| 21. |
- Hayden, M. R., et al.
(author)
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The Gaia-ESO Survey : Churning through the Milky Way
- 2018
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In: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 609
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Journal article (peer-reviewed)abstract
- Context: There have been conflicting results with respect to the extent that radial migration has played in the evolution of the Galaxy. Additionally, observations of the solar neighborhood have shown evidence of a merger in the past history of the Milky Way that drives enhanced radial migration.Aims: We attempt to determine the relative fraction of stars that have undergone significant radial migration by studying the orbital properties of metal-rich ([Fe/H] > 0.1) stars within 2 kpc of the Sun. We also aim to investigate the kinematic properties, such as velocity dispersion and orbital parameters, of stellar populations near the Sun as a function of [Mg/Fe] and [Fe/H], which could show evidence of a major merger in the past history of the Milky Way.Methods: We used a sample of more than 3000 stars selected from the fourth internal data release of the Gaia-ESO Survey. We used the stellar parameters from the Gaia-ESO Survey along with proper motions from PPMXL to determine distances, kinematics, and orbital properties for these stars to analyze the chemodynamic properties of stellar populations near the Sun.Results: Analyzing the kinematics of the most metal-rich stars ([Fe/H] > 0 : 1), we find that more than half have small eccentricities (e < 0 : 2) or are on nearly circular orbits. Slightly more than 20% of the metal-rich stars have perigalacticons R-p > 7 kpc. We find that the highest [Mg/ Fe], metal-poor populations have lower vertical and radial velocity dispersions compared to lower [Mg/Fe] populations of similar metallicity by similar to 10 km s(-1). The median eccentricity increases linearly with [Mg/Fe] across all metallicities, while the perigalacticon decreases with increasing [Mg/Fe] for all metallicities. Finally, the most [Mg/Fe]-rich stars are found to have significant asymmetric drift and rotate more than 40 km s(-1) slower than stars with lower [Mg/Fe] ratios.Conclusions: While our results cannot constrain how far stars have migrated, we propose that migration processes are likely to have played an important role in the evolution of the Milky Way, with metal-rich stars migrating from the inner disk toward to solar neighborhood and past mergers potentially driving enhanced migration of older stellar populations in the disk.
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| 22. |
- Kushniruk, I., et al.
(author)
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Kinematic structures of the solar neighbourhood revealed by Gaia DR1/TGAS and RAVE
- 2017
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 608
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Journal article (peer-reviewed)abstract
- Context. The velocity distribution of stars in the solar neighbourhood is inhomogeneous and rich with stellar streams and kinematic structures. These may retain important clues regarding the formation and dynamical history of the Milky Way. However, the nature and origin of many of the streams and structures is unclear, hindering our understanding of how the Milky Way formed and evolved. Aims. We aim to study the velocity distribution of stars of the solar neighbourhood and investigate the properties of individual kinematic structures in order to improve our understanding of their origins. Methods. Using the astrometric data provided by Gaia DR1/TGAS and radial velocities from RAVE DR5 we perform a wavelet analysis with the à trous algorithm of 55 831 stars that have U and V velocity uncertainties less than 4 km s-1. An auto-convolution histogram method is used to filter the output data, and we then run Monte Carlo simulations to verify that the detected structures are real and are not caused by noise due to velocity uncertainties. Additionally we analysed our stellar sample by splitting all stars into a nearby sample (<300 pc) and a distant sample (>300 pc), and two chemically defined samples that to a first degree represent the thin and the thick disks. Results. We detect 19 kinematic structures in the solar neighbourhood in the range of scales 3-16 km s-1 at the 3σ confidence level. Among them we identified well-known groups (such as Hercules, Sirius, Coma Berenices, Pleiades, and Wolf 630), confirmed recently detected groups (such as Antoja12 and Bobylev16), and detected a new structure at (U; V) ≈ (37; 8) km s-1. Another three new groups are tentatively detected, but require further confirmation. Some of the detected groups show clear dependence on distance in the sense that they are only present in the nearby sample (<300 pc), and others appear to be correlated with chemistry as they are only present in one of the chemically defined thin and thick disk samples. Conclusions. With the much enlarged stellar sample and much increased precision in distances and proper motions, provided by Gaia DR1/TGAS we have shown that the velocity distribution of stars in the solar neighbourhood contains more structures than previously known. A new feature is discovered and three recently detected groups are confirmed at high confidence level. Dividing the sample based on distance and/or metallicity shows that there are variety of structures which form large-scale and small-scale groups; some of them have clear trends on metallicities, others are a mixture of both disk stars. Based on these findings we discuss possible origins of each group.
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| 23. |
- Magrini, L., et al.
