On the absolute age of the metal-rich globular M71 (NGC 6838). I. Optical photometry
- INAF—Osservatorio Astronomico di Teramo, via M. Maggini, I-64100, Teramo (Italy)
- Universitá di Roma Tor Vergata, via della Ricerca Scientifica, I-00133, Rome (Italy)
- Universitá di Pisa, Largo Pontecorvo 3, I-56127, Pisa (Italy)
- CNRS—Centre de Recherche Astrophysique de Lyon, UMR 5574, Universit de Lyon, cole Normale Suprieure de Lyon, 46 Alle d’Italie, F-69364 Lyon Cedex 07 (France)
- Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council, 5071 West Saanich Road, Victoria, BC V9E 2E7 (Canada)
- INAF—Osservatorio Astronomico di Roma, Via di Frascati 33, I-00040 Monte Porzio Catone, Rome (Italy)
- Instituto de Astrofísica de Canarias, Calle Via Lactea, E-38200 La Laguna, Tenerife (Spain)
- INAF—Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, I-40131, Trieste (Italy)
We investigated the absolute age of the Galactic globular cluster M71 (NGC 6838) using optical ground-based images (u{sup ′},g{sup ′},r{sup ′},i{sup ′},z{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 ′},u{sup ′}−g{sup ′},g{sup ′}−r{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 ′}−g{sup ′}, and in the r{sup ′},g{sup ′}−r{sup ′} CMD, thus supporting previous theoretical predictions by Borysow et al. The key advantage in using the Δ{sub TO}{sup Knee} 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 Δ{sub TO}{sup Knee} 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.
- OSTI ID:
- 22879437
- Journal Information:
- The Astronomical Journal (Online), Vol. 150, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 1538-3881
- Country of Publication:
- United States
- Language:
- English
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