O I lines in the sun and stars. I - Understanding the resonance lines

The use of O I lines as spectral diagnostics of conditions in the chromospheres of the sun and cool stars is addressed, focusing on the resonance lines. The important processes influencing these lines are identified and an attempt it made to understand the detailed line transfer calculations in terms of simpler analytical models. How the lines respond to changes in uncertain atomic parameters is illustrated, and it is shown how the O I line flux densities depend simply and sensitively on the radiative transfer solution for hydrogen. Approximate analytical formulas are derived for the O I line flux densities in terms of the hydrogen number densities.