2013ApJ...779...81M

We have measured absolute proper motions for the three populations intercepted in the direction of the Galactic globular cluster NGC 6681: the cluster itself, the Sagittarius dwarf spheroidal galaxy, and the field. For this, we used Hubble Space Telescope ACS/WFC and WFC3/UVIS optical imaging data separated by a temporal baseline of 5.464 yr. Five background galaxies were used to determine the zero point of the absolute-motion reference frame. The resulting absolute proper motion of NGC 6681 is (µ_α cos δ, µ_δ) = (1.58±0.18, -4.57±0.16) mas/yr. This is the first estimate ever made for this cluster. For the Sgr dSph we obtain (µ_α cos δ, µ_δ) = -2.54±0.18, -1.19±0.16) mas/yr, consistent with previous measurements and with the values predicted by theoretical models. The absolute proper motion of the Galaxy population in our field of view is (µ_αcos δ, µ_δ) = (- 1.21±0.27, -4.39±0.26) mas/yr. In this study we also use background Sagittarius Dwarf Spheroidal stars to determine the rotation of the globular cluster in the plane of the sky and find that NGC 6681 is not rotating significantly: v_rot= 0.82±1.02 km/s at a distance of 1' from the cluster center.