Young [α/Fe]-enhanced stars discovered by CoRoT and APOGEE: What is their origin?^⋆

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: Cristina.Chiappini@aip.de
² Laboratório Interinstitucional de e-Astronomia, – LIneA, Rua Gal. José Cristino 77, 20921-400 Rio de Janeiro, Brazil
³ Osservatorio Astronomico di Padova – INAF, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ Dipartimento di Fisica e Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁵ School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
⁶ LESIA, Université Pierre et Marie Curie, Université Denis Diderot, Obs. de Paris, 92195 Meudon Cedex, France
⁷ Institut d’Astrophysique et de Geophysique, Allée du 6 août, 17 – Bât. B5c, 4000 Liège 1 ( Sart-Tilman), Belgium
⁸ Instituto de Física, Universidade Federal do Rio Grande do Sul, Caixa Postal 15051, 91501-970 Porto Alegre, Brazil
⁹ Observatoire de la Côte d’Azur, Laboratoire Lagrange, CNRS UMR 7923, BP 4229, 06304 Nice Cedex, France
¹⁰ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹¹ Observatório Nacional, Rua Gal. José Cristino 77, 20921-400 Rio de Janeiro, Brazil
¹² Instituto de Astrofísica de Canarias, 38205, La Laguna, Tenerife, Spain
¹³ Universidad de La Laguna, Departamento de Astrofísica, 38206, La Laguna, Tenerife, Spain
¹⁴ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
¹⁵ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
¹⁶ Institut für Astronomie, Universität Wien, Türkenschanzstr. 17, 1180 Wien, Austria
¹⁷ Laboratoire AIM, CEA/DSM – CNRS – Univ. Paris Diderot – IRFU/SAp, Centre de Saclay, 91191 Gif-sur-Yvette Cedex, France
¹⁸ Space Science Institute, 4750 Walnut Street Suite 205, Boulder CO 80301, USA
¹⁹ Institut dÁstrophysique Spatiale, UMR8617, CNRS, Université Paris XI, Bâtiment 121, 91405 Orsay Cedex, France
²⁰ Dept. of Physics and JINA-CEE: Joint Institute for Nuclear Astrophysics – Center for the Evolution of the Elements, Univ. of Notre Dame, Notre Dame, IN 46530, USA
²¹ 57 Department of Astronomy, Case Western Reserve University, Cleveland, OH 44106, USA
²² New Mexico State University, Las Cruces, NM 88003, USA
²³ Department of Astronomy, University of Virginia, PO Box 400325, Charlottesville VA 22904-4325, USA
²⁴ ELTE Gothard Astrophysical Observatory, 9704 Szombathely, Szent Imre herceg st. 112, Hungary
²⁵ Department of Astronomy, Indiana University, Bloomington, IN 47405, USA
²⁶ Dept. of Astronomy, University of Michigan, Ann Arbor, MI 48104, USA
²⁷ Apache Point Observatory PO Box 59, Sunspot, NM 88349, USA
²⁸ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park 146 Brownlow Hill Liverpool L3 5RF, UK
²⁹ McDonald Observatory, University of Texas at Austin, HC75 Box 1337-MCD, Fort Davis, TX 79734, USA
³⁰ Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
³¹ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
³² Vanderbilt University, Dept. of Physics & Astronomy, VU Station B 1807, Nashville, TN 37235, USA

Received: 11 February 2015
Accepted: 18 March 2015

Abstract

We report the discovery of a group of apparently young CoRoT red-giant stars exhibiting enhanced [α/Fe] abundance ratios (as determined from APOGEE spectra) with respect to solar values. Their existence is not explained bystandard chemical evolution models of the Milky Way, and shows that the chemical-enrichment history of the Galactic disc is more complex. We find similar stars in previously published samples for which isochrone-ages could be reliably obtained, although in smaller relative numbers. This might explain why these stars have not previously received attention. The young [α/Fe]-rich stars are much more numerous in the CoRoT-APOGEE (CoRoGEE) inner-field sample than in any other high-resolution sample available at present because only CoRoGEE can explore the inner-disc regions and provide ages for its field stars. The kinematic properties of the young [α/Fe]-rich stars are not clearly thick-disc like, despite their rather large distances from the Galactic mid-plane. Our tentative interpretation of these and previous intriguing observations in the Milky Way is that these stars were formed close to the end of the Galactic bar, near corotation – a region where gas can be kept inert for longer times than in other regions that are more frequently shocked by the passage of spiral arms. Moreover, this is where the mass return from older inner-disc stellar generations is expected to be highest (according to an inside-out disc-formation scenario), which additionally dilutes the in-situ gas. Other possibilities to explain these observations (e.g., a recent gas-accretion event) are also discussed.