Carbon in the Interstellar Medium

碳是宇宙中含量第四丰富的元素，一氧化碳（CO）、中性碳（CI）和单电离碳（CII）以及中性氢（HI）的排放物是星系中原子和分子云最重要的示踪剂和诊断剂。然而，目前我们还没有完全了解云中的基本碳化学，或者如何解释CO， CI和CII的排放。该项目由Mark Wolfire博士领导，采用了最先进的理论建模工具来解决星际介质（ISM）中碳化学和线发射的几个问题。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，并提供建模工具来解释银河系和银河系外的[CII]， [CI]和CO排放的观测结果。本项目主要解决以下问题：1)恒星形成所需的分子气体储层对于理解星系演化具有重要意义。这种质量通常是用CO旋转线发射来测量的。然而，也有分子氢（H2）气体不含明亮的CO，预计这种气体的质量在低金属丰度水平下占主导地位。这种暗分子气体对金属丰度的依赖是什么？如何通过[CII]和[CI]线发射来追踪它？2)分子云中CO的估计丰度通常比漫射ISM中测量到的碳丰度低5-10倍；然而，预计几乎所有的非耐火碳都应该以CO的形式存在。碳在哪里？这个研究小组将使用观察和模型来对碳预算的理解趋于一致。分子云中的能量产生和流动是ISM物理学中一个长期存在的问题。中高跃迁CO线发射对云表面附近的温度敏感，但通常比模式预测的更亮。是否可以通过辐射加热加上新的化学途径来理解CO线的排放，或者是否需要机械加热？不透明云中氧(O)、碳(c)和氮(N)分子的冻结带来了许多理论问题。例如，水冰的冻结产生高C/O气相比，从根本上改变了碳的化学性质。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的理解，枯竭和解吸过程，以及何时适用稳态或时间依赖描述-这对校准崩塌岩心的化学时钟很重要。所有的漫射云模型都大大低估了观测到的丰度，即使是简单的碳基分子。冲击或湍流能量耗散产生的高温可能是必需的，但对化学速率系数的探索能否导致其他解释？这个项目的广泛影响包括几个方面。考夫曼博士是研究团队的一员，他在一所大型公立大学教授天文学，该校的学生中有许多是第一代学生，也有许多来自代表性不足的群体的学生，该校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结果。对学生助理的支持是有预算的，以便本科生或研究生，最好是来自代表性不足的群体的学生，可以参与研究项目。考夫曼吗?哈佛大学的系最近（在捐助者的资助下）创建了一个新的师生合作中心，名为“天文/物理计算和可视化实验室”，旨在为学生提供一个研究计算研究问题的空间。在这里，他们培养了计算、分析和表达方面的技能，这些技能将很好地为他们服务，无论他们最终的职业道路如何。这与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是一致的。Wolfire博士还将带领研究生进行项目研究。研究结果将通过一个著名的网站PDR工具箱提供。这个网站已经被数百名天文学家使用最先进的模型来分析他们的观测结果。代码将在天体物理源代码库（ASCL）上列出并提供。

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).