New Clues to the Impact Broadening Mystery in Radio Recombination Lines

TL;DR

This paper investigates the mysterious narrowing of radio recombination lines at certain frequencies, linking it to the increasing spacing of adjacent n-transitions and suggesting a potential new impact broadening effect.

Contribution

It demonstrates that the line narrowing is source-related and correlates with the transition density, proposing a new perspective on impact broadening in radio recombination lines.

Findings

Line narrowing begins when transition density exceeds ~11.6 transitions per GHz.

The narrowing is unlikely caused by data processing errors.

The phenomenon is tied to the increasing spacing of adjacent n-transitions with frequency.

Abstract

Problems where impact broadened radio recombination lines appeared narrower than predicted first showed up ~40 years ago at frequencies below ~3 GHz. But it was soon found that the observations could be explained by throwing out the uniform density models and replacing them with variable density ones. However, this problem re-appeared recently when a mysterious line narrowing above quantum numbers of (n,\Delta n) = (202,8) was reported from sensitive observations of Orion and W51 near 6 GHz. Here it is demonstrated that the narrowing is unlikely to be caused by the data processing technique and therefore must be source related. It is further demonstrated that the observed line narrowing can be tied to one of the fundamental properties of radio recombination lines; namely the fact that the spacing of adjacent n-transitions increases with frequency. The line narrowing is observed to begin when the n-transition density, D_{n}, exceeds ~11.6 transitions per GHz. This may imply that it is somehow related either to a previously overlooked effect in the impact broadening process, or to some unknown parallel process, that is tied to the separation between adjacent n-transitions. Based on these results it can be concluded, as has also been concluded in several theoretical investigations, that the observed line narrowing is not tied to a fixed range of either n or \Delta n.