5. UKS 1

5.1. Colour magnitude diagrams

In Fig. 8 (click here) is shown the I vs. (I-z) diagram for the whole frame. We note that the Gunn z magnitudes are instrumental, and consequently the zero-point of (I-z) is arbitrary. Differently from the Danish CMD fields shown in Figs. 4 (click here) and 6 (click here), the NTT frame corresponds to a much smaller field, and the cluster occupies most of the frame (Fig. 3 (click here)). The RGB should correspond to that of the cluster plus bulge.

An extraction of radius r < 16'' centred on the cluster is shown in Fig. 9 (click here). The cluster RGB, which basically coincides with that of the bulge plus cluster (Fig. 8 (click here)), is clear. The brightest giants are found at . Interestingly, this value is similar to that of Liller 1, which is reported to have almost the same reddening (Ortolani et al. 1996a; Frogel et al. 1995).

  
Figure 8: UKS 1: I vs. (I-z) colour-magnitude diagram for the NTT whole frame. Note that the Gunn z magnitudes are instrumental

  
Figure 9: UKS 1: I vs. (I-z) colour-magnitude diagram for an extraction of radius r < 16'' from the NTT data

5.2. Metallicity

In the UKS 1 discovery work, Malkan et al. (1980) pointed out that the cluster might be metal-rich or metal-poor, each assumption led to A_V = 9.7 and 11.4 respectively. All subsequent metallicity derivations in the literature are based on Malkan's (1982) integrated infrared photometry, which suggested .

However, the similarity of the cluster and field CMDs (Figs. 8 (click here), 9 (click here)), together with the coincidence of reddening and bright giants I magnitude of Liller 1, indicates that UKS 1 might be metal-rich. Moreover, an integrated spectrum by Bica et al. (1997) indicates . We adopt .

5.3. Reddening and distance

We assume a reddening of E(B-V) = 3.1 (see Sect. 1). We use 47 Tuc as reference for the distance. The bright giants in 47 Tuc are at , and with E(B-V) = 0.04, together with A_I/A_V = 0.61, there results an intrinsic magnitude I₀ = 9.92. Using the cluster distance modulus (m-M)₀ = 13.31, we obtain an absolute magnitude for the brightest giants in 47 Tuc of M_I = -3.39.

With the prescriptions of Sect. 3, R = 3.46, A_V = 10.73 and A_I = 6.54. Since for UKS 1 the bright giants level is , the intrinsic magnitude of brightest giants of UKS 1 is I₀ = 10.96. Therefore, the true distance modulus of UKS 1 is (m-M)₀ = 14.35, and the distance  kpc. The Galactocentric coordinates are X = 0.6, Y = 0.7 and Z = 0.1 kpc.