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RESEARCH PRODUCT

Multi-Object Spectroscopy of Open Clusters with FLAMES: Preliminary GTO Results

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We report on preliminary results of VLT/FLAMES observations of the old open clusters NGC 2506, Mel 66 and Cr 261, obtained as part of our Guaranteed Time on this instrument. We focus in particular on the very old cluster Cr 261, one of the oldest open clusters in the Galaxy. We compare the derived Li abundances with those of other old clusters, and we discuss briefly Li depletion on the main-sequence from the age of the Hyades to ∼8 Gyr. 1 The GTO Program on Open Clusters With the advent of the multi-object spectrograph FLAMES at the VLT [13], the study of open clusters has received a great boost due both to the larger telescope area and to the multiplexing capability offered by the new instrument. Not only it is now possible to reach at intermediate resolution (R∼20,000) fainter objects and more distant clusters, but it is also possible to obtain in one shot spectra of ∼130 objects, with an increase in observational efficiency of several hundreds with respect to single-object spectrographs on 4m-class telescopes. Open clusters are among the most suitable targets for observations with FLAMES since they provide homogeneous samples of stars of the same age and chemical composition over projected areas in the sky typically smaller, except for the closest clusters, than the FLAMES field-of-view (25 arcmin diameter). In return to the Italian participation in the development of FLAMES, the Ital-FLAMES Consortium obtained 11 guaranteed nights that were devoted to various science programs. 3 of these nights were used for observations of open clusters of different ages and metallicities (P.I. R. Pallavicini) with the purpose of investigating Li abundances, metallicities and membership with GIRAFFE, and Fe and α-element abundances with the fiber link to UVES. Our sample includes young clusters such as the Orion Nebula Cluster, NGC 6530 (a few Myr in age) and Blanco 1 (∼100 Myr), all observed also in X-rays with Chandra and/or XMM-Newton, as well as the old clusters NGC 2506, Mel 66 and Cr 261 (all older than the Hyades). Here we focus on the old clusters which offer a unique opportunity for investigating Li depletion and non-standard mixing mechanisms in dwarf stars during their main-sequence (MS) evolution. 182 R. Pallavicini et al. 2 Li in Old Clusters The scientific motivations of our study were reviewed recently by several authors (e.g. [1]; [6]; [20]; [12]). In brief, observations of clusters of different ages and metallicities, carried out mostly with 4m-class telescopes and, to a much less extent so far, with HIRES at Keck and UVES at the VLT, have shown that solar-type stars suffer Li depletion on the MS, contrary to expectations of standard models, i.e. models that assume convection as the only mixing mechanism on the MS. Moreover, while the Hyades (age ∼600 Myr) and most clusters older than the Hyades observed so far (e.g. [22]; [18]; [19]; [21]) show a tight Li vs. Teff relationship, with little or no scatter, a large Li spread is observed in the solar-age cluster M 67 ([15]; [7]) as well as in field stars [14]. In particular, the low Li abundance of the present Sun, which is two orders of magnitude lower than the Li abundance of T-Tauri stars and of the meteorites, remains totally unexplained. This feature, together with the presence of a Li spread in M 67 and of Li depletion on the MS, with little or no observed dependence on metallicity ([6]; [21]), indicates the occurrence of non-standard mixing mechanisms (e.g. rotational mixing, diffusion, mass loss, gravitational waves) in addition to convection (e.g. [17]; [1]; and references therein). As part of our GTO program on FLAMES, we have observed the open clusters NGC 2506 (age ∼2 Gyr, [9]), Mel 66 (∼4 Gyr, [8]) and Cr 261 (∼8 Gyr, [4]; [2]). The observations were carried out in May and Dec 2003 (for Cr 261 and Mel 66, respectively) and Feb 2004 (for NGC 2506). Astrometry and photometry were taken from EIS imaging observations [11] for NGC 2506 and Mel 66 and from dedicated WFI observations at the ESO/MPA 2.2m telescope for Cr 261. The data, which comprise more than 100 objects for each cluster, were obtained with the GIRAFFE spectrograph in the MEDUSA mode (R∼20,000) using the grating setting HR15 centered at 679.7 nm (i.e. in the Li region). 7 bright members of each cluster were observed at a higher resolution (R∼50,000) with the fiber-link to UVES. The GIRAFFE spectra were reduced with the standard pipeline and are now being analysed to derive radial velocities, Li abundances, and metallicities for turn-off (TO) and lower MS stars. Preliminary results on Li abundances in Cr 261 are presented here. Results for the other clusters will be reported elsewhere (Spano et al., in preparation). 3 The Old Open Cluster Cr 261 Cr 261 is one of the oldest open clusters in the Galaxy, with an estimated age ranging from ∼7 to ∼11 Gyr ([5]; [16]; [4]). The metallicity, derived from both low-resolution and high-resolution spectra, is slightly subsolar ([Fe/H]=−0.2; [3]; [2]) and the reddening is highly uncertain, with quoted E(B − V ) values ranging from 0.22 to 0.34 ([5]; [10]; [4]). Proper motion and/or radial velocity memberships are not known, and the contamination by field stars is high. However, the TO and MS are clearly visible in the color-magnitude diagrams with the TO at V 16.7, (B−V ) 0.85, (V −I) 0.95 (cf. [4]). We have observed with Multi-Object Spectroscopy of Open Clusters with FLAMES 183 0.50 0.55 0.60 0.65 0.70 0.75 0 50 10 0 15 0 (B − V)o (reddening=0.34) E .W . L iI+ F eI ( m A ) Fig. 1. Li equivalent widths vs. dereddened (B − V )o color for MS stars of Cr 261. Teff (K) lo g A (L i) + 12 6400 6200 6000 5800 5600 5400 5200 5000 0. 0 0. 5 1. 0 1. 5 2. 0 2. 5 3. 0 Collinder 261 (7−11 Gyr), this study E(B−V)=0.34 Hyades (600 Myr), Thorburn et al. 1993 M 67 (5 Gyr), Jones et al. 1999 Fig. 2. Li abundances vs. Teff for MS stars in Cr 261 (filled circles), compared to the Hyades (asterisks, [22]) and M 67 (squares, [7]). A reddening E(B−V )=0.34 has been