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Astron. Astrophys. Suppl. Ser. 136, 237-243

Colour-magnitude diagrams of the post-core collapse globular clusters NGC 6256 and NGC 6717 (Palomar 9)[*]

S. Ortolani1 - B. Barbuy2 - E. Bica3

1 - Università di Padova, Dept. di Astronomia, Vicolo dell'Osservatorio 5, I-35122 Padova, Italy
2 - Universidade de São Paulo, CP 3386, São Paulo 01060-970, Brazil
3 - Universidade Federal do Rio Grande do Sul, Dept. de Astronomia, CP 15051, Porto Alegre 91500-970, Brazil

Received June 10; accepted October 8, 1998


We present photometry of the globular clusters NGC 6256 in V, I, NGC 6717, NGC 6256 in V, I, and NGC 6717 in B, V, obtaining cluster parameters. NGC 6256 has a post core-collapse structure, while such feature is suspected for NGC 6717. Both clusters are projected on the bulge.

From the colour-magnitude diagrams for NGC 6256 we derive reliable cluster parameters which considerably revise its properties, with respect to previous literature. It has a blue-extended Horizontal Branch already observed in other post-core collapse clusters in the bulge, such as HP1. We derive a reddening of E(B-V) = 1.10, a distance $d_{\odot} \approx 6.4$ kpc, and an intermediate metallicity for NGC 6256.

NGC 6717 presents a blue Horizontal Branch. We derive a reddening of E(B-V) = 0.23 and a distance $d_{\odot} \approx 7.1$ kpc, locating the cluster in the bulge. The photometry is deep and we study the luminosity function, which presents some evidence of depletion of Main Sequence stars. This effect is not unexpected for such a poorly populated Palomar cluster in the bulge.

We also estimate the cluster age from the magnitude difference between the horizontal branch and the turnoff. We obtain $\Delta V^{\rm TO}_{\rm HB} = 3.7$, which would place this intermediate metallicity bulge globular as coeval with the halo.

Key words: Galaxy: globular clusters: NGC 6256; NGC 6717 (Palomar 9) -- HR diagram

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