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arXiv · 1402.0446

Stroemgren - near-infrared photometry of the Baade's Window. I. The bulge globular cluster NGC6528 and the surrounding field

Annalisa Calamida·Giuseppe Bono·Edoardo P. Lagioia·Antonino P. Milone·Michele Fabrizio·Ivo Saviane·Christian Moni Bidin·Francesco Mauro·Roberto Buonanno·Ivan Ferraro·Giacinto Iannicola·Manuela Zoccali
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Abstract

We present Stroemgren-NIR photometry of NGC6528 and its surroundings in the Baade's Window. uvby images were collected with EFOSC2@NTT, while NIR catalogs are based on VIRCAM@VISTA and SOFI@NTT data. The matching with HST photometry allowed us to obtain proper-motion-cleaned samples of cluster and bulge stars. The huge color sensitivity of Stroemgren-NIR CMDs helped us in disentangling age and metallicity effects. The RGB of NGC6528 is reproduced by scaled-solar isochrones with solar abundance or alpha-enhanced isochrones with the same iron content, and an age of t = 11+/-1 Gyr. These findings support literature age estimates for NGC6528. We also performed a theoretical metallicity calibration based on the Stroemgren index m1 and on visual-NIR colors for RGs, by adopting scaled-solar and alpha-enhanced models. We applied the calibration to estimate the metallicity of NGC6528, finding [Fe/H] = -0.04+/-0.02, with an intrinsic dispersion of 0.27 dex (by averaging abundances based on the scaled-solar [m], y - J and [m], y - K Metallicity-Index-Color relations), and of -0.11+/-0.01 (sig = 0.27 dex), by using the m1, y - J and m1, y - K relations. These findings support the results of Zoccali et al. (2004) which give [Fe/H] = -0.10+/-0.2, and a low alpha-enhancement, [alpha/Fe] = 0.1, and of Carretta et al. (2001), that find [Fe/H] = 0.07+/-0.01, with [alpha/Fe] = 0.2. By applying the scaled-solar MIC relations to Baade's window RGs, we find a metallicity distribution extending from [Fe/H] ~ -1.0 to ~ 1 dex, with peaks at [Fe/H] ~ -0.2 and +0.55 ([m], y - J and [m], y - K relations), and [Fe/H] ~ -0.25 and +0.4 (m1, y - J and m1, y - K relations). These findings are in good agreement with the spectroscopic studies of Hill et al. (2011) for the Baade's window, of Uttenthaler et al. (2012) for a region centered at (l,b) = (0, -10), and with the results of the ARGOS survey (Ness et al. 2013a).

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Annalisa Calamida, Giuseppe Bono, Edoardo P. Lagioia, Antonino P. Milone, Michele Fabrizio, Ivo Saviane, Christian Moni Bidin, Francesco Mauro, Roberto Buonanno, Ivan Ferraro, Giacinto Iannicola, Manuela Zoccali. 2014-02-03. Stroemgren - near-infrared photometry of the Baade's Window. I. The bulge globular cluster NGC6528 and the surrounding field. https://doi.org/10.1051/0004-6361/201323081

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