4 The globular cluster ESO 452-SC11

4.1 Colour-magnitude diagrams

For comparison purposes, we present in Fig. 5 a very deep CMD of the intermediate metallicity reference cluster NGC 6752, to be used as template. Notice the MS extending along 6 magnitudes.

  

Figure 5: NGC 6752: V vs. (V-I) CMD which is a combination of bright sequences (Rosino et al. 1997) and the present data containing a deep main sequence

  

Figure 6: ESO 452-SC11: V vs. (V-I) NTT CMD for a) whole frame; b) extraction of r < 65'' around the cluster center, where the mean locus of NGC 6752 is overplotted; c) Danish CMD with an extraction r < 94''

 

Figure 6: continued

  

Figure 7: Field around ESO 452-SC11, for r > 188'' (Danish frame)

In Fig. 6a is shown the whole frame V vs. (V-I) NTT CMD for ESO 452-SC11, which is clearly dominated by the cluster features. The MS extends to V $\approx$ 23, about 3 magnitudes below the TO. A poorly populated giant branch is present together with an HB which contains a red component, and some bluer stars.

In Fig. 6b is shown an extraction of r < 65'' where the mean locus of NGC 6752 is overplotted.

In Fig. 6c we show a Danish CMD corresponding to an extraction of r < 94'', a slightly larger area than that presented in Fig. 6a. In this sampling there appear two additional bright giants, and one probable asymptotic giant branch (AGB) star.

In Fig. 7 the field surrounding the cluster is given, corresponding to r > 188'' (Danish frame). Since the cluster Galactic latitude is only b = +12.07$^\circ$,a disk field contamination is not unexpected. In fact, a field MS extends along 16.5 < V < 19.0. Below this the cluster TO is evident, which is somewhat bluer than the field stars. This shows that cluster stars are still detected at such radii.

4.2 Cluster metallicity

The morphology of the HB is not purely red, indicating that this is not a metal-rich cluster like 47 Tuc. In Fig. 6b is shown the mean locus of NGC 6752 (Rosino et al. 1997) overplotted on the NTT extraction of the cluster. The RGB and subgiant branch are well matched. Although the HB morphology is different, a metallicity similar to that of NGC 6752 ([Fe/H] = -1.54, Zinn 1985) can be assigned to ESO 452-SC11. Webbink (1985) gives [Fe/H] $\approx$ -1.0, probably based on the HB morphology.

4.3 Reddening and distance

We calculate the cluster reddening taking NGC 6752 as reference. The (V-I) colour of the RGB at the HB level for ESO 452-SC11 is $1.66\pm0.03$, whereas for NGC 6752 it is $(V-I) = 0.95\pm0.04$, giving a difference of $\Delta (V-I) = 0.71$. Adopting E(B-V) = 0.04 for NGC 6752 (Zinn 1980), and E(V-I)/E(B-V) = 1.31 (Dean et al. 1978), we get E(V-I) = 0.76 which converts to E(B-V) = 0.58 and $A_V=1.80\ (R = A_V/E(B-V) = 3.1$, Savage & Mathis 1979) for ESO 452-SC11.

The HB level in ESO 452-SC11 is located at $V_{\rm HB}$ = 16.55 $\pm$ 0.15. The HB absolute magnitude for [Fe/H] = -1.5 is M_V = 0.7 (Buonanno et al. 1989; Lee et al. 1990; Sandage & Cacciari 1990), and there results an absolute distance modulus of (m-M)₀ = 14.05 $\pm$ 0.25, and a distance of $d_{\odot}$ = 6.5 $\pm$ 0.7 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 = -1.71, Y = -0.90 and Z = 1.36 kpc. We also show in Fig. 4 the |Z| vs. r location of ESO 452-SC11. The cluster is at the bulge distance, but slightly above the bulk of the bulge population.

4.4 Age

It can be seen in Fig. 6a that the cluster CMD reaches several magnitudes below the TO. We measured a $\Delta V^{\rm TO}_{\rm HB}$ = 3.6 $\pm$ 0.2, which means that this is an old globular cluster of age comparable to the average of the halo clusters, for which a mean of $\Delta V^{\rm TO}_{\rm HB}$ = 3.6 was found by Buonanno et al. (1989). The overplot of the mean locus of NGC 6752, with $\Delta V^{\rm TO}_{\rm HB}$ = 3.65 (Buonanno et al. 1989) shows that their TO morphologies are slightly different, and that ESO 452-SC11 would be somewhat younger than this reference cluster.

ESO 452-SC11 could belong to the halo considering its age and metallicity. On the other hand, from its location in the Galaxy (Fig. 4), it could also be a bulge member at the extreme lower end of the metallicity distribution (McWilliam & Rich 1994), in which case the old age is also compatible (Ortolani et al. 1995b).

Kinematical properties of the cluster would be necessary to decide about its subsystem membership in the Galaxy.

4.5 Depletion of low mass stars?

It appears from the CMD (Figs. 6) that ESO 452-SC11 shows a depopulated MS, as compared to the reference cluster NGC 6752 (Fig. 5). In Fig. 8 we present the luminosity function of ESO 452-SC11 compared to that of NGC 6752, where the numbers for ESO 452-SC11 are normalized to those for NGC 6752 in the interval 18 < V < 20. Notice a drop for V > 20.5. We carried out artificial stars experiments, in order to check the completeness by counting retrieved stars. The results show a 100% completeness at V < 20.6, going down to 84% at V = 21.0, and to 81% at V = 22.0. The last bin at V = 23.0 shows 77%. These completeness corrections cannot account for the drop in the luminosity function, which appears to be significantly affected by low mass stars segregation. The stellar mass corresponding to the drop in the luminosity function is close to the turnoff mass. We might be facing a case of a cluster evaporation, similarly to that of E3 (see Veronesi et al. 1996 for a recent discussion). Some evidence of this was also found for NGC 6717 (Palomar 9) - see Ortolani et al. (1999).

  

Figure 8: Luminosity function of ESO 452-SC11 (dotted lines) compared to that for NGC 6752 (solid lines). Notice the depletion at fainter magnitudes for ESO 452-SC11