Data and results

Our strategy for high-resolution spectroscopy is to obtain spectra of a few stars per cluster, choosing stars known to be cluster members (see Figure 10.1). We concentrate on clump stars, as an optimal compromise between high luminosity (for a high signal-to-noise ratio) and temperatures that are not too cold (to minimize problems of line crowding and uncertainties in the model atmospheres). Furthermore, our choice of a single evolutionary status increases the homogeneity of the analysis.

Table 10.1. Metal-rich open clusters examined so far in the BOCCE sample

Number of Radial velocity

Table 10.1. Metal-rich open clusters examined so far in the BOCCE sample

Number of Radial velocity

Cluster

Age (Gyr)

stars

(km s_1)

Spectrograph

[Fe/H]

Reference

NGC 6819

2

3

4.2 ± 1.1

SARG

+0.07

Bragaglia etal, (2001)

IC 4651

1.7

5

-29.7 ± 0.5

FEROS

+0.11

Carretta et al, (2004)

NGC 6134

0.7

6

-24.5 ± 0.2

FEROS/UVES

+0.15

Carretta et al, (2004)

NGC 6791

9

4

-47.2 ± 0.8

SARG

+0.47

Gratton et al, (2006)

NGC 6253

3

4

-28.3 ± 0.3

FEROS/UVES

+0.46

Carretta et al, (2007)

Figure 10.2. Spectra for stars in the two most metal-rich BOCCE open clusters, NGC 6791 and NGC 6253, shown together with the well-studied, metal-rich field giant |-Leo (e.g. Gratton & Sneden 1990). The arrows indicate Fe lines. Star NGC6253-2508 is most probably a binary, since the radial velocity measured on the present spectrum differs by about 10a from the cluster average and from that measured on an old, lower-quality spectrum (Carretta et al. 2000), while its membership probability based on proper motion is high (M. Montalto and S. Desidera, private communication).

Figure 10.2. Spectra for stars in the two most metal-rich BOCCE open clusters, NGC 6791 and NGC 6253, shown together with the well-studied, metal-rich field giant |-Leo (e.g. Gratton & Sneden 1990). The arrows indicate Fe lines. Star NGC6253-2508 is most probably a binary, since the radial velocity measured on the present spectrum differs by about 10a from the cluster average and from that measured on an old, lower-quality spectrum (Carretta et al. 2000), while its membership probability based on proper motion is high (M. Montalto and S. Desidera, private communication).

We have already collected data for 15 clusters, using various telescopes and spectrographs; in particular, we employed SARG at the TNG, FEROS at the 1.5-m ESO, and UVES at the VLT for the clusters presented here (see Table 10.1, where information on the five clusters can be found, and Figure 10.2 for examples of spectra). We will later enlarge our sample using archive data and the spectra collected in a companion survey with FLAMES at the VLT; see Randich et al. (2005) and Chapter 9. With the latter we will also be able to understand possible systematic differences resulting from the various analysis procedures and assumptions.

We make every effort to keep our analysis procedure as homogeneous as possible: the model grids used to derive abundances, the line lists, the oscillator strengths, the Solar reference abundances, and the method of measurement of equivalent

Figure 10.3. A comparison between the run of a-elements with [Fe/H] of field thin-disk stars (Soubiran & Girard 2005) and BOCCE clusters (some values are still provisional).

widths are always the same. Abundances are usually derived using equivalent widths (EWs). Spectrum synthesis is used as a check, see Carretta et al. (2004) for a discussion, except for the most metal-rich OCs, for which lines are too blended to allow us to obtain reliable EWs at the resolution we have (R = 30,000-48,000), in which cases we use synthesis of selected lines.

We have obtained the metallicity (see Table 10.1) and have measured (or are working on) the elemental ratios of light, a, Fe-group, and heavier elements. Our results are generally in agreement with existing determinations, e.g. Pasquini et al. (2004) for IC 4651; and Peterson & Green (1998), Carraro et al. (2006), and Origlia etal. (2006) for NGC 6791.

