Properties of metalrich clusters

The ten clusters with spectroscopic [Fe/H] > 0.1 currently known are listed in Table 9.1, together with their properties and references. In Figure 9.2 we show the distribution of ages of OCs with [Fe/H] below and above 0.1. Since the sample of clusters for which spectroscopic metallicity data are available is not complete, we cannot carry out a comparison on a statistical basis. Nevertheless, Figure 9.2 suggests that the sample with [Fe/H] > 0.1 has a flatter age distribution than the other one, which peaks at ages older than ~1 Gyr. Metal-rich clusters are not necessarily young, in contrast to what one would naively expect, and they are evenly distributed

Table 9.1. Properties of OCs with [Fe/H] > 0.1

Cluster

[Fe/H]

Age (Gyr)

Rgc (kpc)

z (pc)

Reference

IC 4725

+0.15

0.09

7.9

-47.9

Luck (1994)

NGC 6705

+0.10

0.2

6.9

-91

González & Wallerstein (2000)

NGC 6475

+0.14

0.25

8.2

-24

Sestito et al. (2003)

Hyades

+0.13

0.6

8.5

-17.1

Paulson et al. (2003)

NGC 6134

+0.15

0.69

7.6

-3.2

Carretta et al. (2004)

NGC 5822

+0.10

1.2

7.8

57.4

Luck (1994)

IC 4651

+0.10

1.7

7.7

-122.2

Pasquini et al. (2004)

NGC 6253

+0.36

3.0

7.0

-164

Sestito et al. (2007)

NGC 6791

+0.40

8.0

8.2

800

Carraro et al. (2006)

Praesepe

+0.27

0.6

8.6

85

Pace et al. (2008)

Figure 9.2. Age distributions of clusters with metallicities [Fe/H] < 0.1 and [Fe/H] > 0.1 (left- and right-hand panels). Ages have been taken from the catalog of Dias et al. (2002).

in the age interval between ~0.1 and 8 Gyr: NGC 6791, the most metal-rich cluster, is indeed among the oldest Galactic OCs. Conversely, and surprisingly, we note the lack of metal-rich OCs younger than 100 Myr. Figures 9.3 and 9.4 are similar to Figure 9.2, but the distribution of Galactocentric distances and heights above the Galactic plane are shown. Again, there seems to be a difference between metal-rich clusters and those with [Fe/H] < 0.1. Namely, OCs with [Fe/H] > 0.1 are all located at Rgc < 9 kpc, with more than half of them being closer than 8.5 kpc (Rgc of the Sun) to the Galactic Center; the low-metallicity sample instead covers a much larger range of Rgc values and most of them are located at Rgc larger than 10 kpc. In other words, no known metal-rich clusters exist in the outer regions of the Galactic disk, at least when the spectroscopic sample is considered. Regarding z values, with the exception of NGC 6791, all metal-rich clusters are located within 200 pc of the

5 10

5 10

Rgc(kpc)

Rgc(kpc)

Figure 9.3. The same samples in as Figure 9.2, but showing the distributions of Galactocentric distances.

1500

1500

2000

200 400

800 1000 1200

200 400

800 1000 1200

Figure 9.4. The same samples as in Figures 9.2 and 9.3, but showing the distributions of absolute distances from the Galactic plane. In each panel the solid line represents the empirical distribution, while the dashed curve denotes the best fit to it.

Galactic plane: their scale height is 100 pc, compared with the value of 450 pc for the non-metal-rich sample

Was this article helpful?

0 0

Post a comment