3 The open cluster ESO 93-SC08

3.1 Colour-magnitude diagrams

In Figs. 3a and b we show respectively the NTT CMDs corresponding to whole frame and an extraction of r < 33''. In Fig. 3a a bright disk MS is present, whereas in Fig. 3b the cluster sequences become clearer. The red clump, typical of intermediate age clusters, is located at V = 18.6 and (V-I) = 1.85. A few bright giants are detected above the clump. The cluster TO is reached as well as $\approx 2$ magnitudes of MS. The CMD of ESO 93-SC08 appears to be rather similar to that of M 67 (e.g. Demarque et al. 1992).

  

Figure 3: ESO 93-SC08: V vs. (V-I) CMD for the a) NTT whole frame; b) r < 33''

3.2 Age

The magnitude difference between the TO and the clump is an age indicator (Janes & Phelps 1994). We have measured in Demarque et al.'s (1992) CMDs this value for the reference intermediate age clusters M 67 and NGC 188, resulting $\Delta V^{\rm TO}_{\rm clump}$ = 2.2 and 2.75 respectively. For ESO 93-SC08 we obtain $\Delta V^{\rm TO}_{\rm clump}$ = 2.3 $\pm$ 0.20 (Fig. 3b). Adopting Demarque et al.'s ages of 4 Gyr (M 67) and 6.5 Gyr (NGC 188), there results 4 - 5 Gyr for the present cluster. Therefore ESO 93-SC08 adds to the list of the about 20 old open clusters with ages > 4 Gyr (Friel 1995; Carraro et al. 1998).

3.3 Reddening and distance

We take as reference point in the CMD the dereddened locus of the clump in intermediate age clusters; Garnavich et al. (1994) indicates M_V = 0.94 and ($V-I)_{\rm 0}$ = 1.0 for M 67. With values for ESO 93-SC08 from Sect. 3.1 we obtain E(V-I) = 0.85, which converts to E(B-V) = 0.64 (Dean et al. 1978) for ESO 93-SC08. This reddening value is larger than that obtained from the integrated spectrum by Bica et al. (1998), where there is probably an effect of less reddened foreground stars contaminating this poorly populated cluster, and also the integrated light must be affected by statistical fluctuations owing to the small number of bright member stars.

Adopting a selective-to-total absorption of R = 3.1, we get A_V = 1.98. An absolute distance modulus of $(m-M)_{\rm 0} = 15.68\pm 0.2$ and a distance from the Sun $d_{\odot}$ = 13.5 $\pm$ 0.6 kpc.

The Galactocentric coordinates of the cluster, assuming a distance of the Sun to the Galactic center of $R_{\odot}$ = 8.0 kpc (Reid 1993), are X = -2.6 (X < 0 refers to our side of the Galaxy), Y = -12.5 kpc and Z = -1.0 kpc. The galactocentric distance projected on the plane is r = 12.8 kpc, indicating that this is probably the most distant open cluster relative to the Galactic center.

The location of the cluster with respect to the open clusters older than 2 Gyr is shown in Fig. 4, where the data given in Friel (1995) are adopted, using a distance to the Galactic center of 8 kpc (Reid 1993). ESO 93-SC08 is among the farthest, and is also among the open clusters more distant from the Galactic plane. Such outlyers may occur as far as the disk border in HI (Kulkarni et al. 1982), HII regions (Flich & Blitz 1984), and luminous stars (Moffat et al. 1979), which point to an extent of the disk of 20 - 30 kpc.

It is also interesting to discuss the galactic location of old open star clusters and three metal rich globular clusters with a structure similar to that of open clusters. Two of them were until recently classified as open clusters, but analyses by means of CMDs, spectroscopy of individual stars and integrated spectroscopy, point to a globular cluster nature: Lynga 7 (Ortolani et al. 1993; Tavares & Friel 1995), and BH 176 (Ortolani et al. 1995a). Besides, the globular cluster candidate Palomar 10, which has a loose structure, shows a CMD compatible with that of a metal-rich globular cluster (Kaisler et al. 1997). Integrated spectroscopy of Lynga 7, BH 176 and Palomar 10, which are all projected on the disk, indicates that they are similar to metal-rich globular clusters (Bica et al. 1998). In Fig. 4 are also indicated the positions in the Galaxy of these three objects.

  

Figure 4: Distance from the plane |z| vs. Galactocentric radial distance projected on the plane r for known open clusters of ages > 2 Gyr (Friel 1995) and ESO 93-SC08 (open circles), where are also plotted ESO 452-SC11, besides Lynga 7, Palomar 10 and BH 176 (classified as globulars) (filled circles)