It has long been recognized that thermal energy charge transfer (CT) of singly- and doubly charged ions with atomic hydrogen plays an important role in determining the ionization structure, thermal structure, emission spectrum, and absorption spectrum of planetary nebulae, H II regions, Lya clouds, the intergalactic medium (IGM), and shocks in supernova remnants and Herbig-Haro objects. Ground-based spectroscopic observations of these sources are used to address many fundamental questions in astrophysics such as the primordial He abundance, the chemical evolution of the universe, the shape of the metagalactic radiation field as a function of redshift, galactic chemical evolution, and stellar nucleosynthesis. Of particular importance to address these issues are reliable thermal energy CT rate coefficients for singly- and doubly-charged ions of C, N, and O. The majority of thermal CT data used in astrophysics has been calculated with the Landau- Zener (LZ) method. Dr. Daniel Wolf Savin's laboratory results have demonstrated that LZ calculations of thermal CT cross section can be off by up to an order of magnitude. A few state-of-the-art molecular orbital close-coupling (MOCC) calculations also exist. But laboratory work has shown that without benchmark measurements, MOCC thermal CT cross section calculations can be off by up to a factor of 3.
Dr. Savin and colleagues will carry out a combined program of laboratory measurements and modeling studies for CT of C2+, N+, N2+, O+, and O2+ on atomic H at thermal energies. They have already carried out such measurements for C+. The new measurements will be carried out using the Oak Ridge National Laboratory ion-atom merged-beams apparatus which is the only existing facility capable of carrying out the proposed thermal energy cross section measurements. The results will be used to benchmark state-of-the-art MOCC calculations. The measurements, in combination with benchmarked theory if needed, will also be used to produce CT rate coefficients with an estimated accuracy of 20%. The group will publish simple fits to derived rate coefficients so that the astrophysics community can use the new data in their studies of cosmic plasmas. Concurrent with the measurements, they will carry out modeling studies using CLOUDY to investigate the astrophysical implication of the new data as well as the implication due to any inferred uncertainties in the unmeasured CT data for other ions. Some of the issues to be investigated include the role that CT plays in determining the ionization correction factors used to infer the primordial He abundance from H II regions. The group will also study the role of CT in Lya clouds and the IGM, observations of which are used to constrain the chemical evolution of the universe and the shape of the metagalactic radiation field as a function of redshift.
A large portion of this research project will be carried out by a Columbia University graduate student in partial fulfillment of the requirements for her/his Ph.D. Teaching and training of the student will be overseen by Dr. Savin and collaborators at Oak Ridge National Laboratory (ORNL), the University of Kentucky, and the University of Georgia-Athens. The research will thereby result in the education and training of a student to be a future scientist. In addition this work will enhance the Columbia/ORNL infrastructure for research and education which Dr. Savin has recently established with his collaborators at ORNL. The measurements will be carried out in collaboration with ORNL scientists using a unique ORNL facility. Lastly, to enhance scientific and technical understanding the group will broadly disseminate the results at conferences and publish them in the appropriate scientific journals. ***
