3 Colour-magnitude diagrams

In Fig. 2 we show the whole frame V vs. B-V CMD. We see a main sequence (MS) with a curved turnoff, and evidence for a clump of giants at the same magnitude level as the turnoff. The spread of the MS appears to be 2-3 times wider than the photometric errors, indicating a true spread, probably due to differential reddening and field contamination.

In Fig. 3 the V vs. B-V diagram for a cluster extraction of r < 0.8' (r < 200 pixels) is shown, where about 2/3 are cluster stars and 1/3 belong to the field. The clump of giants becomes clear at $V \approx 16.2$ and B-V $\approx$ 1.55. The spread decreases relative to Fig. 2, making the overall appearance of the cluster CMD clearer.

In Fig. 4 an extraction of the field of radius r > 1.6' (r > 400 pixels) is shown. No stars are seen in the giant clump region, confirming that those in Fig. 3 belong to the cluster. The MS is brighter, showing no curved turnoff, suggesting a somewhat younger age with respect to that of the cluster, assuming that the bulk of the field stars are at a comparable distance.

Figure 2: V vs. B-V diagram of the whole frame $4' \times 2.4'$

Figure 3: V vs. B-V of UKS 2 corresponding to an extraction of r < 0.8' (r < 200 pixels). The five high probability member giants are labeled

Figure 4: V vs. B-V diagram for surrounding field at r > 1.6' (r > 400 pixels)

3.1 Isochrone fitting

The Padova isochrones (Bertelli et al. 1994) have been updated (Girardi et al. 2000), where for solar metallicity, both overshooting and no-overshooting models are available.

In Fig. 5 we overimpose no-overshooting models of 0.5, 1.0 and 3.16 Gyr to the same cluster extraction of Fig. 2. Clearly the oldest isochrone can be ruled out, since the magnitude difference between the giant clump and turnoff is too large with respect to that of the cluster. Taking into account the location and shape of the turnoff, location of giants, and MS locus and spread, a compromise fit is shown in this Fig. 5. An age intermediate between 0.5 and 1 Gyr is suggested, around 800 Myr. This fit leads to E(B-V) = 0.39, and a distance modulus of (V-M_V) = 15.55. Adopting a standard total-to-selective absorption R = 3.1, we obtain A_V = 1.21, a true distance modulus (m-M)₀ = 14.34 and a distance from the Sun $d_{\rm\odot}$ = 7.4 kpc.

In Fig. 6 the overshooting models are fitted to the cluster CMD. Basically the conclusions are the same as above, because the intrinsic spread and underpopulation of this cluster do not provide enough constraints. We derive the same age of 800 Myr, a reddening E(B-V) = 0.42 and A_V = 1.30, (V-M_V) = 15.45. The true distance modulus is (m-M)₀ = 14.15 and the distance from the Sun is $d_{\rm\odot}$ = 6.8 kpc.

We adopt an age of 800 Myr thus comparable to that of the Hyades. Therefore UKS 2 can be classified as an old disk cluster, according to Friel (1995 and references therein), where old means ages of 700 Myr and older. Such objects are often referred to as intermediate age clusters (IACs).

The sample of confirmed old open clusters by means of CMDs is steadily growing in recent years. Considering the compilations by Janes & Phelps (1994), Friel (1995), Carraro et al. (1998) and the more recent one by Dutra & Bica (2000), their number amounts to approximately 100. UKS 2 is a new entry in the family of old open clusters.

Figure 5: No-overshooting Padova isochrones of solar metallicity overimposed on the cluster CMD. Ages are: 0.5 (solid), 1.0 (dashed), 3.16 (dotted) Gyrs

Figure 6: Overshooting Padova isochrones of solar metallicity overimposed on the cluster CMD. Ages are: 0.5 (solid), 1.0 (dashed), 3.16 (dotted) Gyrs

We conclude that UKS 2 has a reddening E(B-V) = 0.40 and a distance of $d_{\rm\odot}$ = 7.0 $\pm$ 1.0 kpc. The age is compatible with that derived from the integrated spectrum by Bica et al. (1998), and the reddening is higher than their value, probably because of contamination by foreground stars in the integrated spectra.

We assume the distance of the Sun to the Galaxy center to be $R_{\odot}$ = 8.0 kpc by Reid (1993). The Galactocentric coordinates are then X = -8.73 kpc (X < 0refers to our side of the Galaxy), Y = -6.95 kpc and Z = -0.37 kpc. The distance from the Galactic center is $R_{\rm GC}$ = 11.2 kpc, and the distance projected on the plane $r_{\rm GC}$ is about the same. We are dealing with an old open cluster outside the solar radius, as expected since essentially all the sample is found there. A possible explanation for this distribution is that inner radius old open clusters would have been dissolved by tidal encounters with giant molecular clouds (Friel 1995 and references therein).

The location of the cluster in the |z| vs. distance from the galactic center projected on the plane $r_{\rm GC}$ (see Fig. 4 of Bica et al. 1999) shows that it is relatively close to the plane (370 pc), while the oldest open clusters (age > 2 Gyr) often attain heights from the plane of 1 kpc or more.

The age of UKS 2, together with the distance from the plane, can be used for a simple estimation of the vertical velocity. Assuming that it is in its initial orbit, |v_z| $\sim$ 0.4 km s^-1. For the oldest open clusters that estimation gives a comparable value. This is consistent with the stellar disk thickness being supported by vertical motions. Proper motions and radial velocity studies are required to shed more light on formation and evolution of disk substructures.

3.2 Red giants

Since UKS 2 is a relatively distant faint open cluster, it can be an important contribution to the old disk sample of clusters, in terms of understanding their kinematics, metallicities and age distributions. In a recent study in this direction, Scott et al. (1995) determined radial velocities for 11 old open clusters, and compiled data for additional 24 such objects. This means about one third of the verified old open clusters. It is thus important to extend radial velocity and metallicity determinations for more open clusters. For this purpose, we identify the giants in UKS 2 in view of future studies to observe these stars individually. In the CCD image (Fig. 1) are marked the 5 red giants identified in the CMD of Fig. 3. In Table 4 are given their identification, V and B-V colours. Four of the giants appear to be typical clump giants, and the coolest one might be a Hayashi line red giant.

   

Table 4: Red giants

Star	X	Y	V	B-V
1	393.58	495.50	15.424	1.755
2	529.41	386.05	15.824	1.577
3	484.20	569.93	16.220	1.548
4	426.07	408.99	16.351	1.554
5	316.54	429.61	16.579	1.530