Examinando por Autor "Mikolaitis S."
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Ítem The Gaia -ESO Survey: Open clusters in Gaia -DR1: A way forward to stellar age calibration(EDP Sciences, 2018-04) Randich S.; Tognelli E.; Jackson R.; Jeffries R.D.; Degl'Innocenti S.; Pancino E.; Fiorentin, P. Re; Spagna A.; Sacco G.; Bragaglia A.; Magrini L.; Moroni, P. G. Prada; Alfaro E.; Franciosini E.; Morbidelli L.; Roccatagliata V.; Bouy H.; Bravi L.; Jiménez-Esteban F.M.; Jordi C.; Zari E.l.; Tautvaišiene G.; Drazdauskas A.; Mikolaitis S.; Gilmore G.; Feltzing S.; Vallenari A.; Bensby T.; Koposov S.; Korn A.; Lanzafame A.; Smiljanic R.; Bayo A.; Carraro G.; Costado M.T.; Heiter U.; Hourihane A.; Jofré P.; Lewis J.; Monaco L.; Prisinzano L.; Sbordone L.; Sousa S.G.; Worley C.C.; Zaggia S.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. © ESO 2018.Ítem The Gaia -ESO Survey: The N/O abundance ratio in the Milky Way?(EDP Sciences, 2018-10) Magrini L.; Vincenzo F.; Randich S.; Pancino E.; Casali G.; Tautvaišien G.; Drazdauskas A.; Mikolaitis S.; Minkevičiūt R.; Stonkut E.; Chorniy Y.; Bagdonas V.; Kordopatis G.; Friel E.; Roccatagliata V.; Jiménez-Esteban F.M.; Gilmore G.; Vallenari A.; Bensby T.; Bragaglia A.I.; Korn A.J.; Lanzafame A.C.; Smiljanic R.; Bayo A.; Casey A.R.; Costado M.T.; Franciosini E.; Jofré P.; Lewis J.; Mónaco L.; Morbidelli L.; Sacco G.; Worley C.; Hourihane A.Context. The abundance ratio N/O is a useful tool to study the interplay of galactic processes, for example star formation e ciency, 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 e ciency 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 e ect 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 e ects of the star formation e ciency, infall timescale, and di erential outflow. Results. With our samples, we map the metallicity range0:6 [Fe/H] 0.3 with a corresponding1:2 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 di erent parts of the Galaxy. Conclusions. Our spatially resolved results allow us to distinguish di erences 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 e ciency. 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. © ESO 2018.Ítem The Gaia-ESO Survey: Churning through the Milky Way(EDP Sciences, 2018-01) Hayden M.R.; Recio-Blanco A.; De Laverny P.; Mikolaitis S.; Guiglion G.; Randich S.; Bayo A.; Bensby T.; Bergemann M.; Bragaglia A.; Casey A.; Costado M.; Feltzing S.; Franciosini E.; Hourihane A.; Jofre P.; Koposov S.; Kordopatis G.; Lanzafame A.; Lardo C.; Lewis J.; Lind K.; Magrini L.; Monaco L.; Morbidelli L.; Pancino E.; Sacco G.; Stonkute E.; Worley C.C.; Zwitter T.; Hill V.; Gilmore G.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 Rp> 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 ~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. © ESO, 2018.Ítem The Gaia-ESO Survey: Low-α element stars in the Galactic bulge(EDP Sciences, 2017-07) Recio-Blanco A.; Rojas-Arriagada A.; De Laverny P.; Mikolaitis S.; Hill V.; Zoccali M.; Fernández-Trincado J.G.; Robin A.C.; Babusiaux C.; Gilmore G.; Randich S.; Alfaro E.; Allende Prieto C.; Bragaglia A.; Carraro G.; Jofré P.; Lardo C.; Monaco L.; Morbidelli L.; Zaggia S.We take advantage of the Gaia-ESO Survey iDR4 bulge data to search for abundance anomalies that could shed light on the composite nature of the Milky Way bulge. The α-element (Mg, Si, and whenever available, Ca) abundances, and their trends with Fe abundances have been analysed for a total of 776 bulge stars. In addition, the aluminum abundances and their ratio to Fe and Mg have also been examined. Our analysis reveals the existence of low-α element abundance stars with respect to the standard bulge sequence in the [α/ Fe] versus [Fe/H] plane. Eighteen objects present deviations in [α/ Fe] ranging from 2.1 to 5.3σ with respect to the median standard value. Those stars do not show Mg-Al anti-correlation patterns. Incidentally, this sign of the existence of multiple stellar populations is reported firmly for the first time for the bulge globular cluster NGC 6522. The identified low-α abundance stars have chemical patterns that are compatible with those of the thin disc. Their link with massive dwarf galaxies accretion seems unlikely, as larger deviations in α abundance and Al would be expected. The vision of a bulge composite nature and a complex formation process is reinforced by our results. The approach used, which is a multi-method and model-driven analysis of high resolution data, seems crucial to reveal this complexity. © ESO, 2017.