Resolving the Structure of Photodissociation Regions at Low Metallicity

光解离区（PDR）——其性质由远紫外线辐射决定的星际气体——控制着寒冷、致密的分子气体的形成，在那里可以发生恒星的形成。 该研究项目将利用阿塔卡马大型毫米/亚毫米阵列 (ALMA) 的高质量观​​测来解析和表征小麦哲伦星云 (SMC) 中 PDR 内的不同气体层。 PDR 内的过程对重元素的丰度很敏感，而 SMC 相对于银河系来说缺乏重元素，因此该项目的结果将有助于揭示 PDR 内的物理条件如何依赖于气体的“金属丰度”。 该项目将培训一名研究生高级天文学研究方法，并将制定一项计划，以促进即将入学的社区学院转学生参与本科生研究。研究人员获得了新的 ALMA 观测结果，解析了 SMC 低金属丰度环境中 PDR 的结构。他们将采取三项努力来描述观察到的 PDR 的特征。 首先，他们将使用一氧化碳 (CO) 的旋转发射线和中性碳 (C) 的精细结构线测量气体的温度，并将测量结果与 PDR 模型进行比较。接下来，他们将比较原子到分子氢（H/H2）转变与C/C/CO转变的位置，以研究“CO-暗H2气体”的性质——PDR层，其中氢是分子（H2），而碳是电离（C）或中性。 最后，他们将利用 ALMA 的高速分辨率来研究 PDR 中气体的动力学和湍流。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Photodissociation regions (PDRs) --- interstellar gas whose properties are determined by far-ultraviolet radiation --- govern the formation of cold, dense, molecular gas where star formation can occur. This research project will use high quality observations from the Atacama Large Millimeter/submillimeter Array (ALMA) to resolve and characterize the different layers of gas within PDRs in the Small Magellanic Cloud (SMC). The processes within PDRs are sensitive to the abundance of heavy elements, and the SMC is poor in heavy elements relative to the Milky Way, so the results from this project will help shed light on how physical conditions within PDRs depend on the “metallicity” of the gas. The project will train a graduate student in methods of advanced astronomical research, and it will develop a program to facilitate the participation of incoming community college transfer students in undergraduate research. The investigators have obtained new ALMA observations that resolve the structure of a PDR in the low metallicity environment of the SMC. They will undertake three efforts to characterize the observed PDR. First, they will measure the temperature of the gas using the rotational emission lines of carbon monoxide (CO) and the fine structure lines of neutral carbon (C), comparing the measurements to PDR models. Next, they will compare the location of the atomic-to-molecular hydrogen (H/H2) transition to the C+/C/CO transition to study the nature of “CO-dark H2 gas” --- PDR layers where hydrogen is molecular (H2) while carbon is either ionized (C+) or neutral. Finally, they will use the high velocity resolution possible with ALMA to study the dynamics and turbulence of gas in the PDR.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.