A New Technique for Measuring Impact‐broadened Radio Recombination Lines in HiiRegions: Confrontation with Theory at High Principal Quantum Numbers

Abstract

ABSTRACT. New, low-noise receivers have allowed detection, in several giant H II regions, of RydbergRydberg transitions of hydrogen that cover a large range of *n-values in a single observing window. This, in turn, allows lines covering a large range in principal quantum number n to be detected simultaneously with the same antenna beam. We have employed a new frequency-switching technique which allows a very precise determination of the line widths. We have used this technique with the NRAO 140 foot telescope to observe lines in W51 and Orion A near 6 GHz, with *n-values that vary by a factor of D21 (*n \ 1ÈD21) and corresponding n-values that vary by a factor of 2.7 (n \ 102È274). By generating Voigt line proÐles using GriemÏs theory of impact broadening by electrons, inserting them into a telescope data Ðle, and processing them in a manner identical to that of the telescope data, we have been able to examine how the observing and reduction techniques a †ect both the line widths and line areas as n increases. For n ¹ 180, *n ¹ 6, our restored line widths and areas give densities of and 4000 cm~3 in W51 and Orion A, respectively. N e \ 2500 These densities are higher than reported previously with a 5@ beamwidth. For higher n-values we are unable to Ðt our data using GriemÏs theory. For n [ 180, *n [ 6, our telescope-measured line widths fall rapidly below predicted values, while the line areas simultaneously increase above predicted values. This behavior of the line area as the line widths decrease is inconsistent with GriemÏs theory or an instrumental e †ect. Observations of Orion A at 17.6 GHz, with a beamwidth, require a density in excess of cm~3 1@ .7 N e \ 20,000 to Ðt. Although the detected lines cover a range in n and *n from 71 to 177 and 1 to 17, respectively, there is no evidence for a line width decrease at the high *n-values. We conclude from this that the line narrowing seen at 6 GHz is related to the principal quantum number.

Exact expression of the impact broadening operator for hydrogen Stark broadening

“…However, when the ion density is very low, ions can no longer be considered static during the typical dipole relaxation time. As already pointed out by Griem (2005) -see also Griem (1967Griem ( , 1974 -, this happens, for example, for the conditions of radio recombination lines studied in Bell et al (2000), where ions effect can be considered as "impact". In those cases, then, the dominant broadening is ionic impact, as their fields, as they are slower, have a correlation integral (42) larger than that of electrons 3 .…”

“…Recent measurements of the width of radiofrequency spectral lines (Bell et al 2000), together with discrepancies between experimental data (Smirnov et al 1984;Smirnov 1985;Wilson & Jäger 1987;Sorochenko 1989;Bell et al 2000) and conventional models (Baranger 1958a-c;Griem et al 1959;Griem 1967Griem , 1974 have stimulated the interest in this type of transitions. In this framework a controversy (see Griem 2005 and works cited therein) in explaining these disagreements without changing the usual frame of Stark broadening theories occur.…”

New Source of the Red Shift of Highly-Excited Hydrogenic Spectral Lines in Astrophysical and Laboratory Plasmas

“…As for the RRL of n ~ 200 and higher observed by Bell et al, [27] and Alexander and Gulyaev [28] point out the following. Bell et al [27] applied the frequency switching method to the same spectrum six times successively, which made their method increasingly insensitive to line broadening as the line width increased and exceeded the frequency switching offset parameter.…”

“…In the well-known paper by Bell et al [27], the authors measured widths of Radio Recombination Lines (RRL) from several H II regions, including Orion A. Their primary finding was that the width of RRL of the principal quantum number n up 180 increased with the growing n, but for RRL of n > 180 the widths decreased with the growing n. Recently Alexander and Gulyaev [28] presented the newest observations of RRL from Orion nebula.…”

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).…”