The eye of Gaia on globular clusters structure: tidal tails
  I analyse the projected density distribution of member stars over a wide area
surrounding 18 Galactic globular clusters using the photometric and astrometric
information provided by the second data release of the Gaia mission. A 5D
mixture modelling technique has been employed to optimally isolate the signal
of the cluster stellar population from the contamination of the Galactic field,
taking advantage of its different distribution in the space formed by colours,
magnitudes, parallaxes and proper motions. In 7 clusters I detect collimated
overdensities at a >3 sigma level above the background density extending well
beyond the cluster tidal radius, consistent with the distortion expected as a
result of the tidal interaction with the Milky Way potential. In five of these
clusters (NGC288, NGC2298, NGC5139, NGC6341 and NGC7099) spectacular tidal
tails extend up to the border of the analysed field of view at 5 degrees from
the centre. At large distances from the cluster centre, the orientation of the
detected overdensities appears to be systematically aligned with the cluster
orbital path, in agreement with the predictions of N-body simulations. The
fraction of stars contained in the tidal tails of these clusters is also used
to determine the first observational estimate of their present-day destruction
rates.
