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).
