University of Kentucky
Influence of Dielectronic Recombination upon the Ionization State of Interstellar and Intergalactic Clouds
Institution
University of Kentucky
Faculty Advisor/ Mentor
Gary Ferland
Abstract
Dielectronic recombination is the dominant process by which free electrons recombine with ions in astrophysics. Upon the collision of a free electron with an atom, the energy of the electron is converted into internal energy of excitation of a bound electron, producing an intermediate ion that has more energy than the ionization potential of the element. Most such doubly-excited ions will then autoionize, a process in which one electron goes back into the continuum and the other returns to the ground state. However, in some small fraction of the cases one of the excited electrons will decay to a lower orbit and the ion will become stable. When this happens, energy is released in the form of electromagnetic radiation, which contributes to the observed emission spectrum of the photoionized gas. I will describe some typical photoionization calculations in which I have updated the rate coefficients to include this physics in Cloudy, a large scale numerical code used to predict the observed spectra of the ionized gasses present in interstellar and intergalactic clouds. Badnell and collaborators have recently recomputed a large number of the needed dielectronic recombination rates and provided fits to the rate coefficients. I will present results obtained for interstellar and intergalactic clouds that have been ionized by the background radiation field. Finally, I will discuss the changes caused by the improved dielectronic recombination data sets.
Influence of Dielectronic Recombination upon the Ionization State of Interstellar and Intergalactic Clouds
Dielectronic recombination is the dominant process by which free electrons recombine with ions in astrophysics. Upon the collision of a free electron with an atom, the energy of the electron is converted into internal energy of excitation of a bound electron, producing an intermediate ion that has more energy than the ionization potential of the element. Most such doubly-excited ions will then autoionize, a process in which one electron goes back into the continuum and the other returns to the ground state. However, in some small fraction of the cases one of the excited electrons will decay to a lower orbit and the ion will become stable. When this happens, energy is released in the form of electromagnetic radiation, which contributes to the observed emission spectrum of the photoionized gas. I will describe some typical photoionization calculations in which I have updated the rate coefficients to include this physics in Cloudy, a large scale numerical code used to predict the observed spectra of the ionized gasses present in interstellar and intergalactic clouds. Badnell and collaborators have recently recomputed a large number of the needed dielectronic recombination rates and provided fits to the rate coefficients. I will present results obtained for interstellar and intergalactic clouds that have been ionized by the background radiation field. Finally, I will discuss the changes caused by the improved dielectronic recombination data sets.