4 The globular cluster NGC 6717 (Palomar 9)

4.1 Colour-magnitude diagrams

Brocato et al. (1996) studied the brighter CMD sequences of this cluster in detail, while the aim of the present study is to explore the HB and lower sequences.

  

Figure 5: NGC 6717: a) whole frame; b) extraction of 19'' < r < 94'', with mean locus of NGC 6752 overplotted

Figures 5a, b show V vs. (B-V) CMD spatial extractions for the cluster, corresponding to the whole frame $(4.0'\times 2.5')$,and to a ring with 19'' < r < 94'' (in Fig. 5b the cluster crowded core is avoided), respectively. In Fig. 5a the field contamination is rather important while in Fig. 5b the cluster sequences are well defined. The mean locus of NGC 6752 from Desidera et al. (1997) is superimposed on the diagram in Fig. 5b.

Again, like in the case of NGC 6256, we use NGC 6752 as reference. In Fig. 5b is superimposed the mean locus of the post core-collapse globular cluster NGC 6752 from Buonanno et al. (1986). A good agreement is obtained for the cluster sequences. Bica et al. (1998) spectroscopy gives $\rm [Fe/H] = -1.05$. Likewise for NGC 6256, we adopt a compromise value for the cluster metallicity of $\rm [Fe/H] \approx -1.3$ for NGC 6717.

4.2 Reddening and distance

The brighter parts of the blue HB of NGC 6717 are located at $V = 15.55 \pm 0.15$.

By matching the fiducial sequence of NGC 6752 (Buonanno et al. 1986; Desidera et al. 1997) to the cluster CMD (Fig. 5b) we obtain $\Delta (B-V) = 0.20$. With E(B-V) = 0.03 for NGC 6752 there results E(B-V) = 0.23, which corresponds to A_V = 0.71. Bica et al. (1998) using the continuum distribution of a near-IR integrated spectrum gives E(B-V) = 0.21. The results are comparable within uncertainties.

Adopting an absolute magnitude of the HB for NGC 6752 of M_V = 0.7 (Buonanno et al. 1989), we derive an absolute distance modulus $(m-M)_0 = 14.14\pm 0.2$. This corresponds to a distance of $d_{\odot} = 6.7\pm 0.6$ kpc for NGC 6717. Assuming an absolute distance modulus of NGC 6752 as determined by Renzini et al. (1996) we obtain a distance of $d_{\odot}$ = 7.5 kpc for the present cluster. We adopt a compromise value of $d_{\odot} = 7.1 \pm 0.5$ kpc for NGC 6717. The Galactocentric coordinates are X = -1.2, Y = 1.5 kpc and Z = -1.3 kpc. Therefore NGC 6717 is located in the bulge.

4.3 Age

The magnitude difference between the horizontal branch and the turnoff has been pointed out as a good age indicator (Iben 1974; Sandage 1986 and references therein).

For NGC 6717 we obtain from Fig. 5b $\Delta V^{\rm TO}_{\rm HB}\approx 3.7 \pm 0.2$, comparable to the mean value for the halo of $\Delta V^{\rm TO}_{\rm HB}\approx 3.55$ by Buonanno et al. (1989). This indicates that NGC 6717 is coeval with the halo, within uncertainties.

Adopting the present reddening, distance modulus, and apparent total V magnitude $V_{\rm t} = 9.28$ (Harris 1996), we derive an absolute integrated magnitude for this cluster M_V = -5.57. NGC 6717 is not an extremely low mass globular cluster like E3 (McClure et al. 1985; Gratton & Ortolani 1987), but it is comparable to halo Palomar clusters. This is a rare case of such type of clusters in the bulge.

4.4 Variable stars

Goranskii (1979) discovered an RR Lyrae star in this cluster, and constructed a light curve. The RR Lyrae is outside our frame. Placing his data for this star of V = 15.7 and (B-V) = 0.5 on our CMD, we see that it is at the correct magnitude level of the HB, and at the RR Lyrae gap, and can be considered a member. Goranskii determined a period of 0.5752 days, so that the cluster can be tentatively classified as Oosterhoff type I.