HST absolute Proper Motions of NGC 6681 (M70) and the Sagittarius Dwarf Spheroidal Galaxy
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 years. 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 ($μ_α\cosδ, μ_δ$)=($1.58\pm0.18, -4.57\pm0.16$) \masyr. This is the first estimate ever made for this cluster. For the Sgr dSph we obtain ($μ_α\cosδ, μ_δ)=(-2.54\pm0.18, -1.19\pm0.16$) \masyr, 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 ($μ_α\cosδ, μ_δ)=(-1.21\pm0.27, -4.39\pm0.26$) \masyr. 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:\ $v_{\rm rot}=0.82\pm1.02$ km$\,$s$^{-1}$ at a distance of 1 arcmin from the cluster center.