One final consideration: are OCs really good tracers of the disk abundances? To check this issue, we made a comparison of abundance ratios obtained for OCs in the BOCCE sample and thin-disk field stars by Soubiran & Girard (2005). The run with [Fe/H] of a-elements and other elements considered is encouraging (Figure 10.3): the two populations seem to follow the same trends. The only notable exception is Na, which seems to be overabundant in OCs with respect to field stars. The same result has been found by others, see e.g. Yong et al. (2005). The difference is worth investigating and it could have something to do with the different evolutionary status of stars in the two samples (dwarfs for the field one and giants for the OC one), which could be either real or due to some systematic difference in the analysis (e.g. Mishenina et al. 2006).

Further considerations are postponed until a larger number of clusters has been examined (both the photometric and the spectroscopic data), forming a truly homogeneous sample on which to draw reliable conclusions.

Figure 10.3. A comparison between the run of a-elements with [Fe/H] of field thin-disk stars (Soubiran & Girard 2005) and BOCCE clusters (some values are still provisional).

References

Anthony-Twarog, B. J., & Twarog, B. A. (2000), AJ110, 2282-2295 Bragaglia, A., & Tosi, M. (2006), AJ 131, 1544-1558

Bragaglia, A., Tessicini, G., Tosi, M., Marconi, G., & Munari, U. (1997), MNRAS 284, 477-488

Bragaglia, A., Carretta, E., Gratton, R. G. et al. (2001), AJ 121, 327-336

Bruntt, H., Frandsen, S., Kjeldsen, H., & Andersen, A. I. (1999), A&A 140, 135-143

Carraro, G., Villanova, S., Demarque, P., McSwain, M. V., Piotto, G., & Bedin, L. R.

(2006), ApJ 643, 1151-1159 Carretta, E., Bragaglia, A., Tosi, M., & Marconi, G. (2000), in Stellar Clusters and

Associations: Convection, Rotation, and Dynamos, eds. R. Pallavicine, G. Micela, & S. Sciortiono (San Francisco, CA, Astronomical Society of the Pacific), pp. 273-276 Carretta, E., Bragaglia, A., Gratton, R. G., & Tosi, M. (2004), A&A 422, 951-962 Carretta, E., Bragaglia, A., & Gratton, R. G. (2007), A&A 473, 129 D'Orazi, V., Bragaglia, A., Tosi, M., Di Fabrizio, L., & Held, E. V. (2006), MNRAS 368, 471-478

Friel, E. D., Janes, K. A., Tavarez, M. et al. (2002), AJ 124, 2693-2720

Gratton, R. G., & Sneden C. (1990), A&A 234, 366-386

Gratton, R. G., Bragaglia, A., Carretta, E., & Tosi, M. (2006), ApJ 642, 462-469

Mishenina, T. V., Bienayme, O., Gorbaneva, T. I. et al. (2006), A&A 456, 1109-1120

Montgomery, K. A., Janes, K. A., & Phelps, R. L. (1994), AJ 108, 585-593

Origlia, L., Valenti, E., Rich, M. R., & Ferraro, F. R. (2006), ApJ 646, 499-504

Pasquini, L., Randich, S., Zoccali, M., Hill, V., Charbonnel, C., & Nordstrom, B. (2004),

A&A 424, 951-963 Peterson, R. C. & Green, E. M. (1998), ApJ 502, L39

Randich, S., Bragaglia, A., Pastori, L. et al. (2005), The Messenger 121, 18-22 Rosvick, J. M., & VandenBerg, D. A. (1998), AJ 115, 1516-1523 Soubiran, C., & Girard, P. (2005), A&A 438, 139-151

Twarog, B. A., Ashman, K. M., & Anthony-Twarog, B. J. (1997), AJ 114, 2556-2585 Yong, D., Carney, B. W., & Teixera de Almeida, M. L. (2005), AJ 130, 597-625

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