Parkes observations for project P1014 semester 2019APRS_04

The spatial distribution of neutral gas is important for understanding the interstellar medium (ISM) evolution and star formation. However, the distribution of Tiny Scale Atomic Structures (TSAS), especially the extremely small scale structures with spatial scales ranging from a few to hundreds of AU are largely unexplored. The formation and evolution of TSAS are still on an active debate. Multi-epoch observations of HI absorption against pulsars are a tested, practical observational method to probe the TSAS, due to pulsar’s nature on-off state and proper motions of 1 to 100 AU per year. The new Parkes UWL receiver has a powerful capability to simultaneously observe not only the HI line, but nearly all the CH lines, the four L-band OH lines, several dozen C and H recombination lines, and possibly even denser tracers such as ammonia and formaldehyde. Detecting any of those along with the HI absorption would represent a major leap in our understanding of TSAS. A particularly interesting possibility is that we might discover molecular absorption associated with TSAS, which would then imply that TSAS are not transient structures. We thus propose two epoch observations, which are separated by about five months, for four bright pulsars with the new Parkes UWL receiver using multiple spectral windows for probing structures ranging from 0.5 to 500 AU, monitoring the evolution of a known TSAS traced by PSR B1557-50, and try and detect other interstellar molecules in the TSAS.

中性气体的空间分布对于理解星际介质（interstellar medium, ISM）的演化与恒星形成过程至关重要。然而，极小尺度原子结构（Tiny Scale Atomic Structures, TSAS）的分布，尤其是空间尺度为数天文单位至数百天文单位的超小型结构，目前仍未得到充分探索。学界对TSAS的形成与演化仍存在激烈争论。利用脉冲星开展氢（HI）吸收的多历元观测，是一种经过验证的实用探测TSAS的观测方法——这得益于脉冲星的通断状态特性，以及每年1至100天文单位的自行运动。新型帕克斯（Parkes）UWL接收机具备强大的观测能力，不仅能够同时观测氢（HI）谱线，还可覆盖几乎所有CH谱线、四条L波段OH谱线、数十条碳与氢复合线，甚至可能探测到氨、甲醛等更致密的星际示踪物。若能同时探测到上述任一谱线与HI吸收，都将是我们认知TSAS过程中的重大突破。一个尤为引人关注的可能性是，我们或可发现与TSAS相关联的分子吸收信号，这将意味着TSAS并非瞬态结构。因此，我们提议利用新型帕克斯（Parkes）UWL接收机，针对四颗明亮脉冲星开展两次间隔约五个月的历元观测：采用多频谱窗口以探测0.5至500 AU尺度的结构，监测由PSR B1557-50示踪的已知TSAS的演化，并尝试探测TSAS中的其他星际分子。