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.

中文翻译：

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在 H ii 区域 W 28 A2 触发大质量恒星形成：云-云碰撞场景

我们通过使用 12CO 和 13CO (J=1-0) 和 12CO (J =2-1) 由 NANTEN2 和 Mopra 观测获得的数据。这些分子云由三个速度分量组成，CO 强度峰值位于 V_LSR ~ -4 km s$^{-1}$、9 km s$^{-1}$ 和 16 km s$^{-1}$。在 V_LSR ~ 9 km s$^{-1}$ 处检测到最高的 CO 强度，其中嵌入了光谱类型为 O6.5-B0.5 的大质量恒星。我们发现了将这些云连接到令人兴奋的来源方向的桥接特征。气体分布与无线电连续谱发射和 8 um 红外发射的比较显示了空间重合/反重合，表明气体和激发源之间的物理联系。对于-4 km s$^{-1}$ 和+9 km s$^{-1}$ 的激发源，12CO J=2-1 与1-0 的强度比显示出高值（> 0.8）云，可能是由于大质量恒星加热，而 CO 强度峰值 (V_LSR ~ 9 km s$^{-1}$) 处的强度比降低至 ~0.6，表明被高密度气体自吸收+9 公里 s$^{-1}$ 云的近侧。我们发现 -4 公里 s$^{-1}$ 和 +9 公里 s$^{-1}$ 云以及 -4 公里 s$^{-1}$ 和 +16 公里之间存在部分互补的气体分布s$^{-1}$ 云。令人兴奋的源位于-4 km s$^{-1}$ 和+9 km s$^{-1}$ 云中的重叠区域。在银河系大质量星团 RCW 38 和 NGC 6334 中发现了类似的气体特性，建议在云碰撞的早期阶段触发恒星形成。基于这些结果，我们讨论了由云-云碰撞引发的 W28A2 区域形成大质量恒星的可能性。