High-mass star formation possibly triggered by cloud-cloud collision in
  the HII region RCW 34
  We report a possibility that the high-mass star located in the HII region RCW
34 was formed by a triggering induced by a collision of molecular clouds.
Molecular gas distributions of the $^{12}$CO and $^{13}$CO $J=$2-1, and
$^{12}$CO $J=$3-2 lines toward RCW 34 were measured by using the NANTEN2 and
ASTE telescopes. We found two clouds with the velocity ranges of 0-10 km
s$^{-1}$ and 10-14 km s$^{-1}$. Whereas the former cloud as massive as ~2.7 x
10$^{4}$ Msun has a morphology similar to the ring-like structure observed in
the infrared wavelengths, the latter cloud with the mass of ~10$^{3}$ Msun,
which has not been recognized by previous observations, distributes just likely
to cover the bubble enclosed by the other cloud. The high-mass star with the
spectral types of O8.5V is located near the boundary of the two clouds. The
line intensity ratio of $^{12}$CO $J=$3-2 / $J=$2-1 yields high values (~1.5)
in the neighborhood of the high-mass star, suggesting that these clouds are
associated with the massive star. We also confirmed that the obtained
position-velocity diagram shows a similar distribution with that derived by a
numerical simulation of the supersonic collision of two clouds. Using the
relative velocity between the two clouds (~5 km s$^{-1}$), the collisional time
scale is estimated to be $\sim$0.2 Myr with the assumption of the distance of
2.5 kpc. These results suggest that the high-mass star in RCW 34 was formed
rapidly within a time scale of ~0.2 Myr via a triggering of cloud-cloud
collision.
