Phase Interchange of Galactic Interstellar Gas Explored by the Terahertz Heterodyne Sensing

本研究以银河系星际气体的相交换为重点，进行了以下两项研究：第一项是利用超导HEB混合器研制THz外差接收机，以探索氮离子线（1.46THz）的分布。利用这条谱线，我们可以研究等离子体气体在分子云形成和演化中的作用。利用野边山射电天文台的设备，我们制作了一个扩散冷却的高电子束混频元件，它具有尺寸为几个100纳米的铌微桥结构。经过多次试验，我们最终确定了使用剥离方法的制造工艺。混频器元件安装在波导混频器块上进行RF测试，我们已经确认混频器在800 GHz下工作。虽然还存在一些问题，但为实现太赫兹天文学的HEB混频器接收器奠定了坚实的基础。沿着上述开发研究，我们利用富士山亚毫米波望远镜观测碳原子的精细结构线，研究了分子云的形成和演化。特别是，我们观察到明亮的边缘球，这是由附近的OB星强烈的紫外线辐射照明。在一个年轻的明亮边缘球状体IC 1396中，我们发现碳原子的分布遍及整个云，而不是集中在明亮的边缘。另一方面，我们发现在一个相当古老的球状体λ Ori的亮边位置有一个碳原子发射峰。这表明，随着云的演化，原子碳逐渐转变为CO，并且它只存在于OB星照明的PDR层中。从而进一步证实了碳原子可以作为研究分子云形成和演化的良好示踪剂。

In this study, we focused on the phase interchange of interstellar gas in the Galaxy, and conducted the following two researches.The first one is a development of the THz heterodyne receiver using the superconductive HEB mixer in order to explore the distribution of the nitrogen ion line (1.46THz). With this line, we can study a role of plasma gas in formation and evolution of molecular clouds. By using the facilities at Nobeyama Radio Observatory, we have fabricated a diffusion cooled HEB mixer element which has a microbridge structure of Nb with a size of a few 100nm. After many trials, we finally established the fabrication process using the liftoff method. The mixer element was mounted on the waveguide mixer block for the RF test, and we have confirmed the mixer operation at 800GHz. Although we have still some problems to overcome, we were able to form a strong base toward realizing the HEB mixer receiver for the THz astronomy.Along with the above development study, we studied formation and evolution of molecular clouds by observing the fine structure line of the atomic carbon with the Mount Fuji submillimeter-wave telescope. Particularly, we observed bright rimmed globules, which are illuminated by strong UV radiation from a nearby OB stars. In a young bright rimmed globule, IC1396, we found that the distribution of the atomic carbon extends over the entire of the clouds, not concentrating at the bright rim. On the other hand, we found a peak of the atomic carbon emission at the bright rim position in a rather old globule, λ Ori. This indicates that the atomic carbon gradually changes into CO as the cloud evolves, and that it only exists in the PDR layer illuminated by OB stars. Therefore we further confirmed that the atomic carbon can be a good tracer to study formation and evolution of molecular clouds.