The local spiral arm in the LAMOST-Gaia common stars?
  Using the LAMOST-Gaia common stars, we demonstrate that the in-plane velocity
field for the nearby young stars are significantly different from that for the
old ones. For the young stars, the probably perturbed velocities similar to the
old population are mostly removed from the velocity maps in the $X$--$Y$ plane.
The residual velocity field shows that the young stars consistently move along
$Y$ with faster $v_\phi$ at the trailing side of the local arm, while at the
leading side, they move slower in azimuth direction. At both sides, the young
stars averagely move inward with $v_R$ of $-5\sim-3$ km s$^{-1}$. The
divergence of the velocity in $Y$ direction implies that the young stars are
associated with a density wave nearby the local arm. We therefore suggest that
the young stars may reflect the formation of the local spiral arm by
correlating themselves with a density wave. The range of the age for the young
stars is around 2 Gyr, which is sensible since the transient spiral arm can
sustain that long. We also point out that alternative explanations of the
peculiar velocity field for the young population cannot be ruled out only from
this observed data.
