We performed new comprehensive $^{13}$CO($J$=2--1) observations toward NGC 2024, the most active star forming region in Orion B, with an angular resolution of $\sim$100'' obtained with NANTEN2. We found that the associated cloud consists of two independent velocity components. The components are physically connected to the H{\sc ii} region as evidenced by their close correlation with the dark lanes and the emission nebulosity. The two components show complementary distribution with a displacement of $\sim$0.6 pc. Such complementary distribution is typical to colliding clouds discovered in regions of high-mass star formation. We hypothesize that a cloud-cloud collision between the two components triggered the formation of the late O-type stars and early B stars localized within 0.3 pc of the cloud peak. The duration time of the collision is estimated to be 0.3 million years from a ratio of the displacement and the relative velocity $\sim$3 km s$^{-1}$ corrected for probable projection. The high column density of the colliding cloud $\sim$10$^{23}$ cm$^{-2}$ is similar to those in the other high-mass star clusters in RCW 38, Westerlund 2, NGC 3603, and M42, which are likely formed under trigger by cloud-cloud collision. The present results provide an additional piece of evidence favorable to high-mass star formation by a major cloud-cloud collision in Orion.

中文翻译：

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云-云碰撞引发猎户座 B 中的大质量恒星形成：NGC 2024 中分子云的合并

我们对猎户座 B 中最活跃的恒星形成区域 NGC 2024 进行了新的综合 $^{13}$CO($J$=2--1) 观测，使用 NANTEN2 获得的角分辨率为 $\sim$100'' . 我们发现相关联的云由两个独立的速度分量组成。这些组件物理连接到 H{\sc ii} 区域，这可以通过它们与暗通道和发射星云的密切相关性来证明。两个分量显示互补分布，位移为 $\sim$0.6 pc。这种互补分布是在大质量恒星形成区域发现的碰撞云的典型特征。我们假设两个成分之间的云-云碰撞触发了晚期 O 型星和早期 B 型星的形成，这些星定位在云峰的 0.3 pc 内。根据位移与相对速度之比 $\sim$3 km s$^{-1}$ 估计碰撞的持续时间为 30 万年，并根据可能的预测进行了修正。碰撞云的高柱密度 $\sim$10$^{23}$ cm$^{-2}$ 与 RCW 38、Westerlund 2、NGC 3603 和 M42 中其他大质量星团的密度相似，这很可能是在云云碰撞触发下形成的。目前的结果提供了一个额外的证据，有利于猎户座的一次主要云云碰撞形成大质量恒星。Westerlund 2、NGC 3603和M42，很可能是在云云碰撞触发下形成的。目前的结果提供了一个额外的证据，有利于猎户座的一次主要云云碰撞形成大质量恒星。Westerlund 2、NGC 3603和M42，很可能是在云云碰撞触发下形成的。目前的结果提供了一个额外的证据，有利于猎户座的一次主要云云碰撞形成大质量恒星。