A&A 430, L13–L16 (2005) Astronomy DOI: 10.1051/0004-6361:200400118 & c ESO 2005 Astrophysics Editor UVES observations of the Canis Major overdensity the to L. Sbordone1,3,P.Bonifacio2, G. Marconi1,S.Zaggia2, and R. Buonanno3 1 ESO - European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago de Chile e-mail: [email protected] 2 INAF - Osservatorio Astronomico di Trieste, via G. B. Tiepolo, Trieste, Italy Letter 3 Universitá di Roma “Tor Vergata”, via Della Ricerca Scientifica 2, Roma, Italy Received 21 October 2004 / Accepted 2 December 2004 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: [α/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. Key words. stars: abundances – stars: atmospheres – galaxies: abundances – galaxies: evolution – galaxies: dwarf 1. Introduction The authors situate the structure at about 7 kpc from the Sun and about 16 kpc from the Galactic Centre, and estimate a mass In the framework of the hierarchical merging scenario for the 7 of about 10 M, which would make it the nearest known ex- galaxy formation, dwarf galaxies play the role of “building ternal galaxy. They also associate it to the ring-like structure blocks” of the larger structures like the Milky Way (MW). known as Monoceros Ring (Newberg et al. 2002), Ring (Ibata Nevertheless, the present day dwarf galaxies in the Local et al. 2003) or GASS (Galactic Anticentre Stellar Structure, see Group (LG) appear to be somewhat undesirable building Crane et al. 2003 and Frinchaboy et al. 2004). Bellazzini et al. blocks: their chemistry is significantly different from the one (2004) also inferred a possible connection with some Galactic found both in the MW Disc and Halo systems (Venn et al. globular clusters, among others NGC 2808. Shortly afterwards, 2004). This is not surprising, since a long evolution took place, Momany et al. (2004) questioned the effective existence and after the main merging phase, in the “survived” dwarf galaxies size of CMa, claiming that the anomaly could be explained, (Lanfranchi & Matteucci 2003). Nevertheless, merging events to a large extent, by properly taking into account the Galactic are still taking place in the MW, as testified by the discov- disc warp, which is maximum in the CMa direction. Bellazzini ery in the Halo of the stream related to the Sagittarius dwarf et al. (2004) examined and rejected this hypothesis, and so did Spheroidal galaxy (Sgr dSph, see Ibata et al. 2001; Majewski Martin et al. (2004b), deriving for the centre of the structure a et al. 2003). The Sgr dSph itself displays a peculiar chemical − radial velocity of 109 km s 1, with a low velocity dispersion of composition (Bonifacio et al. 2000a, 2004), leading to think − 13 km s 1, both difficult to reconcile with the dynamics of the that chemically peculiar subpopulations, traces of past or ongo- local disc. ing merging events, should be identifiable in the Galactic Disc or Halo. Recently Martin et al. (2004a) claimed the discovery of 2. Data reduction and analysis the core of a tidally disrupted dwarf galaxy, still recogniz- Shortly after the announcement of the discovery of CMa, able as an overdensity in the external Galactic disc in Canis we obtained Director’s Discretionary Time (DDT) at VLT- Major (Canis Major Overdensity or CMa from now on). FLAMES with the aim of probing the dynamics and chemi- In a subsequent paper (Bellazzini et al. 2004) they rec- cal composition of the newly discovered structure. Bellazzini ognized the same population also in the background of the ◦ et al. (2004) detected the CMa population in the background Galactic open cluster NGC 2477 at 13 from the CMa