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3 The color magnitude diagrams

The CMD derived from optical photometry is shown in the upper left panel of Fig. 2. The global morphology resembles the CMD of an intermediate age open cluster like NGC 2477 (Hartwick et al. 1972), Tombaugh 1 (Carraro & Patat 1995) and NGC 6603 (Bica et al. 1993). The MS extends vertically down to $V \approx
22$, and the Turn Off point (TO) is situated at V = 15.5, (V-I) = 1.3. The brightest stars in the red region of the diagram can represent the Red Giant (RG) clump of core He-burning stars. Most of these are cluster members, on the basis of their radial velocities (Scott et al. 1995). The Hertzsprung gap is clearly defined. The population of stars on the right side of the MS are probably interlopers, showing that the CMD is contaminated by stars from the field of the Galactic Disk. Also the stars above the TO probably belong to the field.

The CMD from IR photometry is shown in the lower panel of Fig. 2, and appears significantly different. In fact it is quite similar to the CMD of old open clusters like M 67 (Montgomery et al. 1993) and Berkeley 39 (Kassis et al. 1997). The MS extends down to K = 16.5, and the TO is located at K = 12.5, (J-K) = 0.70. The region above the TO might contain some interlopers (see Fig. 3).

Apparently there is no Hertzsprung gap, and the RGB is sparsely populated, like in all clusters with age greater that 4 Gyr. The stars at $K \approx 10.3$, $(J-K) \approx 1.1$might represent the clump of core He burners, quite small in this age range. We recall that IR photometry covers all the cluster, and the foreground contamination is negligible, as it can be seen in Fig. 3, where two comparison fields $15^{\prime }$ eastwards and $5^{\prime }$ northwards, respectively, are shown.


  \begin{figure}{\psfig{file=ds9007f4.eps,width=8.8cm} }
\end{figure} Figure 4: Two colors diagram for MS stars in King 5. The solid line is a Zero Age MS for [Fe/H] = -0.38. In the lower left corner the reddening vector is shown. See text for any detail


  \begin{figure}{\psfig{file=ds9007f5.eps,width=8.8cm} }
\end{figure} Figure 5: Two colors diagram for MS stars in King 5. The solid line is a Zero Age MS for [Fe/H] = -0.38. In the lower left corner the reddening vector is shown. See text for any detail

In order to clarify the nature and age range of this cluster, and whether the differences are simply due to the use of different colors, we have combined optical and IR data to construct the CMD in V - (V-K), which is shown in the lower right panel Fig. 2. This CMD looks quite similar to the optical CMD shown in the same figure, demonstrating that King 5 is an intermediate age open cluster. To better assess this point, in the next sections we are going to determine in details the cluster basic parameters.


  \begin{figure}\par {\psfig{file=ds9007f6.eps,width=8.8cm} }
\end{figure} Figure 6: Age determination for King 5. Overimposed on the CMDs is a Z=0.008 isochrone for an age of 1 Gyr. Lower panel shows the fit in the plane K -(J-K), upper left panel in the plane V -(V-I), and upper right panel in the plane V -(V-K). See the text for any detail


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