Galactic rotation in Gaia DR1
  The spatial variations of the velocity field of local stars provide direct
evidence of Galactic differential rotation. The local divergence, shear, and
vorticity of the velocity field---the traditional Oort constants---can be
measured based purely on astrometric measurements and in particular depend
linearly on proper motion and parallax. I use data for 304,267 main-sequence
stars from the Gaia DR1 Tycho-Gaia Astrometric Solution to perform a local,
precise measurement of the Oort constants at a typical heliocentric distance of
230 pc. The pattern of proper motions for these stars clearly displays the
expected effects from differential rotation. I measure the Oort constants to
be: A = 15.3+/-0.4 km/s/kpc, B = -11.9+/-0.4 km/s/kpc, C = -3.2+/-0.4 km/s/kpc
and K = -3.3+/-0.6 km/s/kpc, with no color trend over a wide range of stellar
populations. These first confident measurements of C and K clearly demonstrate
the importance of non-axisymmetry for the velocity field of local stars and
they provide strong constraints on non-axisymmetric models of the Milky Way.
