Globular clusters with Gaia
  The treatment of crowded fields in Gaia data will only be a reality in a few
years from now. In particular, for globular clusters, only the end-of-mission
data (public in 2022-2023) will have the necessary full crowding treatment and
will reach sufficient quality for the faintest stars. As a consequence, the
work on the deblending and decontamination pipelines is still ongoing. We
describe the present status of the pipelines for different Gaia instruments,
and we model the end-of-mission crowding errors on the basis of available
information. We then apply the nominal post-launch Gaia performances,
appropriately worsened by the estimated crowding errors, to a set of 18
simulated globular clusters with different concentration, distance, and field
contamination. We conclude that there will be 103-104 stars with astrometric
performances virtually untouched by crowding (contaminated by <1 mmag) in the
majority of clusters. The most limiting factor will be field crowding, not
cluster crowding: the most contaminated clusters will only contain 10-100 clean
stars. We also conclude that: (i) the systemic proper motions and parallaxes
will be determined to 1% or better up to 15 kpc, and the nearby clusters will
have radial velocities to a few km/s ; (ii) internal kinematics will be of
unprecendented quality, cluster masses will be determined to 10% up to 15 kpc
and beyond, and it will be possible to identify differences of a few km/s or
less in the kinematics (if any) of cluster sub-populations up to 10 kpc and
beyond; (iii) the brightest stars (V<17 mag) will have space-quality,
wide-field photometry (mmag errors), and all Gaia photometry will have 1-3%
errors on the absolute photometric calibration.
