Open clusters as tracers of the evolution of the abundance gradients

People involved at OAB: Bragaglia, Carretta, Cignoni, Pancino, Tosi

Open clusters are very useful tracers of the properties of the Galactic disk; in the context of the BOCCE project (the Bologna Open Cluster Chemical Evolution project) Bragaglia and Tosi proceeded with the analysis of photometric data. We studied the old, anticentre cluster Tombaugh 2, possibly associated with the Canis Major overdensity (in collaboration with Andreuzzi, TNG and Marconi, ESO; a paper is in preparation).

We obtained LBC@LBT data for three more open clusters (King 8, Berkeley 23, and Berkeley 31) in September 2008, determining their age, distance, metallicity, and radial distribution, in collaboration with G. Beccari (ESTEC) and a paper is in preparation (see Fig. 2).

Bragaglia, Carretta and Tosi completed and published the analysis of FLAMES/UVES spectra of a sample of 10 open clusters, and of the Li abundance in Berkeley 32 in collaboration with Randich, Sestito, and others (Bragaglia et al. 2008; Sestito et al. 2008; Randich et al. 2009). We also continued with the acquisition of SARG@TNG spectra of clusters in the framework of the BOCCE project and in collaboration with Gratton (OA-Pd).

Bragaglia and Carretta submitted in 2008 a proposal to observe giant stars in NGC6791 with Hydra@WIYN (Kitt Peak), in collaboration with Sneden (Univ. Austin), Gratton and Lucatello (OA-Pd). We intend to derive the Na and O abundances to see whether the Na-O anticorrelation is really seen exclusively in globular clusters. Spectra were aquired in June 2009 and analysis in under way (see Fig. 3). The web page displays all the OCs in our sample already studied, with the relative bibliography; the photometry tables can be downloaded from there.

This research is in collaboration with Gratton, Lucatello (INAF-Padova Obs.), Marconi (ESO, Chile), Andreuzzi (INAF-Roma Obs., TNG), Beccari (ESTEC, NL), Randich, Sestito (INAF-Arcetri Obs.), Sneden (Univ. Texas, USA).

Figure: The Colour-Magnitude Diagrams of the three anticentre open clusters Be 23, Be 31, and King 8, obtained with LBC@LBT. In the right panels only the central regions for each of them is plotted, with the best-fit isochrones.

We have also collected several high-resolution spectra of open clusters using FOCES@CAHA2.2 m and UVES@VLT. Three red clump stars have been observed in each of the following clusters, spanning a range of metallicities and ages, some of which have never been studied before with high resolution spectroscopy: Cr 110, M 67, NGC 2099, NGC 2420, NGC 7789, Trumpler 5, Berkeley 39, M 11, NGC 2141, Berkeley 32, NGC 752 plus the Hyades and Praesepe. All spectra are of a suitable quality ( $\rm R\sim30000$ and $\rm S/N\sim100$ per pixel) to derive accurate abundances of iron peak elements, $\alpha$-elements, heavy s-process elements and light elements. The data reduction is now complete and the first paper of the series has been recently submitted (Pancino et al. 2009). Moreover, the detailed abundance analysis of the second group of clusters is presently under way (Carrera et al., in preparation).

This research is in collaboration with C. Gallart and R. Carrera (IAC Tenerife, Spain) and R. Zinn (Yale University, USA)

Figure: Spectral synthesis for Na I lines in the high-metallicity ([Fe/H]${}=+0.45$) open cluster NGC 6791 observed with Hydra@WIYN; as a comparison, we also show the same for Arcturus ([Fe/H]${} =-0.5$)