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Astron. Astrophys. 331, 70-80 (1998)

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Infrared array photometry of bulge globular clusters

I. Combined ground based JK and HST VI photometry of NGC 6553 *

M.D. Guarnieri 1, 2, S. Ortolani 3, P. Montegriffo 4, A. Renzini 1, 4, B. Barbuy 5, E. Bica 6 and A. Moneti 7

1 European Southern Observatory, Karl-Schwarzschild 2, D-85748 Garching b. München, Germany
2 Osservatorio Astronomico di Pino Torinese, I-10025 Torino, Italy
3 University of Padova, Vicolo dell'Osservatorio 5, I-35123 Padova, Italy
4 Dipartimento di Astronomia di Bologna, Bologna, Italy
5 IAG-USP, CP 9638, 01065-970 São Paulo, Brazil
6 UFRGS, CP 15051, 91501-970 Porto Alegre, Brazil
7 SERCO/ESA-Astrophysics Division, Noordwijk, The Netherlands

Received 26 July 1996 / Accepted 6 January 1997


JK infrared photometry for the bulge globular cluster NGC 6553, combined with high resolution visual VI observations are presented and discussed in connection with the bulge metal rich population. The infrared data were taken with IRAC2 at ESO 2.2m telescope while the optical counterparts are from Hubble Space Telescope (HST).

We find a mean magnitude for the horizontal branch of [FORMULA], and [FORMULA]. Assuming a reddening of [FORMULA], the mean distance modulus is [FORMULA] giving an heliocentric distance of 5.2 Kpc. This distance is slightly higher than that previously published by Ortolani et al. (1990, hereafter OBB90).

The brightest giants have been detected at [FORMULA] and [FORMULA] transforming to [FORMULA] and [FORMULA], which are exceptionally high values for a globular cluster.

The metallicity has been found to be consistent with previous values and an average of [FORMULA] has been adopted. The helium abundance estimated via the [FORMULA] method yields [FORMULA].

Key words: globular clusters: NGC 6553 – Galaxy: center – blue stragglers – HR diagram

* Based on observations obtained at ESO-La Silla, Chile and with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, operated by AURA Inc. under contract to NASA. Appendix only available in electronic form at the CDS (Strasbourg) via anonymous ftp to cdsarc.u-strasbg.fr ( or via http://cdsweb.u-strasbg.fr/Abstract.html

Send offprint requests to: Maria Donata Guarnieri

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© European Southern Observatory (ESO) 1998

Online publication: February 4, 1998