Gaia DR1 evidence of disrupting Perseus Arm
  We have discovered a clear sign of the disruption phase of the Perseus arm in
the Milky Way using Cepheid variables, taking advantage of the accurately
measured distances of Cepheids and the proper motions from Gaia Data Release 1.
Both the Galactocentric radial and rotation velocities of 77 Cepheids within
1.5 kpc of the Perseus arm are correlated with their distances from the locus
of the Perseus arm, as the trailing side is rotating faster and moving inward
compared to the leading side. We also found a negative vertex deviation for the
Cepheids on the trailing side, $-27.6\pm2.4$ deg, in contrast to the positive
vertex deviation in the solar neighborhood. This is, to our knowledge, the
first direct evidence that the vertex deviation around the Perseus arm is
affected by the spiral arm. We compared these observational trends with our
$N$-body/hydrodynamics simulations based on a static density-wave spiral
scenario and those based on a transient dynamic spiral scenario. Although our
comparisons are limited to qualitative trends, they strongly favor a conclusion
that the Perseus arm is in the disruption phase of a transient arm.
