Proper Motions in Terzan 5: Membership of the Multi-iron Subpopulations and First Constraint on the OrbitMassari, D.; Dalessandro, E.; Ferraro, F. R.; Miocchi, P.; Bellini, A.; Origlia, L.; Lanzoni, B.; Rich, R. M.; Mucciarelli, A.

By exploiting two sets of high-resolution images obtained with the Hubble Space Telescope Advanced Camera for Surveys/Wide Field Channel over a baseline of ∼10 years, we have measured relative proper motions (PMs) of ∼70,000 stars in the stellar system Terzan 5. The results confirm the membership of the three subpopulations with different iron abundances discovered in the system. The orbit of the system has been derived from a first estimate of its absolute PM, obtained by using bulge stars as a 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.