3 Terzan 3

3.1 Colour-magnitude diagrams

In Fig. 3a and Fig. 3b we show V vs. (B-V) diagrams for the NTT and Danish whole frames respectively. Both diagrams are deep attaining the cluster turn-off (TO), the NTT one being better defined. In turn, the Danish CMD presents a more populated red giant branch (RGB) as expected from the larger frame size.

  

Figure 3: V vs. (B-V) CMD of Terzan 3: a) NTT data, b) Danish data, where the solid line is a fit of the mean locus of 47 Tuc

The horizontal branch (HB) is red, suggesting that Terzan 3 is metal-rich. It is slightly elongated and tilted, which suggests some amount of differential reddening. The similarity to 47 Tuc ([Fe/H] = -0.71, Zinn 1985) is shown in Fig. 3b where the mean locus of 47 Tuc as given in Aurière & Ortolani (1988) and Desidera & Ortolani (1997) is superimposed in Fig. 3b. The subgiant and giant branches of Terzan 3 are steeper than those of the more metal-rich clusters such as NGC 6528 and NGC 6553 (Ortolani et al. 1995). From these comparisons we conclude that the metallicity of Terzan 3 is about the same as that of 47 Tuc ([Fe/H] $\approx -0.7$).

The cluster HB is located at $V \approx$ 17.3 $\pm$ 0.1 and the (B-V) colour of the RGB at the HB level is $\approx$ 1.63 $\pm$ 0.05 (Fig. 3). The reddening is derived with respect to 47 Tuc for which $(B-V)\rm _g$ = 0.95 and E(B-V) = 0.04 (Hesser et al. 1987; Aurière & Ortolani (1988)). There results E(B-V) = 0.72 for Terzan 3, a value considerably higher than previous determinations (Sect. 1).

Absolute magnitudes of the horizontal branch $M_{ V}^{\rm HB}$ are metallicity dependent. We adopted Jones et al. (1992)'s relation, slightly modifying the zero point in order to fit the results by Guarnieri et al. (1998) for NGC 6553: $M_{ V}^{\rm HB}$ = 0.16[Fe/H] + 0.98. The metallicity of Terzan 3 implies $M_{ V}^{\rm HB}$ = 0.87, and the observed distance modulus is (m-M) _V = 16.43. The selective-to-total absorption dependence on the reddening and metallicity as discussed in Barbuy et al. (1998), results for Terzan 3 the value R = 3.3 and A _V = 2.38. The true distance modulus is then $(m-M)_{\rm 0}$ = 14.05 and the distance to the Sun $d_{\odot}$ $\approx$ 6.5 kpc.

Assuming the distance of the Sun to the Galaxy center of $R_{\odot}$ = 8.0 kpc (Reid 1993), the Galactocentric coordinates are X = -1.8 (X< 0 refers to our side of the Galaxy), Y = -1.7 kpc and Z = 1.0 kpc. This indicates that Terzan 3 is not far in the halo as the early determinations suggested (Sect. 1), but it is instead located in the bulge. We conclude that it is a bulge cluster which is revealed by its location and metallicity.

3.2 Field

In Fig. 4 is shown the CMD of a field 31' north of Terzan 3. There are too few stars in the brighter sequences but the bulge TO is well populated. Considering the high latitude of this field ($b \approx 10^\circ$), not much contamination from the disk is expected.

  

Figure 4: V vs. (B-V) CMD of the field 31' North of Terzan 3 (Danish data)

Assuming 47 Tuc as reference, the mean locus of Hesser et al. (1987) indicates an E(B-V) = 0.54 for this field, corresponding to A _V = 1.78. The reddening results slightly lower than that of the cluster. This difference is not unexpected, given that this field is slightly higher in b than the cluster itself. The TO shows some spread, with a mean value around $V \approx$ 20.4, corresponding to a HB of $V \approx$ 16.9. The resulting true distance modulus of (m-M)₀ = 14.25 gives a distance from the Sun $d_{\odot}$ $\approx$ 7.1 kpc. This value implies that the bulk of these field stars is located slightly closer to us than the Galactic center, as expected from its l and b values.