(author)
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The Gaia-ESO Survey : The N/O abundance ratio in the Milky Way
- 2018
-
In: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 618
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Journal article (peer-reviewed)abstract
- Context. The abundance ratio N/O is a useful tool to study the interplay of galactic processes, for example star formation efficiency, timescale of infall, and outflow loading factor. Aims. We aim to trace log(N/O) versus [Fe/H] in the Milky Way and to compare this ratio with a set of chemical evolution models to understand the role of infall, outflow, and star formation efficiency in the building up of the Galactic disc. Methods. We used the abundances from IDR2-3, IDR4, IDR5 data releases of the Gaia-ESO Survey both for Galactic field and open cluster stars. We determined membership and average composition of open clusters and we separated thin and thick disc field stars. We considered the effect of mixing in the abundance of N in giant stars. We computed a grid of chemical evolution models, suited to reproduce the main features of our Galaxy, exploring the effects of the star formation efficiency, infall timescale, and differential outflow. Results. With our samples, we map the metallicity range -0. 6 <= [Fe/H] <= 0.3 with a corresponding -1.2 <= log(N/O) <= -0.2, where the secondary production of N dominates. Thanks to the wide range of Galactocentric distances covered by our samples, we can distinguish the behaviour of log(N/O) in different parts of the Galaxy. Conclusions. Our spatially resolved results allow us to distinguish differences in the evolution of N/O with Galactocentric radius. Comparing the data with our models, we can characterise the radial regions of our Galaxy. A shorter infall timescale is needed in the inner regions, while the outer regions need a longer infall timescale, coupled with a higher star formation efficiency. We compare our results with nebular abundances obtained in MaNGA galaxies, finding in our Galaxy a much wider range of log(N/O) than in integrated observations of external galaxies of similar stellar mass, but similar to the ranges found in studies of individual H II regions.
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| 24. |
- Magrini, L., et al.
(author)
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The Gaia-ESO Survey: Insights into the inner-disc evolution from open clusters
- 2015
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In: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 580
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Journal article (peer-reviewed)abstract
- Context. The inner disc, which links the thin disc with the bulge, has been somewhat neglected in the past because of the intrinsic difficulties in its study, among which crowding and high extinction. Open clusters located in the inner disc are among the best tracers of its chemistry at different ages and distances. Aims. We analyse the chemical patterns of four open clusters located within 7 kpc of the Galactic centre and of field stars to infer the properties of the inner disc with the Gaia-ESO survey idr2/3 data release. Methods. We derive the parameters of the newly observed cluster, Berkeley 81, finding an age of about 1 Gyr and a Galactocentric distance of similar to 5.4 kpc. We construct the chemical patterns of clusters and we compare them with those of field stars in the solar neighbourhood and in the inner-disc samples. Results. Comparing the three populations we observe that inner-disc clusters and field stars are both, on average, enhanced in [O/Fe], [Mg/Fe], and [Si/Fe]. Using the idr2/3 results of M67, we estimate the non-local thermodynamic equilibrium (NLTE) effect on the abundances of Mg and Si in giant stars. After empirically correcting for NLTE effects, we note that NGC 6705 and Be 81 still have a high [alpha/Fe]. Conclusions. The location of the four open clusters and of the field population reveals that the evolution of the metallicity [Fe/H] and of [alpha/Fe] can be explained within the framework of a simple chemical evolution model: both [Fe/H] and [alpha/Fe] of Trumpler 20 and of NGC 4815 are in agreement with expectations from a simple chemical evolution model. On the other hand, NGC 6705, and to a lesser degree Berkeley 81, have higher [alpha/Fe] than expected for their ages, location in the disc, and metallicity. These differences might originate from local enrichment processes as explained in the inhomogeneous evolution framework.
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| 25. |
- Puspitarini, L., et al.
