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--支配着寒冷、致密的分子气体的形成，在那里可以形成恒星。这一研究项目将利用阿塔卡马大毫米/亚毫米阵列(ALMA)的高质量观测结果来分辨和描述小麦哲伦星云(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相对于银河系来说重元素含量较低，因此该项目的结果将有助于阐明PDR内的物理条件如何取决于气体的“金属丰度”。该项目将培训一名研究生进行高级天文学研究的方法，并将制定一项计划，以促进即将到来的社区大学转学学生参与本科生的研究。研究人员获得了新的ALMA观测结果，解决了SMC低金属丰度环境中PDR的结构。他们将采取三项努力来确定观察到的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)的精细结构线来测量气体的温度，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)转变和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)，而碳是电离的(C)或中性的。最后，他们将使用ALMA可能的高速度分辨率来研究PDR中气体的动力学和湍流。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.