Carbon in the Interstellar Medium

星际介质中的碳

• 批准号: 1411827
• 负责人: Mark Wolfire
• 金额: $ 41.27万
• 依托单位: University of Maryland, College Park
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-08-15 至 2019-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1411827&HistoricalAwards=false
• 关键词: Carbon; Interstellar; Medium

Carbon is the 4th most abundant element in the universe, and emissions from carbon monoxide (CO), neutral carbon (CI) and singly ionized carbon (CII), along with neutral hydrogen (HI), are the most important tracers and diagnostics of atomic and molecular clouds in galaxies. However, at present we do not fully understand basic carbon chemistry in clouds, or how to interpret CO, CI, and CII emission. This project, led by Dr. Mark Wolfire, brings to bear state-of-the-art theoretical modeling tools to address several problems in carbon chemistry and line emission in the interstellar medium (ISM). The result will be a fundamentally better understanding of carbon-based chemistry in atomic and molecular gas plus modeling tools to interpret Galactic and extragalactic observations of [CII], [CI], and CO emission.The following problems are addressed in this project: 1) The reservoir of molecular gas available for star formation is important for understanding galaxy evolution. This mass is usually measured by CO rotational line emission. However, there is also molecular hydrogen (H2) gas that does not contain bright CO, and the mass of this gas is expected to dominate at low levels of metallicity. What is the dependence of this dark molecular gas on metallicity, and how can it be traced by [CII], and [CI] line emission? 2)The estimated abundance of CO in molecular clouds is typically 5-10 times lower than the measured carbon abundance in the diffuse ISM; however, it is expected that nearly all of the non-refractory carbon should be in the form of CO. Where is the carbon? This research team will use observations and models to converge on an understanding of the carbon budget. 3)The energy generation and flow in molecular clouds is a long standing problem in ISM physics. Mid- and high-transition CO line emission is sensitive to temperature near the cloud surface but is often brighter than predicted by models. Can the CO line emission be understood by radiative heating plus novel chemical pathways or are mechanical sources of heating required? 4)The freeze-out of oxygen (O), carbon, and nitrogen (N) molecules in opaque clouds presents a host of theoretical problems. For example, the freeze-out of water ice produces a high C/O gas-phase ratio that radically changes the carbon chemistry. The models developed by Dr. Wolfire will further our understanding of the physical grain and gas conditions, depletion and desorption processes, and when a steady-state or time dependent description applies - a distinction important for calibrating a chemical clock for collapsing cores. 5) All models of diffuse clouds vastly underestimate the observed abundance of even simple carbon-based molecules. High temperatures produced by shocks or the dissipation of turbulent energy might be required, but can an exploration of chemical rate coefficients lead to alternative explanations?The broader impacts of this project cover several areas. Dr. Kaufman, a member of the research team, teaches astronomy at a large, public university with a student body that includes many first generation students, many students from underrepresented groups, and an institutional commitment to STEM education for students in all majors. Kaufman will disseminate the results of this work in general education courses for non-science majors, advanced courses for majors, and student research projects. Support for a student assistant is budgeted so that an undergraduate or graduate student, preferably one from an underrepresented group, may participate in the research project. Kaufman?s department has recently created (with funding from donors) a new center for faculty/student collaboration called the "Astro/Physics Computation and Visualization Lab," designed as a space for students to work on computational research problems. Here they build skills in computation, analysis and presentation that will serve them well, regardless of their ultimate career path. This is in line with the NSF goal of creating a broad, skilled, scientific workforce for the nation. Dr. Wolfire will also lead graduate students in project research. The results will be made available through a well-known web site, the PDR Toolbox. This site has been used by hundreds of astronomers to analyze their observations using state-of-the-art models. The code will be listed and made available on the ASCL (Astrophysics Source Code Library).

碳是宇宙中第四丰富的元素，一氧化碳 (CO)、中性碳 (CI) 和单电离碳 (CII) 以及中性氢 (HI) 的排放是原子最重要的示踪剂和诊断剂。和星系中的分子云。然而，目前我们还不完全了解云中的基本碳化学，或者如何解释 CO、CI 和 CII 排放。该项目由 Mark Wolfire 博士领导，采用最先进的理论建模工具来解决碳化学和星际介质 (ISM) 中的线发射的几个问题。结果将从根本上更好地理解原子和分子气体中的碳基化学，以及解释银河系和河外星系 [CII]、[CI] 和 CO 排放观测结果的建模工具。该项目解决了以下问题：1 ）可用于恒星形成的分子气体库对于理解星系演化非常重要。该质量通常通过 CO 旋转线排放来测量。然而，也存在不含光亮CO的分子氢(H2)气体，并且这种气体的质量预计在低金属丰度下占主导地位。这种暗分子气体对金属丰度的依赖性是什么？如何通过 [CII] 和 [CI] 线发射来追踪它？ 2）分子云中CO的估计丰度通常比弥散ISM中测量的碳丰度低5-10倍；然而，预计几乎所有的非难熔碳都应该以CO的形式存在。碳在哪里？该研究团队将利用观察和模型来加深对碳预算的理解。 3）分子云中能量的产生和流动是ISM物理学中长期存在的问题。中高跃迁二氧化碳线排放对云表面附近的温度敏感，但通常比模型预测的更亮。 CO 线排放是否可以通过辐射加热加上新颖的化学途径来理解，或者是否需要机械加热源？ 4）不透明云中氧（O）、碳和氮（N）分子的冻结提出了许多理论问题。例如，水冰的冻结会产生高 C/O 气相比，从而从根本上改变碳化学性质。 Wolfire 博士开发的模型将进一步加深我们对物理颗粒和气体条件、消耗和解吸过程的理解，以及何时应用稳态或时间相关描述——这一区别对于校准坍缩核心的化学时钟非常重要。 5）所有弥散云模型都大大低估了观测到的丰度，甚至是简单的碳基分子。可能需要由冲击或湍流能量耗散产生的高温，但对化学速率系数的探索能否得出替代解释？该项目的更广泛影响涵盖多个领域。考夫曼博士是研究团队的成员之一，在一所大型公立大学教授天文学，该大学的学生群体包括许多第一代学生和许多来自弱势群体的学生，并且机构致力于为所有专业的学生提供 STEM 教育。考夫曼将在非科学专业的通识教育课程、专业的高级课程和学生研究项目中传播这项工作的成果。对学生助理的支持已列入预算，以便本科生或研究生（最好是来自代表性不足的群体）可以参与研究项目。考夫曼的系最近（在捐助者的资助下）创建了一个新的教师/学生合作中心，称为“天文/物理计算和可视化实验室”，旨在为学生解决计算研究问题提供空间。在这里，他们培养计算、分析和演示方面的技能，无论他们的最终职业道路如何，这些技能都将对他们很有帮助。这符合国家科学基金会为国家创造一支广泛的、熟练的、科学的劳动力队伍的目标。 Wolfire 博士还将带领研究生进行项目研究。结果将通过著名网站 PDR Toolbox 提供。该网站已被数百名天文学家使用最先进的模型分析他们的观测结果。该代码将在 ASCL（天体物理学源代码库）上列出并提供。