New Clues to the Impact Broadening Mystery in Radio Recombination Lines
  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.
