Pressure Broadening Effects on the Emission of He Recombination Lines

Abstract

The first rigorous treatement of pressure broadening of hydrogenlike emitters of radio recombination lines is due to Griem [1967]. The calculations showed that most of the radio observations that were available at that time [mostly 109 and lower frequency transitions] must have been affected by pressure broadening. This conclusion was ignored or contested by most of the people involved with the observations, on the ground that there was no evidence showing any observed amount of broadening. Indeed the widths of recombination lines are remarkably constant over several frequency octaves, Brocklehurst and Seaton [1972] were the first to show that theory and observations were not necessarily incompatible. They considered a model source of variable density, At a given frequency there are two regions of Interest in the source: the dense region that emits broadened lines, and the less dense regions, that emit unbroadened, gaussian looking lines. The combined spectrum would be an almost gaussian line, sitting on the extended wings of the broadened line, The usual expedient of fitting baselines to any odd looking spectra removed the extended wings and left just the gaussian looking profile. Obviously, this profile carries a small fraction of the total line radiation emited by the source. The relevance of this fact in the understanding of LINE/CONT or ALFA/ BETA ratios will be certainly discussed In this workshop.