Parkes observations for project P1014 semester 2021OCTS_01

The detections of overdense and overpressured Tiny Scale Atomic Structures (TSAS) challenge the standard picture of cooling and heating mechanisms in the interstellar medium (ISM). Multi-epoch observations of HI absorption against pulsars are a tested, practical observational method to probe the TSAS. Based on our observations in the 2019APR, 2019OCT, and 2020APR, we successfully detected a TSAS towards PSR B1557-50 with a spatial scale of 17 AU and significant dispersion measurements (DM) variations simultaneously. If TSAS is dust-free and molecule-free with a large fraction of carbon gas and not highly shielded from starlight, most of the carbon is ionized. Such electron column density in the TSAS can be observed in DM variations at a level of ~10^-2 pc cm^-3. Therefore, the ratio of HI column density variations and electron density variations measures the ionization fractions of the TSAS directly. The ionization fractions estimation in such cold atomic gas is very difficult usually through low-frequency carbon radio recombination lines with interferometers. In order to constrain the ionization fractions in TSAS, we propose two epoch observations towards PSR B1557-50 and two epoch observations towards PSR B1749-28. This will help us to better understand the cooling and heating mechanisms in the ISM.

过密超压微尺度原子结构（Tiny Scale Atomic Structures，TSAS）的探测结果，对星际介质（interstellar medium，ISM）中冷却与加热机制的标准理论图景提出了挑战。针对脉冲星的中性氢（HI）吸收多历元观测，是一种经过实践验证的实用TSAS探测方法。基于2019年4月、2019年10月及2020年4月的观测数据，我们成功在脉冲星PSR B1557-50方向探测到一个空间尺度为17天文单位（Astronomical Unit，AU）且伴随显著色散量（dispersion measurements，DM）变化的TSAS。若TSAS不含尘埃与分子、碳气占比高且未被星光强烈遮蔽，则其中大部分碳将处于电离态。此类TSAS中的电子柱密度，可在~10^-2 秒差距（parsec，pc）·厘米^-3量级的DM变化中被观测到。因此，中性氢柱密度变化与电子密度变化的比值，可直接表征TSAS的电离分数。通常，借助干涉仪通过低频碳射电复合线来估算这类冷原子气体的电离分数难度极大。为约束TSAS中的电离分数，我们提议对脉冲星PSR B1557-50与PSR B1749-28各开展两次历元观测。这将有助于我们更深入地理解星际介质中的冷却与加热机制。