Gaia-DR2 distance to the W3 Complex in the Perseus Arm
  The Perseus Arm is the closest Galactic spiral arm from the Sun, offering an
excellent opportunity to study in detail its stellar population. However, its
distance has been controversial with discrepancies by a factor of two.
Kinematic distances are in the range 3.9-4.2 kpc as compared to 1.9-2.3 kpc
from spectrophotometric and trigonometric parallaxes, reinforcing previous
claims that this arm exhibits peculiar velocities. We used the astrometric
information of a sample of 31 OB stars from the star-forming W3 Complex to
identify another 37 W3 members and to derive its distance from their Gaia-DR2
parallaxes with improved accuracy. The Gaia-DR2 distance to the W3
Complex,2.14$^{+0.08}_{-0.07}$ kpc, coincides with the previous stellar
distances of $\sim$ 2 kpc. The Gaia-DR2 parallaxes tentatively show
differential distances for different parts of the W3 Complex: W3 Main, located
to the NE direction, is at 2.30$^{+0.19}_{-0.16}$ kpc, the W3 Cluster (IC
1795), in the central region of the complex, is at 2.17$^{+0.12}_{-0.11}$ kpc,
and W3(OH) is at 2.00$^{+0.29}_{-0.23}$ kpc to the SW direction. The W3 Cluster
is the oldest region, indicating that it triggered the formation of the other
two star-forming regions located at the edges of an expanding shell around the
cluster.
