, for the thin disc, a flat behaviour of the azimuthal velocity, a metallicity gradient equal to -0.058 +/- 0.008 dex kpc(-1) and a very small positive [alpha/Fe] gradient. For the thick disc, flat gradients in [M/H] and [alpha/Fe] are derived. Conclusions. Our chemo-kinematical analysis suggests a picture where the thick disc seems to have experienced a settling process, during which its rotation increased progressively and, possibly, the azimuthal velocity dispersion decreased. At [M/H] approximate to -0.25 dex and [alpha/Fe] approximate to 0.1 dex, the mean characteristics of the thick disc in vertical distance to the Galactic plane, rotation, rotational dispersion, and stellar orbits' eccentricity agree with that of the thin disc stars of the same metallicity, suggesting a possible connection between these two populations at a certain epoch of the disc evolution. Finally, the results presented here, based only on the first months of the Gaia-ESO Survey observations, confirm how crucial large high-resolution spectroscopic surveys outside the solar neighbourhood are today for our understanding of the Milky Way history.
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| 10. |
- Mikolaitis, S., et al.
(author)
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The Gaia-ESO Survey: the chemical structure of the Galactic discs from the first internal data release
- 2014
-
In: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 572
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Journal article (peer-reviewed)abstract
- Aims. Until recently, most high-resolution spectroscopic studies of the Galactic thin and thick discs were mostly confined to objects in the solar vicinity. Here we aim at enlarging the volume in which individual chemical abundances are used to characterise the thin and thick discs, using the first internal data release of the Gaia-ESO survey (GES iDR1). Methods. We used the spectra of around 2000 FGK dwarfs and giants from the GES iDR1, obtained at resolutions of up to R similar to 20 000 with the FLAMES/GIRAFFE spectrograph. We derive and discuss the abundances of eight elements (Mg, Al, Si, Ca, Ti, Fe, Cr, Ni, and Y). Results. We show that the trends of these elemental abundances with iron are very similar to those in the solar neighbourhood. We find a natural division between alpha-rich and alpha-poor stars, best seen in the bimodality of the [Mg/M] distributions in bins of metallicity, which we attribute to thick-and thin-disc sequences, respectively. This separation is visible for most alpha-elements and for aluminium. With the possible exception of Al, the observed dispersion around the trends is well described by the expected errors, leaving little room for astrophysical dispersion. Using previously derived distances from the first paper from this series for our sample, we further find that the thick-disc is more extended vertically and is more centrally concentrated towards the inner Galaxy than the thin-disc, which indicates a shorter scale-length. We derive the radial (4 to 12 kpc) and vertical (0 to 3.5 kpc) gradients in metallicity, iron, four alpha-element abundances, and aluminium for the two populations, taking into account the identified correlation between R-GC and vertical bar Z vertical bar. Similarly to other works, a radial metallicity gradient is found in the thin disc. The positive radial individual [alpha/M] gradients found are at variance from the gradients observed in the RAVE survey. The thin disc also hosts a negative vertical metallicity gradient in the solar cylinder, accompanied by positive individual [alpha/M] and [Al/M] gradients. The thick-disc, on the other hand, presents no radial metallicity gradient, a shallower vertical metallicity gradient than the thin-disc, an alpha-elements-to-iron radial gradient in the opposite sense than that of the thin disc, and positive vertical individual [alpha/M] and [Al/M] gradients. We examine several thick-disc formation scenarii in the light of these radial and vertical trends.
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