New clues to the impact broadening mystery in radio recombination lines

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, ∆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 ∆n.

More on the narrowing of impact broadened radio recombination lines at high principal quantum number

“…A broader coverage was carried out by Gordon and Sorochenko (2002) and radio recombination lines have been a valuable tool for many years for the purpose of studying the physical conditions inside galaxies (Gordon 2008). Using the frequency switching observing technique we have found evidence that the impact broadened linewidths of hydrogen recombination lines near 6 GHz appear to become much narrower than predicted at high quantum numbers, n > 200 (Bell 1997;Bell et al 2000;Bell et al 2011;Bell 2011). Although a possible explanation for this was suggested by Oks (2004), this was refuted by Griem (2005).…”

“…Although a possible explanation for this was suggested by Oks (2004), this was refuted by Griem (2005). It was later demonstrated by Bell (2011) that when observations at other radio frequencies were taken into account the line narrowing appeared to be correlated with the density of recombination lines in frequency space. More recently Hey (2012) has obtained results that may eventually provide a theoretical explanation for the discrepancies seen between what the impact broadening theories predict and what has been observed in these high-n Rydberg-Rydberg recombination spectra from Galactic HII regions.…”

Stark Broadening of High-Order Radio Recombination Lines Toward the Orion Nebula

“…Rood et al (1984) search they detected high-order RRLs from b H114 to q H178 , but did not address Stark broadening. Bell et al (2000Bell et al ( , 2011 observed high-order RRLs at 6 and 17.6 GHz using a novel frequency switching (FS) method. Mathematically, this method is equivalent to applying a finite difference-a discrete analog of differentiation-to the observed spectrum (Alexander & Gulyaev 2012).…”

“…Mathematically, this method is equivalent to applying a finite difference-a discrete analog of differentiation-to the observed spectrum (Alexander & Gulyaev 2012). In Bell et al (2000Bell et al ( , 2011, FS was applied to the same spectrum six times successively (a 6th order finite difference), which made their method increasingly insensitive to line broadening as the line width increased and exceeded the FS offset parameter. Alexander & Gulyaev (2012) demonstrate that the narrowing of RRLs reported by Bell et al (2000Bell et al ( , 2011 is apparent: their method effectively filtered out Stark broadening for  n 200.…”

“…In Bell et al (2000Bell et al ( , 2011, FS was applied to the same spectrum six times successively (a 6th order finite difference), which made their method increasingly insensitive to line broadening as the line width increased and exceeded the FS offset parameter. Alexander & Gulyaev (2012) demonstrate that the narrowing of RRLs reported by Bell et al (2000Bell et al ( , 2011 is apparent: their method effectively filtered out Stark broadening for  n 200. Most of the width measurements reported in Bell et al (2000Bell et al ( , 2011 for  n 200 were below Doppler width and increasingly below s 3 in signal-to-noise ratio (S/N), which is a manifestation of limitations in the use of the multiple FS method.…”