We use deep multi-epoch near-IR images of the VISTA Variables in the Vía Láctea ( VVV ) Survey to measure proper motions ( PMs ) of stars in the Milky Way globular cluster FSR1716 = VVV-GC05 .
The color-magnitude diagram of this object , made using PM selected members , shows an extended horizontal branch , nine confirmed RR Lyrae members in the instability strip , and possibly several hotter stars extending to the blue .
Based on the fundamental-mode ( ab-type ) RR Lyrae stars that move coherently with the cluster , we confirmed that FSR1716 is an Oosterhoff I globular cluster with a mean period \langle P _ { ab } \rangle =0.574 days .
Intriguingly , we detect tidal extensions to both sides of this cluster in the spatial distribution of PM selected member stars .
Also , one of the confirmed RRabs is located \sim 11 arcmin in projection from the cluster center , suggesting that FSR1716 may be losing stars due to the gravitational interaction with the Galaxy .
We also measure radial velocities ( RVs ) for five cluster red giants selected using the PMs .
The combination of RVs and PMs allow us to compute for the first time the orbit of this globular cluster , using an updated Galactic potential .
The orbit results to be confined within { |Z _ { max } | < 2.0 } kpc , and has eccentricity 0.4 < e < 0.6 , with perigalactic distance { 1.5 < R _ { peri } ( kpc ) < 2.3 } , and apogalactic distance { 5.3 < R _ { apo } ( kpc ) < 6.4 } .
We conclude that , in agreement with its relatively low metallicity ( [ Fe/H ] = -1.4 dex ) , this is an inner halo globular cluster plunging into the disk of the Galaxy .
As such , this is a unique object to test the dynamical processes that contribute to the disruption of Galactic globular clusters .