Triggered high-mass star formation in the HII region W28A2: A
  cloud-cloud collision scenario
  We report on a study of the high-mass star formation in the the HII region
W28A2 by investigating the molecular clouds extended over ~5-10 pc from the
exciting stars using the 12CO and 13CO (J=1-0) and 12CO (J=2-1) data taken by
the NANTEN2 and Mopra observations. These molecular clouds consist of three
velocity components with the CO intensity peaks at V_LSR ~ -4 km s$^{-1}$, 9 km
s$^{-1}$ and 16 km s$^{-1}$. The highest CO intensity is detected at V_LSR ~ 9
km s$^{-1}$, where the high-mass stars with the spectral types of O6.5-B0.5 are
embedded. We found bridging features connecting these clouds toward the
directions of the exciting sources. Comparisons of the gas distributions with
the radio continuum emission and 8 um infrared emission show spatial
coincidence/anti-coincidence, suggesting physical associations between the gas
and the exciting sources. The 12CO J=2-1 to 1-0 intensity ratio shows a high
value (> 0.8) toward the exciting sources for the -4 km s$^{-1}$ and +9 km
s$^{-1}$ clouds, possibly due to heating by the high-mass stars, whereas the
intensity ratio at the CO intensity peak (V_LSR ~ 9 km s$^{-1}$) lowers down to
~0.6, suggesting self absorption by the dense gas in the near side of the +9 km
s$^{-1}$ cloud. We found partly complementary gas distributions between the -4
km s$^{-1}$ and +9 km s$^{-1}$ clouds, and the -4 km s$^{-1}$ and +16 km
s$^{-1}$ clouds. The exciting sources are located toward the overlapping region
in the -4 km s$^{-1}$ and +9 km s$^{-1}$ clouds. Similar gas properties are
found in the Galactic massive star clusters, RCW 38 and NGC 6334, where an
early stage of cloud collision to trigger the star formation is suggested.
Based on these results, we discuss a possibility of the formation of high-mass
stars in the W28A2 region triggered by the cloud-cloud collision.
