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arXiv · 1506.01180

On the absolute age of the metal-rich globular M71 (NGC 6838): I. optical photometry

Alessandra Di Cecco·Giuseppe Bono·Pier Giorgio Prada Moroni·Emanuele Tognelli·France Allard·Peter B. Stetson·Roberto Buonanno·Ivan Ferraro·Giacinto Iannicola·Matteo Monelli·Mario Nonino·Luigi Pulone
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Abstract

We investigated the absolute age of the Galactic globular cluster M71 (NGC 6838) by using optical ground-based images (u',g',r',i',z') 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',u'-g',g'-r') Color-Color-Magnitude-Diagram. The comparison between the Color-Magnitude-Diagram 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 (NIR) bands and explained as a consequence of an opacity mechanism (collisionally induced absorption of molecular hydrogen) in the atmosphere of cool low-mass stars (Bono et al. 2010). The same feature was also detected in the r',u'-g' and in the r',g'-r' CMD, thus supporting previous theoretical predictions by Borysow et al. (1997). The key advantage in using the Delta^(Knee)_(TO) 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 Delta^(Knee)_(TO) is more sensitive to the metallicity than the MSTO, but the dependence becomes vanishing using the difference in color between the MSK and the MSTO.

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Alessandra Di Cecco, Giuseppe Bono, Pier Giorgio Prada Moroni, Emanuele Tognelli, France Allard, Peter B. Stetson, Roberto Buonanno, Ivan Ferraro, Giacinto Iannicola, Matteo Monelli, Mario Nonino, Luigi Pulone. 2015-06-03. On the absolute age of the metal-rich globular M71 (NGC 6838): I. optical photometry. https://doi.org/10.1088/0004-6256/150/2/51

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