The outer parts of the Milky Way disk are believed to be one of the main
arenas where the accretion of external material in the form of dwarf galaxies
and subsequent formation of streams is taking place. The Monoceros stream and
the Canis Major and Argo over-densities are notorious examples. VLT high
resolution spectra have been acquired for five distant open clusters. We derive
accurate radial velocities to distinguish field interlopers and cluster
members. For the latter we perform a detailed abundance analysis and derive the
iron abundance [Fe/H] and the abundance ratios of several α elements.
Our analysis confirms previous indications that the radial abundance gradient
in the outer Galactic disk does not follow the expectations extrapolated from
the solar vicinity, but exhibits a shallower slope. By combining the
metallicity of the five program clusters with eight more clusters for which
high resolution spectroscopy is available, we find that the mean metallicity in
the outer disk between 12 and 21 kpc from the Galactic center is [Fe/H]
≈−0.35, with only marginal indications for a radial variation. In
addition, all the program clusters exhibit solar scaled or slightly enhanced
α elements, similar to open clusters in the solar vicinity and thin disk
stars. We investigate whether this outer disk cluster sample might belong to an
extra-galactic population, like the Monoceros ring. However, close scrutiny of
their properties - location, kinematics and chemistry - does not convincingly
favor this hypothesis. On the contrary, they appear more likely genuine
Galactic disk clusters. We finally stress the importance to obtain proper
motion measurements for these clusters to constrain their orbits.Comment: 19 pages, 9 eps figure, in press in A&A, abstract rephrased to fit i
