Guzman Fulgencio, Francisco ; Chatzikos, Marios ; Balser, Dana ; van Hoof, Peter A. M. ; Dehghanian, Maryam ; Ulrich, Otho ; Ferland, Gary

published in American Astronomical Society Meeting Abstracts, 233, pp. 412.08 (2019)

Abstract: Radio recombination lines (RRL) are an important tool in astronomy. They are used to determine the temperature, density and metallicity, and probe the structure of H II regions, active galactic nuclei (AGN), photo-dissociation regions (PDR) and molecular clouds. It is important that the underlying atomic physics theory is accurate in order to correctly interpret the observations. Currently, there are four datasets of collisional excitation rate coefficients based in several levels of theoretical approximation and semi-empirical formulas. However, these available datasets disagree by several order of magnitude at all temperatures. Collisional excitation brings the high Rydberg levels to local thermodynamic equilibrium (LTE), and is the driving mechanism for the distribution of the population in highly excited levels. We will show that these different atomic datasets produce critical disagreements in the predictions of RRL and optical depths in our models for the broad line regions (BLR) of NGC 5548, and the Orion blister nebula. We will also present a method to observationally discriminate between different collisional theories. Finally, we will show that the number of levels chosen to represent the atom is critical for the correct prediction of RRLs.