Through thick and thin: kinematic and chemical components in the solar neighbourhood

JULIO F. NAVARRO, MARIO G. ABADI, KIM A. VENN, K.C. FREEMAN, BORJA ANGUIANO

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2011 vol. 412 p. 1203 - 1209

0035-8711

We search for chemically distinct stellar components in the solar neighbourhood using acompilation of published data. Extending earlier work, we show that when the abundances ofFe, α elements and the r-process element Eu are considered together, stars separate neatly intotwo groups that delineate the traditional thin and thick disc components of the MilkyWay. Thegroup akin to the thin disc is traced by stars with [Fe/H] > −0.7 and [α/Fe] < 0.2. The thickdisc-like group overlaps the thin disc in [Fe/H] but has higher abundances of α elements and Eu.Stars in the range −1.5 < [Fe/H] < −0.7 with low [α/Fe] ratios, however, seem to belong toa separate, dynamically cold, non-rotating component that we associate with tidal debris. Thekinematically hot stellar halo dominates the sample for [Fe/H] < −1.5. These results suggestthat it may be possible to define the main dynamical components of the solar neighbourhoodusing only their chemistry, an approach with a number of interesting consequences.With suchdefinition, the kinematics of thin disc stars is found to be independent of metallicity: theiraverage rotation speed remains roughly constant in the range −0.7 < [Fe/H] < +0.4, a resultthat argues against radial migration having played a substantial role in the evolution of thethin disc. The velocity dispersion of the thin disc is also independent of [Fe/H], implyingthat the familiar increase in velocity dispersion with decreasing metallicity is the result ofthe increasing prevalence of the thick disc at lower metallicities, rather than of the sustainedoperation of a dynamical heating mechanism. The substantial overlap in [Fe/H] and, likely,stellar age, of the various components might affect other reported trends in the properties ofstars in the solar neighbourhood. A purely chemical characterization of these componentswould enable us to scrutinize these trends critically in order to understand which result fromaccretion events and which result from secular changes in the properties of the Galaxy.