UVES observations of the Canis Major overdensity

Abstract

We present the first detailed chemical abundances for three giant stars which are candidate members of the Canis Major overdensity, obtained by using FLAMES-UVES at VLT. The stars, in the background of the open cluster NGC 2477, have radial velocities compatible with a membership to this structure. However, due to Galactic disc contamination, radial velocity by itself is unable to firmly establish membership. The metallicities span the range -0.5 < [Fe/H] < +0.1. Assuming that at least one of the three stars is indeed a member of CMa implies that this structure has undergone a high level of chemical processing, comparable to that of the Galactic disc. The most metal-rich star of the sample, EIS 6631, displays several abundance ratios which are remarkably different from those of Galactic stars: [alpha/Fe] ~-0.2, [Cu/Fe] ~+0.25, [La/Fe]~+0.6, [Ce/Fe]~+0.8 and [Nd/Fe]~+0.6. These ratios make it likely that this star was formed in an external galaxy.