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http://hdl.handle.net/20.500.12386/24104
Title: | On the Absolute Age of the Metal-rich Globular M71 (NGC 6838). I. Optical Photometry | Authors: | Di Cecco, A. Bono, G. Prada Moroni, P. G. Tognelli, E. Allard, F. Stetson, P. B. Buonanno, R. Ferraro, I. Iannicola, G. Monelli, M. NONINO, Mario PULONE, Luigi |
Issue Date: | 2015 | Journal: | THE ASTRONOMICAL JOURNAL | Number: | 150 | Issue: | 2 | First Page: | 51 | Abstract: | We investigated the absolute age of the Galactic globular cluster M71 (NGC 6838) using optical ground-based images (u<SUP>\prime</SUP> ,g<SUP>\prime</SUP> ,r<SUP>\prime</SUP> ,i<SUP>\prime</SUP> ,z<SUP>\prime</SUP> ) collected with the MegaCam camera at the Canada-France-Hawaii Telescope (CFHT). We performed a robust selection of field and cluster stars by applying a new method based on the 3D (r<SUP>\prime</SUP> ,u<SUP>\prime</SUP> -g<SUP>\prime</SUP> ,g<SUP>\prime</SUP> -r<SUP>\prime</SUP> ) color-color-magnitude diagram. A comparison between the color-magnitude diagram (CMD) of the candidate cluster stars and a new set of isochrones at the locus of the main sequence turn-off (MSTO) suggests an absolute age of 12 ± 2 Gyr. The absolute age was also estimated using the difference in magnitude between the MSTO and the so-called main sequence knee, a well-defined bending occurring in the lower main sequence. This feature was originally detected in the near-infrared bands and explained as a consequence of an opacity mechanism (collisionally induced absorption of molecular hydrogen) in the atmosphere of cool low-mass stars. The same feature was also detected in the r′, u<SUP>\prime</SUP> -g<SUP>\prime</SUP> , and in the r<SUP>\prime</SUP> ,g<SUP>\prime</SUP> -r<SUP>\prime</SUP> CMD, thus supporting previous theoretical predictions by Borysow et al. The key advantage in using the {{{∆ }}}<SUB>{TO</SUB>}<SUP>{Knee</SUP>} as an age diagnostic is that it is independent of uncertainties affecting the distance, the reddening, and the photometric zero point. We found an absolute age of 12 ± 1 Gyr that agrees, within the errors, with similar age estimates, but the uncertainty is on average a factor of two smaller. We also found that the {{{∆ }}}<SUB>{TO</SUB>}<SUP>{Knee</SUP>} is more sensitive to the metallicity than the MSTO, but the dependence vanishes when using the difference in color between the MSK and the MSTO. | Acknowledgments: | This investigation was partially supported by PRIN-MIUR (2010LY5N2T) “Chemical and dynamical evolution of the Milky Way and Local Group galaxies” (P.I.: F. Matteucci). It is a pleasure to acknowledge the anonymous referee for very pertinent suggestions on the early version of our paper. A.D.C. thanks the ASI Science Data Center, and in particular Dr. Lucio Angelo Antonelli, for supporting this investigation; G.B. thanks the Carnegie Observatory visitor program for support as a science visitor. This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. | URI: | http://hdl.handle.net/20.500.12386/24104 | URL: | https://iopscience.iop.org/article/10.1088/0004-6256/150/2/51 | ISSN: | 0004-6256 | DOI: | 10.1088/0004-6256/150/2/51 | Bibcode ADS: | 2015AJ....150...51D | Fulltext: | open |
Appears in Collections: | 1.01 Articoli in rivista |
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Cecco_2015_AJ_150_51.pdf | PDF editoriale | 3.18 MB | Adobe PDF | View/Open |
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