  RCW120 is a Galactic HII region having a beautiful ring shape bright in
infrared. Our new CO J=1-0 and J=3-2 observations performed with the NANTEN2,
Mopra, and ASTE telescopes have revealed that two molecular clouds with a
velocity separation of 20km/s are both physically associated with RCW120. The
cloud at -8km/s apparently traces the infrared ring, while the other cloud at
-28km/s is distributed just outside the opening of the infrared ring,
interacting with the HII region as supported by high kinetic temperature of the
molecular gas and by the complementary distribution with the ionized gas. A
spherically expanding shell driven by the HII region is usually discussed as
the origin of the observed ring structure in RCW120. Our observations, however,
indicate no evidence of the expanding motion in the velocity space, being
inconsistent with the expanding shell model. We here postulate an alternative
that, by applying the model introduced by Habe & Ohta (1992), the exciting O
star in RCW120 was formed by a collision between the present two clouds at a
colliding velocity ~30km/s. In the model, the observed infrared ring can be
interpreted as the cavity created in the larger cloud by the collision, whose
inner surface is illuminated by the strong UV radiation after the birth of the
O star. We discuss that the present cloud-cloud collision scenario explains the
observed signatures of RCW120, i.e., its ring morphology, coexistence of the
two clouds and their large velocity separation, and absence of the expanding
motion.
