Parkes observations for project P1014 semester 2020APRS_03

The spatial distribution of neutral gas is important for understanding the interstellar medium (ISM), its 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 is largely unexplored. The formation and evolution of TSAS are still unknown. Multi-epoch observations of HI absorption against pulsars are a tested, practical observational method to probe the TSAS. Based on our observations in the 2019 APRS semester, we successfully processed baseband data and detected the HI absorption variation at over 4-months towards B1557-50. PSR B1451-68 is another ideal source to trace the TSAS because of the intervening cold gas. We thus propose one-epoch observation towards B1557-50 which should be as early in the semester as possible, and two-epoch observations towards B1451-68 to be separated by about 5 months, for monitoring the HI absorption variations and probing structures on a range of scales. This will better constrain the characteristics of variations in HI absorption, thus contribute significantly to understanding the origin of TSAS.