A new look at the molecular gas in M42 and M43; possible evidence for
  cloud-cloud collision which triggered formation of the OB stars in the Orion
  Nebula Cluster
  The Orion Nebula Cluster toward the HII region M42 is the most outstanding
young cluster at the smallest distance 410pc among the rich high-mass stellar
clusters. By newly analyzing the archival molecular data of the 12CO(J=1-0)
emission at 21" resolution, we identified at least three pairs of complementary
distributions between two velocity components at 8km/s and 13km/s. We present a
hypothesis that the two clouds collided with each other and triggered formation
of the high-mass stars, mainly toward two regions including the nearly ten O
stars, theta1 Ori and theta2 Ori, in M42 and the B star, NU Ori, in M43. The
timescale of the collision is estimated to be ~0.1Myr by a ratio of the cloud
size and velocity corrected for projection, which is consistent with the age of
the youngest cluster members less than 0.1Myr. The majority of the low-mass
cluster members were formed prior to the collision in the last one Myr. We
discuss implications of the present hypothesis and the scenario of high-mass
star formation by comparing with the other eight cases of triggered O star
formation via cloud-cloud collision.
