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The Gaia-ESO Survey: characterisation of the [alpha/Fe] sequences in the Milky Way discs

Kordopatis, G.; Wyse, R. F. G.; Gilmore, G.; Recio-Blanco, A.; de Laverny, P.; Hill, V.; Adibekyan, V.; Heiter, U.; Minchev, I. and Famaey, B., et al. (2015) In Astronomy & Astrophysics 582.
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 [alpha/Fe] vs. [Fe/H], possibly suggesting different formation mechanisms for each of these structures. 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-alpha and low-alpha populations. We discuss their compatibility with discs defined by other means, such as metallicity, kinematics, or positions. Methods. This investigation uses two different approaches: in velocity space, for stars located in the extended solar neighbourhood; and, in chemical space, for... (More)
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 [alpha/Fe] vs. [Fe/H], possibly suggesting different formation mechanisms for each of these structures. 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-alpha and low-alpha populations. We discuss their compatibility with discs defined by other means, such as metallicity, kinematics, or positions. 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 alpha-elements, we characterise the spatial variation of the slopes of the [alpha/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. Results. We find that the thick disc, defined as the stars tracing the high-alpha sequence, extends up to super-solar metallicities ([Fe/H] approximate to + 0.2 dex), and the thin disc, defined as the stars tracing the low-alpha sequence, extends at least down to [Fe/H] approximate to 0.8 dex, with hints pointing towards even lower values. Radial and vertical gradients in alpha-abundances are found for the thin disc, with mild spatial variations in its [alpha/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. Conclusions. The small variations in the spatial [alpha/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 [alpha/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. (Less)
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Galaxy: abundances, Galaxy: stellar content, Galaxy: evolution, Galaxy:, kinematics and dynamics, Galaxy: disk, stars: abundances
in
Astronomy & Astrophysics
volume
582
publisher
EDP Sciences
external identifiers
  • wos:000363538500122
  • scopus:84945971805
ISSN
0004-6361
DOI
10.1051/0004-6361/201526258
language
English
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62c5703c-e2eb-4e8f-bd3b-f6c8ff609ead (old id 8383382)
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2015-12-18 14:17:58
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@article{62c5703c-e2eb-4e8f-bd3b-f6c8ff609ead,
  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 [alpha/Fe] vs. [Fe/H], possibly suggesting different formation mechanisms for each of these structures. 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-alpha and low-alpha populations. We discuss their compatibility with discs defined by other means, such as metallicity, kinematics, or positions. 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 alpha-elements, we characterise the spatial variation of the slopes of the [alpha/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. Results. We find that the thick disc, defined as the stars tracing the high-alpha sequence, extends up to super-solar metallicities ([Fe/H] approximate to + 0.2 dex), and the thin disc, defined as the stars tracing the low-alpha sequence, extends at least down to [Fe/H] approximate to 0.8 dex, with hints pointing towards even lower values. Radial and vertical gradients in alpha-abundances are found for the thin disc, with mild spatial variations in its [alpha/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. Conclusions. The small variations in the spatial [alpha/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 [alpha/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.},
  articleno    = {A122},
  author       = {Kordopatis, G. and Wyse, R. F. G. and Gilmore, G. and Recio-Blanco, A. and de Laverny, P. and Hill, V. and Adibekyan, V. and Heiter, U. and Minchev, I. and Famaey, B. and Bensby, Thomas and Feltzing, Sofia and Guiglion, G. and Korn, A. J. and Mikolaitis, S. and Schultheis, M. and Vallenari, A. and Bayo, A. and Carraro, G. and Flaccomio, E. and Franciosini, E. and Hourihane, A. and Jofre, P. and Koposov, S. E. and Lardo, C. and Lewis, J. and Lind, K. and Magrini, L. and Morbidelli, L. and Pancino, E. and Randich, S. and Sacco, G. G. and Worley, C. C. and Zaggia, S.},
  issn         = {0004-6361},
  keyword      = {Galaxy: abundances,Galaxy: stellar content,Galaxy: evolution,Galaxy:,kinematics and dynamics,Galaxy: disk,stars: abundances},
  language     = {eng},
  publisher    = {EDP Sciences},
  series       = {Astronomy & Astrophysics},
  title        = {The Gaia-ESO Survey: characterisation of the [alpha/Fe] sequences in the Milky Way discs},
  url          = {http://dx.doi.org/10.1051/0004-6361/201526258},
  volume       = {582},
  year         = {2015},
}