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We report on a study of the high-mass star formation in the the HII regionW28A2 by investigating the molecular clouds extended over ~5-10 pc from theexciting stars using the 12CO and 13CO (J=1-0) and 12CO (J=2-1) data taken bythe NANTEN2 and Mopra observations. These molecular clouds consist of threevelocity components with the CO intensity peaks at V_LSR ~ -4 km s$^{-1}$, 9 kms$^{-1}$ and 16 km s$^{-1}$. The highest CO intensity is detected at V_LSR ~ 9km s$^{-1}$, where the high-mass stars with the spectral types of O6.5-B0.5 areembedded. We found bridging features connecting these clouds toward thedirections of the exciting sources. Comparisons of the gas distributions withthe radio continuum emission and 8 um infrared emission show spatialcoincidence/anti-coincidence, suggesting physical associations between the gasand the exciting sources. The 12CO J=2-1 to 1-0 intensity ratio shows a highvalue (> 0.8) toward the exciting sources for the -4 km s$^{-1}$ and +9 kms$^{-1}$ clouds, possibly due to heating by the high-mass stars, whereas theintensity 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 kms$^{-1}$ cloud. We found partly complementary gas distributions between the -4km s$^{-1}$ and +9 km s$^{-1}$ clouds, and the -4 km s$^{-1}$ and +16 kms$^{-1}$ clouds. The exciting sources are located toward the overlapping regionin the -4 km s$^{-1}$ and +9 km s$^{-1}$ clouds. Similar gas properties arefound in the Galactic massive star clusters, RCW 38 and NGC 6334, where anearly stage of cloud collision to trigger the star formation is suggested.Based on these results, we discuss a possibility of the formation of high-massstars in the W28A2 region triggered by the cloud-cloud collision.

Triggered high-mass star formation in the HII region W28A2: A cloud-cloud collision scenario