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

Abstract

Recently Alexander and Gulyaev have suggested that the apparent decrease in impact broadening of radio recombination lines seen at high principal quantum number n may be a product of the data reduction process, possibly resulting from the presence of noise on the telescope spectra that is not present on the calculated comparison spectra. This is an interesting proposal. However, there are serious problems with their analysis that need to be pointed out. Perhaps the most important of these is the fact that for principal quantum numbers below n = 200, where the widths are not in question, their processed generated profile widths do not fit the widths of the processed lines obtained at the telescope. After processing, the halfwidths of the generated and telescope profiles must agree below n = 200 if we are to believe that the processed generated linewidths above n = 200 are meaningful. Theirs do not. Furthermore, we find that after applying the linewidth reduction factors found by Alexander and Gulyaev for their noise added profiles to our generated profiles to simulate their noise adding effect, the processed widths we obtain still do not come close to explaining the narrowing seen in the telescope lines for n values in the range 200 < n < 250. It is concluded that what is needed to solve this mystery is a completely new approach using a different observing technique instead of simply a further manipulation of the frequency-switched data.

Does atomic polarizability play a role in hydrogen radio recombination spectra from Galactic H II regions?

“…Line broadening from this colder, denser source was produced mainly by elastic electron collisions and Stark (quasi-static) broadening by plasma ions, unlike the astrophysical case, where collisions by (mainly) electrons and ions are dominant [3,[11][12][13]. Table 2, which displays some corresponding atomic collision data for elastic ion-induced dipole collisions, suggests that, while atomic polarizability becomes significant for much lower levels (n > 40) than in the astrophysical case [7][8][9][10], its role in the laboratory spectra analysed in [55] is very small.…”

“…For the spectra of Bell et al [7,8] For the purposes of integration of a function of η over a range of n-values, as in the appendix of [29], such a procedure would be entirely appropriate. On the other hand, for the astrophysical plasma conditions and atomic levels of present interest [6][7][8][9][10], equation ( 7) yields enormous ηmax -values (some 10 5 ), greatly in excess of the largest values (η ≈ 4) for which the concept of a single 'collision' is meaningful (see appendix A) for application of the criterion of Brocklehurst and Seaton [35]. Moreover, for the spectra of present interest, the first factor within parentheses in equation ( 5) is typically larger than 3.…”

“…For the physical conditions deduced in [7,8] we proceed to use equations ( 1) and (10) to establish the highest quantum principal number n corresponding to a bound state, according to the above treatment. A comparison of the two times as a function of n is shown in tables 1 and 2 for two completely different plasma conditions.…”

“…One of the observing techniques used to remove these confusing baseline features from spectra containing wide radio recombination lines in the GHz range for line shape analysis is the multiple overlap frequency-switching technique [6]. In this paper particular attention is paid to well resolved radio recombination lines obtained by Bell et al [7][8][9][10] with the aid of this method. While the theory of electron and ion impact broadening of such spectra for diagnostic purposes has been treated in [1,3,[11][12][13], our present aim is to discuss additional aspects of the atomic physics of radio recombination line formation and broadening which appear to have been neglected hitherto, as a sequel to studies in [14][15][16][17].…”

“…At the same time, the associated Stark effect of the ion microfield would introduce an additional contribution to the ion-atom interactions through a permanent atomic dipole moment [24], although this was estimated to be less important than the ion-induced dipole interaction for the laboratory spectra [20,23]. We proceed to consider the possible effects of ion-atom (ion-induced dipole) collisions and the ion microfield on the measured H II spectra described in [7][8][9][10].…”