GeMS MCAO observations of the Galactic globular cluster NGC 2808: the
  absolute age
  Globular clusters are the oldest stellar systems in the Milky Way and probe
the early epoch of the Galaxy formation. However, the uncertainties on their
absolute age are still too large to soundly constrain how the Galactic
structures have assembled. The aim of this work is to obtain an accurate
estimate of the absolute age of the globular cluster NGC 2808 using deep IR
data obtained with the multi conjugate adaptive optics system operating at the
Gemini South telescope (GeMS). This exquisite photometry, combined with that
obtained in V and I bands with HST, allowed us the detection of the faint Main
Sequence Knee feature in NGC 2808 colour magnitude diagram. The difference
between this point and the main sequence turn off is a good age estimator and
provides ages with unprecedented accuracy. We found that NGC 2808 has an age of
t=10.9\pm0.7 (intrinsic) \pm0.45 (metallicity term) Gyr. A possible
contamination by He-enhanced population could make the cluster up to 0.25 Gyr
older. Although this age estimate agrees with the age coming from the classical
turn off method (t=11.0 Gyr), its uncertainty is a factor ~3 better, since it
avoids systematics in reddening, distance assumptions and photometric zero
points determination. The final absolute age indicates that NGC 2808 is
slightly younger than other Galactic globular clusters with similar
metallicity.
