The Gaia-ESO Survey: open clusters in Gaia-DR1 - a way forward to
  stellar age calibration
  We describe the methodologies that, taking advantage of Gaia-DR1 and the
Gaia-ESO Survey data, enable the comparison of observed open star cluster
sequences with stellar evolutionary models. The final, long-term goal is the
exploitation of open clusters as age calibrators. We perform a homogeneous
analysis of eight open clusters using the Gaia-DR1 TGAS catalogue for bright
members, and information from the Gaia-ESO Survey for fainter stars. Cluster
membership probabilities for the Gaia-ESO Survey targets are derived based on
several spectroscopic tracers. The Gaia-ESO Survey also provides the cluster
chemical composition. We obtain cluster parallaxes using two methods. The first
one relies on the astrometric selection of a sample of bona fide members, while
the other one fits the parallax distribution of a larger sample of TGAS
sources. Ages and reddening values are recovered through a Bayesian analysis
using the 2MASS magnitudes and three sets of standard models. Lithium depletion
boundary (LDB) ages are also determined using literature observations and the
same models employed for the Bayesian analysis. For all but one cluster,
parallaxes derived by us agree with those presented in Gaia Collaboration et
al. (2017), while a discrepancy is found for NGC 2516; we provide evidence
supporting our own determination. Inferred cluster ages are robust against
models and are generally consistent with literature values. The systematic
parallax errors inherent in the Gaia DR1 data presently limit the precision of
our results. Nevertheless, we have been able to place these eight clusters onto
the same age scale for the first time, with good agreement between isochronal
and LDB ages where there is overlap. Our approach appears promising and
demonstrates the potential of combining Gaia and ground-based spectroscopic
datasets.
