The peculiar velocities in the Galactic outer disk--hints of the
  elliptical disk and the perturbation of the spiral structures
  We present the peculiar in-plane velocities derived from the LAMOST red clump
stars. From the variations of the in-plane velocity with the Galactocentric
radius for the young and old red clump stars, we are able to identify two types
of peculiar velocities: 1) both the two red clump populations show that the
radial velocity is negative within $R=9.0$\,kpc and becomes positive beyond
(denoted as the \emph{long-wave} mode); and 2) the young red clump stars show
larger mean radial velocity than the old population by about 3\,km$\rm s^{-1}$
between $R=9$ and 12\,kpc (denoted as the \emph{short-wave} mode). We find that
the elliptical disk induced by the rotating bar can well explain the
\emph{long-wave} mode peculiar velocity. The axis ratio of the elliptical disk
is around 0.8-0.95 and the disk keeps circular at $R=9.24\pm0.2$\,kpc, which
should be the location of the outer Lindblad resonance radius (OLR). Adopting
the circular speed of 238\,km$\rm s^{-1}$, the pattern speed of the bar is then
derived as $48\pm3$\,km$\rm s^{-1}$kpc$^{-1}$ from the location of OLR. On the
other hand, the \emph{short-wave} mode is likely the perturbation of the spiral
arms as density waves.
