A Kinematic Analysis of the Giant Molecular Complex W3; Possible
  Evidence for Cloud-Cloud Collisions that Triggered OB Star Clusters in W3
  Main and W3(OH)
  The HII region W3 is one of the most outstanding regions of high-mass star
formation. Based on a new analysis of the $^{12}$CO($J$ = 2-1) data obtained at
38$"$ resolution, we have found that each of the two active regions of
high-mass star formation, W3 Main and W3(OH), is associated with two clouds of
different velocities separated by 3-4 km s$^{-1}$, having cloud mass of
2000-4000 $M_\odot$ in each. In the two regions we have found typical
signatures of a cloud-cloud collision, i.e.,the complementary distribution
with/without a displacement between the two clouds and/or a V-shape in the
position-velocity diagram. We frame a hypothesis that a cloud-cloud collision
triggered the high-mass star formation in the two regions. The collision in W3
Main involves a small cloud of $\sim$5 pc in diameter which collided with a
large cloud of 10 pc $\times$ 20 pc. The collision in W3 Main compressed the
gas in the direction of the collision path toward the west over a timescale of
$\sim$1 Myr, where the dense gas W3 core associated with ten O stars are
formed. The collision also produced a cavity in the large cloud having a size
similar to the small cloud. The collision in W3(OH) has a younger timescale of
$\sim$0.5 Myr and the forming-star candidates are heavily embedded in the
clouds. The results reinforce the idea that a cloud-cloud collision is an
essential process in high-mass star formation by rapidly creating the initial
condition of 1 g cm$^{-2}$ in the natal gas.
