2016ApJ...828...40A

We report the results of observations and analyses of 41 Δn=1,...,5 hydrogen radio recombination lines from the Orion nebula (M42) at 5.5 to 6.5 GHz, with a spectral sensitivity (channel-to-channel standard deviation) of ~2 mJy. Observations were conducted at the Australia Telescope Compact Array. A 1 GHz bandwidth allowed simultaneous detection of up to 11 spectral lines of equal Δn that were stacked to enable accurate measurement of line widths. Collisional widths in the range of principal quantum numbers n from 100 to 179 are found to be consistent with predictions of electron impact Stark broadening theory. An Orion nebula model with density inhomogeneities (clumps) and gradients of temperature and density is consistent with our data. We reanalyze the data of Smirnov et al. and Bell et al. and find excellent agreement between all statistically significant measurements and theory. Our findings confirm the absence of line narrowing for n=100...179.