2016A&A...588A.120C

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±0.007dex/kpc in the outer 12kpc of the disk. The [α/Fe] gradient appears flat, with a slope of 0.006±0.007dex/kpc. 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 5Gyr, 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.