Asymmetric metallicity patterns in the stellar velocity space with RAVE

¹ Directorate of Science, European Space Agency (ESA-ESTEC), PO Box 299, 2200 AG Noordwijk, The Netherlands
² Dept. FQA, Institut de Ciencies del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Marti Franques 1, 08028 Barcelona, Spain
e-mail: tantoja@fqa.ub.edu
³ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Bd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
⁵ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁶ Observatoire astronomique de Strasbourg, Université de Strasbourg, CNRS UMR 7550, 11 rue de l’Université, 67000 Strasbourg, France
⁷ The Oskar Klein Centre for Cosmoparticle Physics, Department of Physics, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
⁸ Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles St, Baltimore, MD 21218, USA
⁹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
¹⁰ Sydney Institute for Astronomy, School of Physics A28, University of Sydney, Sydney, NSW 2006, Australia
¹¹ E.A. Milne Centre for Astrophysics, University of Hull, Hull, HU6 7RX, UK
¹² Senior CIfAR Fellow, University of Victoria, Victoria, BC V8P 5C2, Canada
¹³ Department of Physics, Chong Yuet Ming Physics Building, The University of Hong Kong, Pokfulam Road, Hong Kong
¹⁴ Department of Physics and Astronomy, Macquarie University, Sydney, NSW 2109, Australia
¹⁵ Western Sydney University, Locked Bag 1797, Penrith South DC, NSW 1797, Australia
¹⁶ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, RH5 6NT, UK
¹⁷ Dipartimento di Fisica e Astronomia Galileo Galilei, Universita’ di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁸ Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana, Slovenia

Received: 24 July 2016
Accepted: 14 February 2017

Abstract

Context. The chemical abundances of stars encode information on their place and time of origin. Stars formed together in e.g. a cluster, should present chemical homogeneity. Also disk stars influenced by the effects of the bar and the spiral arms might have distinct chemical signatures depending on the type of orbit that they follow, e.g. from the inner versus outer regions of the Milky Way.

Aims. We explore the correlations between velocity and metallicity and the possible distinct chemical signatures of the velocity over-densities of the local Galactic neighbourhood.

Methods. We use the large spectroscopic survey RAVE and the Geneva Copenhagen Survey. We compare the metallicity distribution of regions in the velocity plane (v_R,v_φ) with that of their symmetric counterparts (−v_R,v_φ). We expect similar metallicity distributions if there are no tracers of a sub-population (e.g. a dispersed cluster, accreted stars), if the disk of the Galaxy is axisymmetric, and if the orbital effects of the bar and the spiral arms are weak.

Results. We find that the metallicity-velocity space of the solar neighbourhood is highly patterned. A large fraction of the velocity plane shows differences in the metallicity distribution when comparing symmetric v_R regions. The typical differences in the median metallicity are of 0.05 dex with statistical significant of at least 95% confidence, and with values up to 0.6 dex. For stars with low azimuthal velocity v_φ, the ones moving outwards. These include stars in the Hercules and Hyades moving groups and other velocity branch-like structures. For higher v_φ, the stars moving inwards have higher metallicity than those moving outwards. We have also discovered a positive gradient in v_φ with respect to metallicity at high metallicities, apart from the two known positive and negative gradients for the thick and thin disks.

Conclusions. The most likely interpretation of the metallicity asymmetry is that it is mainly due to the orbital effects of the Galactic bar and the radial metallicity gradient of the disk. We present a simulation that supports this idea.