Abundances and kinematics for ten anticentre open clusters^⋆

¹ Dipartimento di Fisica e Astronomia, Università di Padova, vicolo Osservatorio 3, 35122 Padova, Italy
e-mail: tristan.cantat@oapd.inaf.it
² INAF−Osservatorio Astronomico di Padova, vicolo Osservatorio 5, 35122 Padova, Italy
³ INAF−Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ Dipartimento di Fisica e Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ INAF−Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 12 January 2016
Accepted: 16 February 2016

Abstract

Context. Open clusters are distributed all across the Galactic disk and are convenient tracers of its properties. In particular, outer disk clusters bear a key role in the investigation of the chemical evolution of the Galactic disk.

Aims. The goal of this study is to derive homogeneous elemental abundances for a sample of ten outer disk open clusters, and investigate possible links with disk structures such as the Galactic anticentre stellar structure.

Methods. We analysed high-resolution spectra of red giants, obtained from the Keck-HIRES and VLT-UVES archives. We derived elemental abundances and stellar atmosphere parameters by means of the classical equivalent width method. We also performed orbit integrations using proper motions.

Results. The Fe abundances we derive trace a shallow negative radial metallicity gradient of slope −0.027 ± 0.007 dex kpc^-1 in the outer 12 kpc of the disk. The [α/Fe] gradient appears flat, with a slope of 0.006 ± 0.007 dex kpc^-1. The two outermost clusters (Be 29 and Sau 1) appear to follow elliptical orbits. The cluster Be 20 also exhibits a peculiar orbit with a large excursion above the plane.

Conclusions. The irregular orbits of the three most metal poor clusters (two of which are located at the edge of the Galactic disk), are compatible with an inside-out formation scenario for the Milky Way in which extragalactic material is accreted onto the outer disk. This is the case if the irregular orbits of these clusters are confirmed by more robust astrometric measurements such as those of the Gaia mission. We cannot determine whether Be 20, Be 29, and Sau 1 are of extragalactic origin, as they may be old, genuine Galactic clusters whose orbits were perturbed by accretion events or minor mergers in the past 5 Gyr, or they may be representants of the thick disk population. The nature of these objects is intriguing and deserves further investigation in the near future.