A New Proper Motion Determination of Leo I
  We measure the absolute proper motion of Leo I using a WFPC2/HST data set
that spans up to 10 years, to date the longest time baseline utilized for this
satellite. The measurement relies on ~ 2300 Leo I stars located near the center
of light of the galaxy; the correction to absolute proper motion is based on
174 Gaia EDR3 stars and 10 galaxies. Having generated highly-precise, relative
proper motions for all Gaia EDR3 stars in our WFPC2 field of study, our
correction to the absolute EDR3 system does not rely on these Gaia stars being
Leo I members. This new determination also benefits from a recently improved
astrometric calibration of WFPC2. The resulting proper-motion value, (mu_alpha,
mu_delta) = (-0.007 +- 0.035, -0.119 +-0.026) mas/yr is in agreement with
recent, large-area, Gaia EDR3-based determinations. We discuss all the recent
measurements of Leo I's proper motion and adopt a combined, multi-study average
of (mu_alpha_3meas, mu_delta_3meas) = (-0.036 +- 0.016, -0.130 +- 0.010)
mas/yr. This value of absolute proper motion for Leo I indicates its orbital
pole is well aligned with that of the Vast Polar Structure, defined by the
majority of the brightest dwarf-spheroidal satellites of the Milky Way.
