Examinando por Autor "Koposov S.E."
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Ítem Discovery of a thin stellar stream in the SLAMS survey(Oxford University Press, 2018) Jethwa P.; Torrealba G.; Navarrete C.; Carballo-Bello J.A.; de Boer T.; Erkal D.; Koposov S.E.; Duffau S.; Geisler D.; Catelan M.; Belokurov V.We report the discovery of a thin stellar stream - which we name the Jet stream - crossing the constellations of Hydra and Pyxis. The discovery was made in data from the Search for the Leading Arm of Magellanic Satellites (SLAMS) survey, which comprises deep g and r imaging for a 650 deg2 region above the Galactic disc performed by the CTIO Blanco + DECam. SLAMS photometric catalogues have been made publicly available. The stream is approximately 0.18 deg wide and 10 deg long, though it is truncated by the survey footprint. Its colour-magnitude diagram is consistent with an old, metal-poor stellar population at a heliocentric distance of approximately 29 kpc. We corroborate thismeasurement by identifying a spatially coincident overdensity of likely blue horizontal branch stars at the same distance. There is no obvious candidate for a surviving stream progenitor. © 2018 The Author(s).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: 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.Ítem The Gaia-ESO Survey: The present-day radial metallicity distribution of the Galactic disc probed by pre-main-sequence clusters(EDP Sciences, 2017-05) Spina L.; Randich S.; Magrini L.; Jeffries R.D.; Friel E.D.; Sacco G.G.; Pancino E.; Bonito R.; Bravi L.; Franciosini E.; Klutsch A.; Montes D.; Gilmore G.; Vallenari A.; Bensby T.; Bragaglia A.; 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.; Donati P.; Frasca A.; Hourihane A.; Jofré P.; Lewis J.; Lind K.; Monaco L.; Morbidelli L.; Prisinzano L.; Sousa S.G.; Worley C.C.; Zaggia S. lContext. The radial metallicity distribution in the Galactic thin disc represents a crucial constraint for modelling disc formation and evolution. Open star clusters allow us to derive both the radial metallicity distribution and its evolution over time. Aims. In this paper we perform the first investigation of the present-day radial metallicity distribution based on [Fe/H] determinations in late type members of pre-main-sequence clusters. Because of their youth, these clusters are therefore essential for tracing the current interstellar medium metallicity. Methods. We used the products of the Gaia-ESO Survey analysis of 12 young regions (age < 100 Myr), covering Galactocentric distances from 6.67 to 8.70 kpc. For the first time, we derived the metal content of star forming regions farther than 500 pc from the Sun. Median metallicities were determined through samples of reliable cluster members. For ten clusters the membership analysis is discussed in the present paper, while for other two clusters (i.e. Chamaeleon I and Gamma Velorum) we adopted the members identified in our previous works. Results. All the pre-main-sequence clusters considered in this paper have close-To-solar or slightly sub-solar metallicities. The radial metallicity distribution traced by these clusters is almost flat, with the innermost star forming regions having [Fe/H] values that are 0.10-0.15 dex lower than the majority of the older clusters located at similar Galactocentric radii. Conclusions. This homogeneous study of the present-day radial metallicity distribution in the Galactic thin disc favours models that predict a flattening of the radial gradient over time. On the other hand, the decrease of the average [Fe/H] at young ages is not easily explained by the models. Our results reveal a complex interplay of several processes (e.g. star formation activity, initial mass function, supernova yields, gas flows) that controlled the recent evolution of the Milky Way. © ESO, 2017.