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|Title:||The Gaia-ESO Survey: characterisation of the [α/Fe] sequences in the Milky Way discs||Authors:||Kordopatis, G.
Wyse, R. F. G.
de Laverny, P.
Korn, A. J.
Koposov, S. E.
RANDICH, Maria Sofia
SACCO, GIUSEPPE GERMANO
Worley, C. C.
|Issue Date:||2015||Journal:||ASTRONOMY & ASTROPHYSICS||Number:||582||First Page:||A122||Abstract:||Context. High-resolution spectroscopic surveys of stars indicate that the Milky Way thin and thick discs follow different paths in the chemical space defined by [α/ Fe] vs. [Fe/H], possibly suggesting different formation mechanisms for each of these structures. <BR /> Aims: We investigate, using the Gaia-ESO Survey internal Data-Release 2, the properties of the double sequence of the Milky Way discs, which are defined chemically as the high-α and low-α populations. We discuss their compatibility with discs defined by other means, such as metallicity, kinematics, or positions. <BR /> Methods: This investigation uses two different approaches: in velocity space, for stars located in the extended solar neighbourhood; and, in chemical space, for stars at different ranges of Galactocentric radii and heights from the Galactic mid-plane. The separation we find in velocity space allows us to investigate, using a novel approach, the extent of metallicity of each of the two chemical sequences, without making any assumption about the shape of their metallicity distribution functions. Then, using the separation in chemical space, adopting the magnesium abundance as a tracer of the α-elements, we characterise the spatial variation of the slopes of the [α/ Fe] - [Fe/H] sequences for the thick and thin discs and the way in which the relative proportions of the two discs change across the Galaxy. <BR /> Results: We find that the thick disc, defined as the stars tracing the high-α sequence, extends up to super-solar metallicities ([Fe/H] ≈ +0.2 dex), and the thin disc, defined as the stars tracing the low-α sequence, extends at least down to [Fe/H] ≈ -0.8 dex, with hints pointing towards even lower values. Radial and vertical gradients in α-abundances are found for the thin disc, with mild spatial variations in its [α/ Fe] - [Fe/H] paths, whereas for the thick disc we do not detect any spatial variations of this kind. This is in agreement with results obtained recently from other high-resolution spectroscopic surveys. <BR /> Conclusions: The small variations in the spatial [α/ Fe] - [Fe/H] paths of the thin disc do not allow us to distinguish between formation models of this structure. On the other hand, the lack of radial gradients and [α/ Fe] - [Fe/H] variations for the thick disc indicate that the mechanism responsible for the mixing of metals in the young Galaxy (e.g. radial stellar migration or turbulent gaseous disc) was more efficient before the (present) thin disc started forming. <P />Based on observations collected with the FLAMES spectrograph at the VLT/UT2 telescope (Paranal Observatory, ESO, Chile), for the Gaia-ESO Large Public Survey, programme 188.B-3002.||Acknowledgments:||We thank the anonymous referee for useful comments, which greatly improved the quality of this paper. G.K. acknowledges C. Gonzalez-Fernandez for fruitful comments and suggestions. R.F.G.W. acknowledges support of NSF Grant OIA-1124403 and thanks the Aspen Center for Physics and NSF Grant #1066293 for hospitality during the writing of this paper. T.B. was funded partly by grant No. 621-2009-3911 from The Swedish Research Council (VR) and partly by the project grant “The New Milky Way” from the Knut and Alice Wallenberg foundation. U.H. acknowledges support from the Swedish National Space Board (SNSB). V.A. acknowledges the support from the Fundação para a Ciência e a Tecnologia, FCT (Portugal) in the form of the fellowship SFRH/BPD/70574/2010. This research was partly supported by the Munich Institute for Astro- and Particle Physics (MIAPP) of the DFG cluster of excellence “Origin and Structure of the Universe”. Based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 188.B-3002. These data products have been processed by the Cambridge Astronomy Survey Unit (CASU) at the Institute of Astronomy, University of Cambridge, and by the FLAMES/UVES reduction team at INAF/Osservatorio Astrofisico di Arcetri. These data have been obtained from the Gaia -ESO Survey Data Archive, prepared and hosted by the Wide Field Astronomy Unit, Institute for Astronomy, University of Edinburgh, which is funded by the UK Science and Technology Facilities Council. 2 This work was partly supported by the European Union FP7 programme through ERC grant number 320360 and by the Leverhulme Trust through grant RPG-2012-541. We acknowledge the support from INAF and Ministero dell’ Istruzione, dell’ Università’ e della Ricerca (MIUR) in the form of the grant “Premiale VLT 2012”. The results presented here benefit from discussions held during the Gaia -ESO workshops and conferences supported by the ESF (European Science Foundation) through the GREAT Research Network Programme.||URI:||http://hdl.handle.net/20.500.12386/23072||URL:||https://www.aanda.org/articles/aa/abs/2015/10/aa26258-15/aa26258-15.html||ISSN:||0004-6361||DOI:||10.1051/0004-6361/201526258||Bibcode ADS:||2015A&A...582A.122K||Fulltext:||open|
|Appears in Collections:||1.01 Articoli in rivista|
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