Infrared array photometry of bulge globular clusters I. Combined ground based JK and HST VI photometry of NGC 6553

Abstract

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 nd a mean magnitude for the horizontal branch of VHB = 16.92, and KHB = 12.42 ± 0.01. Assuming a reddening of E(B − V) = 0.7, the mean distance modulus is (m − M)0 = 13.6 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 K ~ 6.5 and (V −K) = 13.10 transforming to MK = −7.34 and (V−K)0 = 11.20, which are exceptionally high values for a globular cluster. The metallicity has been found to be consistent with previous values and an average of [Fe/H] = −0.22±0.05 has been adopted. The helium abundance estimated via the R' method yields Yp ~ 0.28 ± 0.03.