The peculiar in-plane velocities in the outer disc of the Milky Way
  We present the peculiar in-plane velocities derived from the LAMOST red clump
stars, which are purified and separated by a novel approach into two groups
with different ages. The samples are mostly contributed around the Galactic
anti-centre direction such that we are able to map the radial profiles of the
radial and azimuthal velocities in the outer disc. From the variations of the
in-plane velocities with the Galactocentric radius for the younger and older
populations, we find that both radial and azimuthal velocities are not
axisymmetric at $8<R<14\,kpc$. The two red clump populations show that the mean
radial velocity is negative within $R\sim9\,kpc$ and positive beyond. This is
likely because of the perturbation induced by the rotating bar. The cross-zero
radius, $R\sim9$\, kpc, essentially indicates the rough location of the outer
Lindblad resonance (OLR) radius. Given the circular speed of 238\,km$\rm
s^{-1}$, then the pattern speed of the bar can be approximated as $45$\,km$\rm
s^{-1}\rm kpc^{-1}$. The young red clump stars show larger mean radial velocity
than the old population by about 3$\,km\rm s^{-1}$ between $R\sim9$ and
12\,kpc. This is possibly because the younger population is more sensitive to
the perturbation than the older one. The radial profiles of the mean azimuthal
velocity for the two populations show an interesting U-shape, i.e. at
$R<10.5\,kpc$, the azimuthal velocity declines with $R$ by about 10$\,km\rm
s^{-1}$, while at $R>10.5$ it increases with $R$ to 240-245$\,km\rm s^{-1}$. It
is not clear why the mean azimuthal velocity shows the U-shape along the
Galactic anti-centre direction. Meanwhile, the azimuthal velocity for the
younger population is slightly larger than the older one and the difference
moderately declines with $R$. Beyond $R\sim12\,kpc$, the azimuthal velocities
for the two populations are indistinguishable.
