The diatomic free radical methylidyne (CH) is an important tracer of the interstellar medium, and the study of it was critical to our earliest understanding of star formation. Although it is detectable across the electromagnetic spectrum, observations at radio frequencies allow for a study of the kinematics of the diffuse and dense gas in regions of new star formation. There is only two published (single-dish) detections of the low-frequency hyperfine transitions between 700 and 725 MHz, despite the precise frequencies being known. These low-frequency transitions are of particular interest as they are shown in laboratory experiments to be more sensitive to magnetic fields than their high-frequency counterparts (with more pronounced Zeeman splitting). In this work, we take advantage of the radio quiet environment and increased resolution of the Australian Square Kilometre Array Pathfinder (ASKAP) over previous searches to make a pilot interferometric search for CH at 724.7883 MHz (the strongest of the hyperfine transitions) in RCW 38. We found the band is clean of radio frequency interference, but we did not detect the signal from this transition to a five-sigma sensitivity limit of 0.09 Jy, which corresponds to a total column density upper limit of 1.9 $\times 10^{18}$ cm–2 for emission and 1.3 $\times 10^{14}$ cm–2 for absorption with an optical depth limit of 0.95. Achieved within 5 h of integration, this column density sensitivity should have been adequate to detect the emission or absorption in RCW 38, if it had similar properties to the only previous reported detections in W51.

中文翻译：

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首次使用平方公里阵列前驱望远镜搜索低频 CH

双原子自由基亚甲基（CH）是星际介质的重要示踪剂，对它的研究对于我们最早了解恒星形成至关重要。虽然它可以在整个电磁频谱中检测到，但在无线电频率上的观测可以研究新恒星形成区域中扩散和致密气体的运动学。尽管精确的频率是已知的，但在 700 和 725 MHz 之间只有两个已发表的（单碟）低频超精细跃迁检测。这些低频跃迁特别令人感兴趣，因为它们在实验室实验中被证明比高频对应物（具有更明显的塞曼分裂）对磁场更敏感。在这项工作中，我们利用无线电安静环境和提高分辨率澳大利亚平方公里阵列探路者(ASKAP) 在之前的搜索中，在 RCW 38 中对 724.7883 MHz（最强的超精细跃迁）的 CH 进行导频干涉搜索。我们发现该频段没有射频干扰，但我们没有检测到从这个跃迁到5 Sigma 灵敏度限值为 0.09 Jy，对应于 1.9 的总柱密度上限 $\乘以 10^{18}$ 厘米–2对于排放和 1.3 $\乘以 10^{14}$ 厘米–2用于光学深度限制为 0.95 的吸收。在积分的 5 小时内实现，该柱密度灵敏度应该足以检测 RCW 38 中的发射或吸收，如果它具有与 W51 中唯一先前报道的检测相似的特性。