The Proper Motion of Draco II with HST Using Multiple Reference Frames and Methodologies

We present proper motion (PM) measurements for Draco II, an ultrafaint dwarf satellite of the Milky Way. These PMs are measured using two epochs of Hubble Space Telescope (HST)/Advanced Camera for Surveys imaging separated by a 7 yr temporal baseline. Measuring the PMs of low-luminosity systems is difficult due to the low number of member stars, requiring a precise inertial reference frame. We construct reference frames using three different sets of external sources: (1) stars with Gaia Data Release 3 data, (2) stationary background galaxies, and (3) a combination of the two. We show that all three reference frames give consistent PM results. We find that for this sparse, low-luminosity regime including background galaxies into the reference frame improves our measurement by up to ∼2× versus using only Gaia astrometric data. Using 301 background galaxies as a reference frame, we find that Draco II’s systemic PM is ( μ α * , μ δ ) = ( 1.043 ± 0.029 , 0.879 ± 0.028 ) mas yr −1 , which is the most precise measurement of the three we present in this paper.