The bulge galactic globulars Terzan 5, NGC 6528, and NGC 6553

Grebel, E. K. ; Brander, W. ; Richtler, T. ; Subramaniam, A. ; Sagar, R. (1995) The bulge galactic globulars Terzan 5, NGC 6528, and NGC 6553 Bulletin of the American Astronomical Society, 27 . p. 1404. ISSN 0002-7537

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Deep V,I CCD photometry for a large number of stars has been carried out in the region of three bulge globular clusters, namely Terzan 5, NGC 6528 and NGC 6553. These data in combination with recent theoretical stellar evolutionary models have been used to determine reddening, distance, age and metalicity of the clusters. As perhaps characteristic for metal-rich clusters, the red giant branches ers are apparent already from the shape of their red giant branches, which turn over after reaching a peak and extend to V-I colours as red as 7 mag. Depending on which set of isochrones is used, we find metallicities to range from slightly below solar to above solar. The location of the turnoff region for all three clusters indicates that the clusters are ``old'', i.e., have ages of about 12 Gyr or more. While the horizontal branch of NGC 6528 appears clumpy when applying strict selection criteria with respect to radius, the other two clusters show diagonal ``horizontal branches''. We demonstrate that the shape of the horizontal branches is in part caused by reddening effects, but the branches remain diagonal even after correcting for differential reddening. We discuss this effect further in our poster. The cluster strongest affected by extinction is Terzan 5, for which we find a visual extinction of almost 6 magnitudes. We corrected for the differential reddening of this cluster using a cross-correlation method and present a reddening map. Terzan 5 appears to be located 4--5 kpc behind the Galactic center. Published radial velocities in combination with our derived distances for the other two clusters show that they must have highly elliptical orbits.

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