Radio recombination lines

Abstract. New results of the Primordial Helium abundance (Yp) measurement by radio recombination line (RRL) observations from five galactic HII regions are presented. The RRL observations were carried out with two telescopes RT32 (22.4 and 8.3 GHz, Medicina, Italy) and RT22 (36.5 and 22.4 GHz, Pushchino, Russia). The results of the first run of the low frequency RRL observations (408 MHz) with the Croce del Nord radiotelescope (Medicina Observatory, Italy) are also presented. 

Helium-4 can be observed in galactic and extragalactic HII regions - regions of hot and ionized gas - using either optical or radio recombination lines. However the best determinations come from observations of Hell —> Hel recombination lines in extragalactic HII regions. 

Te can be derived using the ratio of the two lines [O III] A4363 and [O III] A5007, which have very different excitation potentials. Other line ratios can also be used as temperature indicators in nebulae, such as [N ii] A5755/6584 and [S III] A6312/9532. The Balmer and Paschen jumps, the radio continuum and radio recombination lines also allow to estimate the electron temperature, but the measurements are more difficult. 

Another interesting correlation is that noticed by Churchwell et al. (1974) between the ratio of the abundances He /H (derived from radio recombination line observations) and the infrared excess. They find that the ratio He /H decreases as the IRE increases, and... 

Radio continuum observations are widely used for the interpretation of IR-observations. We therefore limit the review in Sect. II. to recent high frequency single dish observations (II.1) and aperture synthesis (SRT) observations (II.2). IR-observers probably are less familiar with radio recombination line observations and their application to the interpretation of IR-observations. Therefore more space is devoted in Sect. Ill to recent developments in this field. 

Figure 1 shows the 109a recombination spectrum observed with the MPIfR 100-m telescope at the center of W3, main component. One recognizes the 109a recombination lines of H, He and C as well as the higher order transition H137g. From radio recombination lines one derives the following information ... 

A special symposium on "HII regions and related topics" which summarizes radio observations of HII regions up to 1974 was held in Mittelberg, Austria, January 13 through 17, 1975. The proceedings of this symposium are published and will be referred to as (1975 Lect. Notes Phys. 42). In the following Sect. II (radio continuum) and III (radio recombination lines) we try to emphasize recent observational results not yet covered in these proceedings. 

Zhao, J. H., Desai, K., Goss, W. M., and Yusef-Zadeh, F. 1993, VLA Radio Recombination Line Observations of Ionized Gas in the —30 km s" Molecular Cloud fG0.04 + 0.03j near the Galactic Center I. The Discrete Radio Sources, preprint. 

Fig. 20. Spectra of the 1 j 0 - 1 j j transitions for three isotopic species of formaldehyde seen in absorption against the radio source Sagittarius B2. The ordinate is the ratio of line/continuum antenna temperatures. The profiles are aligned at a velocity of 62 km/sec with respect to LSR. The center of the H141/3 recombination line for a velocity of 62 km/sec is indicated on the H213C16C) profile (from Gardner et al., 1971)...

The existence of molecular species in interstellar space has been known for almost seventy years. The first observations involved the electronic spectra, seen in absorption in the near-ultraviolet, of the CN, CH [28] and CH+ [29] species. Radiofrequency lines due to hydrogen atoms in emission [30] and absorption [31], and from the recombination of H+ ions with electrons were also known. However, molecular radio astronomy started with the observation of the OH radical by Weinreb, Barrett, Meeks and Henry [32] in 1963 in due course, this was followed by the discovery of CO [33]. In the subsequent years over 110 molecules have been observed in a variety of astronomical sources, including some in galaxies other than our own. Nearly a third of these are diatomic molecules, with both closed and open shell electronic ground states, and some were observed by astronomers prior to being detected in the laboratory. 

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