Proper motions in Terzan 5: membership of the multi-iron sub-populations
  and first constrain to the orbit
  By exploiting two sets of high-resolution images obtained with HST ACS/WFC
over a baseline of ~10 years we have measured relative proper motions of
~70,000 stars in the stellar system Terzan 5. The results confirm the
membership of the three sub-populations with different iron abudances
discovered in the system. The orbit of the system has been derived from a first
estimate of its absolute proper motion, obtained by using bulge stars as
reference. The results of the integration of this orbit within an axisymmetric
Galactic model exclude any external accretion origin for this cluster. Terzan 5
is known to have chemistry similar to the Galactic bulge; our findings support
a kinematic link between the cluster and the bulge, further strengthening the
possibility that Terzan 5 is the fossil remnant of one of the pristine clumps
that originated the bulge.
