• DOI:10.1051/0004-6361/201323081
• Corpus ID: 118165130

Strömgren and near-infrared photometry of metal-rich bulge globular clusters - I. NGC 6528 and its surrounding field

@article{Calamida2014StrmgrenAN,
  title={Str{\"o}mgren and near-infrared photometry of metal-rich bulge globular clusters - I. NGC 6528 and its surrounding field},
  author={Annalisa Calamida and Giuseppe Bono and Edoardo P. Lagioia and Antonino P. Milone and Michele Fabrizio and Ivo Saviane and Christian Moni Bidin and Francesco Mauro and Roberto Buonanno and I. Ferraro and G. Iannicola and Manuela Zoccali},
  journal={Astronomy and Astrophysics},
  year={2014},
  volume={565},
  pages={8-25}
}

• A. Calamida, G. Bono, +9 authors M. Zoccali
• Published 3 February 2014
• Physics
• Astronomy and Astrophysics

We present Stromgren and near-infrared (NIR) photometry of the bulge cluster NGC 6528 and its surrounding field in the Ba ade's Window. uvby images have been collected with EFOSC2 on the New Technology Telescope (NTT, La Silla, ESO). The NIR catalogs are based on J, K-band VIRCAM@VISTA (Paranal, ESO) and SOFI@NTT photometry . The matching of the aforementioned data sets with Hubble Space Telescope photometry allowed us to obtain proper-motion-cleaned samples of NGC 6528 and bulge stars. Furthermore, we were able to correct the Stromgren-NIR photometry for differential reddening. The huge color sensitivity of the Stromgren-NIR Color-Magnitude-Diagrams (CMDs) helped us in disentangling age and metallicity effects. The red-giant branch (RGB) of NGC 6528 is well reproduced in all the CMDs by adopting scaled-solar isochrones with solar abundance, i.e. Z = 0.0198, orα-enhanced isochrones with the same iron content, i.e. Z = 0.04, and an age range of t = 10 - 12 Gyr. The same isochrones well reproduce most of the color spread of the Baade's window RGB. These findings support age estimates present in literat ure for NGC 6528. We also performed a new theoretical visual-NIR metallicity calibration based on the Stromgren index m1 and on visual-NIR colors for red-giant (RG) stars. Scaled-solar andα-enhanced models have been adopted and we validated the new Metallicity-Index-Color (MIC) relations by applying them to estimate the photometric metal abundance of a sample of field RGs and of a metal-poor (M 92, (Fe/H)∼ −2.3) and a metal-rich (NGC 6624, (Fe/H)∼ −0.7) globular cluster. We applied the calibration to estimate the mean metal abundance of NGC 6528, finding (Fe/H) = (M/H) =−0.04±0.02, with a mean intrinsic dispersion ofσ = 0.27 dex, by averaging the metallicities obtained with the scaled-solar ( m), y− J and (m), y− K MIC relations, and of−0.11±0.01, withσ = 0.27 dex, by using the m1, y− J and m1, y− K relations. These findings would support the results of Zocca li et al. (2004) based on high-resolution spectroscopy, which give (Fe/H) =−0.10±0.2 for NGC 6528, and a lowα-enhancement of (α/Fe) = 0.1, and of Carretta et al. (2001), that find (Fe /H) = 0.07±0.01, with a modestα-enhancement, (α/Fe) = 0.2. By applying the scaled-solar MIC relations to the sample of Baade's window RGs, we find a metallicity distribution whi ch extend approximately from (Fe/H)∼ −1.0 up to (Fe/H)∼ 1 dex, with two metallicity peaks at (Fe/H)≈−0.2 and (Fe/H)≈ 0.55 ((m), y− J and (m), y− K relations), and (Fe/H)≈−0.25 and (Fe/H)≈ 0.4 (m1, y− J and m1, y− K relations). These findings are in fairly good agreement with the spectroscopic studies of Zoccali et al. (2008), and Hill et al. (2011) for the Baade's window, of Utte nthaler et al. (2012) for a region centered at ( l, b) = (0 ◦ ,−10 ◦ ), and with the recent results of the ARGOS surveys for the bulge (Freeman et al. 2013, Ness et al. 2013a).