High-mass star formation in Orion B triggered by cloud-cloud collision:
  Merging molecular clouds in NGC 2024
  We performed new comprehensive $^{13}$CO($J$=2--1) observations toward NGC
2024, the most active star forming region in Orion B, with an angular
resolution of $\sim$100'' obtained with NANTEN2. We found that the associated
cloud consists of two independent velocity components. The components are
physically connected to the H{\sc ii} region as evidenced by their close
correlation with the dark lanes and the emission nebulosity. The two components
show complementary distribution with a displacement of $\sim$0.6 pc. Such
complementary distribution is typical to colliding clouds discovered in regions
of high-mass star formation. We hypothesize that a cloud-cloud collision
between the two components triggered the formation of the late O-type stars and
early B stars localized within 0.3 pc of the cloud peak. The duration time of
the collision is estimated to be 0.3 million years from a ratio of the
displacement and the relative velocity $\sim$3 km s$^{-1}$ corrected for
probable projection. The high column density of the colliding cloud
$\sim$10$^{23}$ cm$^{-2}$ is similar to those in the other high-mass star
clusters in RCW 38, Westerlund 2, NGC 3603, and M42, which are likely formed
under trigger by cloud-cloud collision. The present results provide an
additional piece of evidence favorable to high-mass star formation by a major
cloud-cloud collision in Orion.
