Constraining the Solar Galactic Reflex Velocity Using Gaia Observations
  of the Sagittarius Stream
  Because of its particular orientation around the Galaxy - i.e., in a plane
nearly perpendicular to the Galactic plane and containing both the Sun and
Galactic center - the Sagittarius (Sgr) stream provides a powerful means by
which to measure the solar reflex velocity, and thereby infer the velocity of
the Local Standard of Rest (LSR), in a way that is independent of assumptions
about the solar Galactocentric distance. Moreover, the solar reflex velocity
with respect to the stream is projected almost entirely into the proper motion
component of Sgr stream stars perpendicular to the Sgr plane, which makes the
inferred velocity relatively immune to most Sgr model assumptions. Using Gaia
DR2 proper motions of ~2,000 stars identified to be Sgr stream candidates in
concert with the Law and Majewski (2010) Sgr N-body models (which provide a
good match to the Gaia observations) we constrain the solar reflex velocity
induced by its orbital motion around the Galaxy to be $\Theta_{\odot} = 253 \pm
6$ km/s. Assuming a solar peculiar motion in the direction of orbital rotation
of 12 km/s, and an LSR velocity of 12 km/s with respect to the local circular
speed, the implied circular speed of the Milky Way at the solar circle is $229
\pm 6$ km/s.
