Stark broadening in radio recombination lines towards the Orion Nebula

An investigation of Stark broadening in hydrogen radio recombination lines was made by observing a series of adjacent higher order lines and a higher frequency unbroadened n-alpha line. Two different diameter telescopes provided the same angular resolution for the complete set of the lines. Collisional widths are proportional to n to the 4.4 power for quantum level n of about 60 to 174, in agreement with the current theory of Stark broadening. Two different sets of lines were observed with 1.5 and 2.6 arcmin beams toward the Orion Nebula to study effects of beam averaging on the observational characteristics of broadening. An increase of the beamwidth from 1.5 to 2.6 arcmin results in a decrease of the n-dependence of the collisional widths from 4.4 to 3.8 which can be explained by a nonhomogeneous model of the Nebula. Calculations of Stark broadening applied to the 1.5 arcmin observations give electron densities of (1 + or - 0.3) 10,000/cu cm.