(author)
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The Gaia-ESO Survey: Extracting diffuse interstellar bands from cool star spectra DIB-based interstellar medium line-of-sight structures at the kpc scale
- 2015
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In: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 573
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Journal article (peer-reviewed)abstract
- Aims. We study how diffuse interstellar bands (DIBs) measured toward distance-distributed target stars can be used to locate dense interstellar (IS) clouds in the Galaxy and probe a line-of-sight (LOS) kinematical structure, a potentially useful tool when gaseous absorption lines are saturated or not available in the spectral range. Cool target stars are numerous enough for this purpose. Methods. We devised automated DIB-fitting methods appropriate for cool star spectra and multiple IS components. The data were fitted with a combination of a synthetic stellar spectrum, a synthetic telluric transmission, and empirical DIB profiles. The initial number of DIB components and their radial velocity were guided by HI 21 cm emission spectra, or, when available in the spectral range, IS neutral sodium absorption lines. For NaI, radial velocities of NaI lines and DIBs were maintained linked during a global simultaneous fit. In parallel, stellar distances and extinctions were estimated self-consistently by means of a 2D Bayesian method from spectroscopically-derived stellar parameters and photometric data. Results. We have analyzed Gaia-ESO Survey (GES) spectra of 225 stars that probe between similar to 2 and 10 kpc long LOS in five different regions of the Milky Way. The targets are the two CoRoT fields, two open clusters (NGC 4815 and gamma Vel), and the Galactic bulge. Two OGLE fields toward the bulge observed before the GES are also included (205 target stars). Depending on the observed spectral intervals, we extracted one or more of the following DIBs: lambda lambda 6283.8, 6613.6, and 8620.4. For each field, we compared the DIB strengths with the Bayesian distances and extinctions, and the DIB Doppler velocities with the HI emission spectra. Conclusions. For all fields, the DIB strength and the target extinction are well correlated. For targets that are widely distributed in distance, marked steps in DIBs and extinction radial distance profiles match each other and broadly correspond to the expected locations of spiral arms. For all fields, the DIB velocity structure agrees with HI emission spectra, and all detected DIBs correspond to strong NaI lines. This illustrates how DIBs can be used to locate the Galactic interstellar gas and to study its kinematics at the kpc scale, as illustrated by Local and Perseus Arm DIBs that differ by greater than or similar to 30 km s(-1), in agreement with HI emission spectra. On the other hand, if most targets are located beyond the main absorber, DIBs can trace the differential reddening within the field.
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| 26. |
- Randich, S., et al.
(author)
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The Gaia-ESO Survey : open clusters in Gaia-DR1 A way forward to stellar age calibration
- 2018
-
In: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 612
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Journal article (peer-reviewed)abstract
- Context. Determination and calibration of the ages of stars, which heavily rely on stellar evolutionary models, are very challenging, while representing a crucial aspect in many astrophysical areas. Aims. We describe the methodologies that, taking advantage of Gaia-DR1 and the Gaia-ESO Survey data, enable the comparison of observed open star cluster sequences with stellar evolutionary models. The final, long-term goal is the exploitation of open clusters as age calibrators.Methods. We perform a homogeneous analysis of eight open clusters using the Gaia-DR1 TGAS catalogue for bright members and information from the Gaia-ESO Survey for fainter stars. Cluster membership probabilities for the Gaia-ESO Survey targets are derived based on several spectroscopic tracers. The Gaia-ESO Survey also provides the cluster chemical composition. We obtain cluster parallaxes using two methods. The first one relies on the astrometric selection of a sample of bona fide members, while the other one fits the parallax distribution of a larger sample of TGAS sources. Ages and reddening values are recovered through a Bayesian analysis using the 2MASS magnitudes and three sets of standard models. Lithium depletion boundary (LDB) ages are also determined using literature observations and the same models employed for the Bayesian analysis.Results. For all but one cluster, parallaxes derived by us agree with those presented in Gaia Collaboration (2017, A&A, 601, A19), while a discrepancy is found for NGC 2516; we provide evidence supporting our own determination. Inferred cluster ages are robust against models and are generally consistent with literature values.Conclusions. The systematic parallax errors inherent in the Gaia DR1 data presently limit the precision of our results. Nevertheless, we have been able to place these eight clusters onto the same age scale for the first time, with good agreement between isochronal and LDB ages where there is overlap. Our approach appears promising and demonstrates the potential of combining Gaia and ground-based spectroscopic datasets.
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| 27. |
- Rojas-Arriagada, A., et al.
(author)
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The Gaia-ESO Survey : Exploring the complex nature and origins of the Galactic bulge populations
- 2017
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 601
-
Journal article (peer-reviewed)abstract
- 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 similar to 2500 red clump stars in 11 bulge fields, sampling the area -10 degrees <= l <= + 8 degrees and -10 degrees <= b <= -4 degrees from the fourth internal data release of the Gaia-ESO survey. A sample of similar to 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 Delta [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 alpha-abundances has been performed in a large area on the basis of a homogeneous, fully spectroscopic analysis of high-resolution, high S/N data.
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| 28. |
- Rojas-Arriagada, A., et al.
