  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.