centre. of NGC 2477 using the EIS pre-FLAMES photometry and as- Based on observations obtained in the ESO Director’s trometry of Momany et al. (2001) which is publicly avail- Discretionary Time program 272-B.5017. able. We therefore observed this field selecting in the EIS Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:200400118 L14 L. Sbordone et al.: Canis Major Table 1. Photometry and physical parameters for the three stars. Star α (J2000.0) δ V (V − I)0 Teff log g [Fe/H] ξ Vrad hms ◦, , mag mag K cgs dex km s−1 km s−1 Editor − + EIS 6631 07 51 36.2 38 31 10 16.35 0.75 5367 3.5 0.15 1.80 135.4 EIS 7873 07 52 37.1 −38 28 01 16.26 0.89 4990 2.3 –0.42 1.80 111.1 EIS 30077 07 51 41.0 −38 38 39 16.51 0.89 4994 2.8 –0.04 1.45 97.0 the to Table 2. Abundance ratios for the three stars. [X/Fe ] is used for neu- tral elements, [X/Fe ] for ionized species, and [Fe/H] for Fe and Fe . Errors are 1σ intervals, “n” is the number of lines used. EIS 6631 EIS 7873 EIS 30077 Letter [X/Fe] n [X/Fe] n [X/Fe] n Na 0.18 ± 0.18 4 0.18 ± 0.10 3 −0.10 ± 0.15 4 Mg −0.49 ± 0.11 3 −0.18 ± 0.23 3 −0.01 ± 0.15 4 Al −0.21 ± 0.14 2 −0.14 ± 0.13 2 −0.24 ± 0.14 2 Si −0.21 ± 0.14 4 0.10 ± 0.11 4 −0.27 ± 0.16 4 Ca −0.22 ± 0.16 11 0.02 ± 0.16 9 −0.08 ± 0.18 7 Sc −0.03 1 −0.28 ± 0.14 2 −0.20 ± 0.20 2 Ti 0.18 ± 0.16 7 −0.02 ± 0.14 5 0.13 ± 0.16 7 V 0.33 ± 0.19 3 0.31 ± 0.12 2 0.49 ± 0.14 2 Mn −0.02 1 −0.12 1 0.12 1 Fe 0.15 ± 0.11 20 −0.42 ± 0.10 15 −0.04 ± 0.13 21 Fe 0.12 ± 0.17 13 −0.37 ± 0.13 8 −0.07 ± 0.08 8 Co 0.25 ± 0.21 2 −0.17 2 0.06 ± 0.13 2 Ni 0.02 ± 0.18 15 −0.09 ± 0.19 13 −0.21 ± 0.15 11 Fig. 1. Spectra of the three most probable CMa stars, in the region Cu 0.25 1 0.23 1 of the Mg b triplet. [Fe/H], log(g)andTeff all increase from bottom Y 0.19 ± 0.21 3 −0.15 ± 0.13 2 −0.61 ± 0.13 3 to top. The spectra are normalized to one, stars 30077 and 6631 are Ba 0.21 1 0.37 1 0.07 1 shifted vertically for display purposes (continuum is at 2.5 for 30077 La 0.61 ± 0.24 2 0.26 ± 0.14 2 0.67 ± 0.10 2 and at 4 for 6631). Ce 0.78 ± 0.25 3 0.16 1 0.21 ± 0.09 2 Nd 0.59 ± 0.20 2 −0.01 ± 0.18 3 0.36 ± 0.13 4 Eu 0.21 1 0.20 1 0.10 1 photometry Red Giant/Clump stars possible CMa members. Observations were performed between January and March 2004 and consisted of 4 × 3045 s exposures, using the HR09 were derived from Schlegel et al. (1998) maps, as corrected setting for GIRAFFE fibers, and the UVES setting centred at by Bonifacio et al. (2000b). Effective temperatures were de- 580 nm. rived from the Alonso et al. (1999) calibration for giant stars. In this letter we describe the detailed chemical analysis of The abundance analysis was performed in a traditional man- 3 of the 7 UVES stars obtained with FLAMES; the analysis of ner by using our Linux porting of the ATLAS, WIDTH and the stars observed with GIRAFFE has been described briefly in SYNTHE codes (see Kurucz 1993 and Sbordone et al. 2004). Zaggia et al. (2004) and will be the object of a separate paper. We noticed that, at variance with the other two stars, the lines of The four spectra of each star have been corrected to heliocentric EIS 7873 appear to be somewhat broader than the instrumental radial velocity and then coadded. Due to the very low S/N ratio, resolution. We derived the final gravity by forcing Fe –Fe they have been convolved with a 5 km s−1 FWHM Gaussian, ionization equilibrium. In this phase, two more stars (EIS 2812 degrading the resolution to about 33 000, reaching a S/N of and EIS 5429) proved to be dwarfs (log g>4.0),and thus in- about 40 per pixel at 580 nm. By combining our UVES and compatible with a heliocentric distance of the order of 7 kpc.