(author)
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The Gaia-ESO Survey : Separating disk chemical substructures with cluster models. Evidence of a separate evolution in the metal-poor thin disk
- 2016
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 586
-
Journal article (peer-reviewed)abstract
- Context. Recent spectroscopic surveys have begun to explore the Galactic disk system on the basis of large data samples, with spatial distributions sampling regions well outside the solar neighborhood. In this way, they provide valuable information for testing spatial and temporal variations of disk structure kinematics and chemical evolution. Aims. The main purposes of this study are to demonstrate the usefulness of a rigorous mathematical approach to separate substructures of a stellar sample in the abundance-metallicity plane, and provide new evidence with which to characterize the nature of the metal-poor end of the thin disk sequence. Methods. We used a Gaussian mixture model algorithm to separate in the [Mg/Fe] vs. [Fe/H] plane a clean disk star subsample (essentially at R-GC < 10 kpc) from the Gaia-ESO survey (GES) internal data release 2 (iDR2). We aim at decomposing it into data groups highlighting number density and/or slope variations in the abundance-metallicity plane. An independent sample of disk red clump stars from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) was used to cross-check the identified features. Results. We find that the sample is separated into five groups associated with major Galactic components; the metal-rich end of the halo, the thick disk, and three subgroups for the thin disk sequence. This is confirmed with the sample of red clump stars from APOGEE. The three thin disk groups served to explore this sequence in more detail. The two metal-intermediate and metal-rich groups of the thin disk decomposition ([Fe/H] > 0 : 25 dex) highlight a change in the slope at solar metallicity. This holds true at different radial regions of the Milky Way. The distribution of Galactocentric radial distances of the metal-poor part of the thin disk ([Fe/H] < 0 : 25 dex) is shifted to larger distances than those of the more metal-rich parts. Moreover, the metal-poor part of the thin disk presents indications of a scale height intermediate between those of the thick and the rest of the thin disk, and it displays higher azimuthal velocities than the latter. These stars might have formed and evolved in parallel and/or dissociated from the inside-out formation taking place in the internal thin disk. Their enhancement levels might be due to their origin from gas pre-enriched by outflows from the thick disk or the inner halo. The smooth trends of their properties (their spatial distribution with respect to the plane, in particular) with [Fe/H] and [Mg/Fe] suggested by the data indicates a quiet dynamical evolution, with no relevant merger events.
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| 29. |
- San Roman, I., et al.
(author)
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The Gaia-ESO Survey : Detailed abundances in the metal-poor globular cluster NGC 4372
- 2015
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 579
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Journal article (peer-reviewed)abstract
- We present the abundance analysis for a sample of 7 red giant branch stars in the metal-poor globular cluster NGC 4372 based on UVES spectra acquired as part of the Gaia-ESO Survey. This is the first extensive study of this cluster from high-resolution spectroscopy. We derive abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, Fe, Cr, Ni, Y, Ba, and La. We find a metallicity of [Fe/H] = -2.19 +/- 0.03 and find no evidence of any metallicity spread. This metallicity makes NGC 4372 one of the most metal-poor Galactic globular clusters. We also find an a-enhancement typical of halo globular clusters at this metallicity. Significant spreads are observed in the abundances of light elements. In particular, we find a Na-O anticorrelation. Abundances of O are relatively high compared with other globular clusters. This could indicate that NGC 4372 was formed in an environment with high O for its metallicity. A Mg-Al spread is also present that spans a range of more than 0.5 dex in Al abundances. Na is correlated with Al and Mg abundances at a lower significance level. This pattern suggests that the Mg-Al burning cycle is active. This behavior can also be seen in giant stars of other massive, metal-poor clusters. A relation between light and heavy s-process elements has been identified.
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| 30. |
- Smiljanic, R., et al.
(author)
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The Gaia-ESO Survey : Inhibited extra mixing in two giants of the open cluster Trumpler 20?
- 2016
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 591
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Journal article (peer-reviewed)abstract
- We report the discovery of two Li-rich giants, with A(Li) ~ 1.50, in an analysis of a sample of 40 giants of the open cluster Trumpler 20 (with turnoff mass ~1.8 M⊙). The cluster was observed in the context of the Gaia-ESO Survey. Methods. The atmospheric parameters and Li abundances were derived using high-resolution UVES spectra. The Li abundances were corrected for nonlocal thermodynamical equilibrium (non-LTE) effects. Results. Only upper limits of the Li abundance could be determined for the majority of the sample. Two giants with detected Li turned out to be Li rich: star MG 340 has A(Li)non-LTE = 1.54 ± 0.21 dex and star MG 591 has A(Li)non-LTE = 1.60 ± 0.21 dex. Star MG 340 is on average ~0.30 dex more rich in Li than stars of similar temperature, while for star MG 591 this difference is on average ~0.80 dex. Carbon and nitrogen abundances indicate that all stars in the sample have completed the first dredge-up. Conclusions. The Li abundances in this unique sample of 40 giants in one open cluster clearly show that extra mixing is the norm in this mass range. Giants with Li abundances in agreement with the predictions of standard models are the exception. To explain the two Li-rich giants, we suggest that all events of extra mixing have been inhibited. This includes rotation-induced mixing during the main sequence and the extra mixing at the red giant branch luminosity bump. Such inhibition has been suggested in the literature to occur because of fossil magnetic fields in red giants that are descendants of main-sequence Ap-type stars.
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