4. The properties of the emission lines The observations of the OH emission lines have similarly opened up a new and complex area of astronomical research. Early attempts to detect OH emission were confined to observations at 1667 MHz because it was thought this would be the strongest transition, but all attempts gave negative results. When OH was finally discovered in emission at 1665 MHz, not 1667 MHz, its properties were so unusual that two out of the first three groups of radio astronomers to observe it did not attribute the line to OH. Australian observers first observed OH emission in June, 1964, when a narrow, intense emission line was detected to the side of an absorption line at 1665 MHz in the direction of the galactic centre [McGee et al., 1965]. However, the effect was apparently thought to be an instrumental effect and was not reported at the time. Six months or more later, OH emission was observed by groups at Harvard and Berkeley, with properties so unexpected that the Berkeley radio astronomers thought they had detected an unidentified micro-wave spectral line; they nicknamed the line “mysterium” until identification could be established. Although “mysterium” was almost immediately identified as OH, the name can hardly be called a misnomer in view of the strange properties exhibited in interstellar OH. The first OH emission sources found, were characterized by intense, narrow emission lines at 1665 MHz, with lines of lesser intensity at the other three OH frequencies. Departures by several orders of magnitude from the expected intensity ratios were the rule; furthermore, the spectrum obtained at one frequency was usually completely uncorrelated with that obtained at another. This early type of OH emission source is now recognized as only one of three distinctly different types of such sources. A second type radiates solely in the 1720 MHz line, while the third type has strongest emission at 1612 MHz but may be accompanied by weaker emission at both 1665 and 1667 MHz. While the peculiarities of these three types of emission source may appear random, in fact this is not so. Observationally, each type is rather well defined with respect to the relative intensities among the four lines, and each type also shows distinct polarization properties. Theoretically, models have been developed which, although probably not unique, do appear to explain the salient properties of each type of source. The objects with which the various types of OH emission source are associated, are a major factor in the models proposed to explain them. Unlike the 21-cm emission of atomic hydrogen which is widely distributed throughout the Milky Way, each type of OH emission source is found only in special regions. The type associated with strong 1665 MHz emission is found only in isolated positions near ionized hydrogen regions, at positions where strong sources of infra-red are often found; while the type associated with strong 1612 MHz lines comes from the atmospheres of cool stars. The sources emitting in only the 1720 MHz line have a more obscure origin, although they are definitely not associated with stellar-like objects. Between 1968 and 1971, a total of five different emission lines have been discovered which arise from transitions within excited rotational energy states of OH [Zuckerman et al., 1968; Yen et al., 1969; Turner et al., 1970]. Many other lines from these states which theoretically ought to be present are not found. This situation offers evidence as strong as that provided by the peculiar properties of the 18-cm lines, that the excitation of OH is in general not consistent with a medium in thermodynamic equilibrium. Observations of excited-state lines have also proved very valuable in distinguishing between the various theoretical models for anomalous excitation of OH. 5. The O,8H and O,7H lines The strong OH absorption found in the direction of the galactic centre has made another experiment feasible. All the results discussed above have referred to the abundant isotopic species OI6H, but the observation of strong absorption suggested the possibility that the isotopic species OI8H and possibly also O|7H could be detected. The frequencies of these isotopes are shifted relative to those of the OI6H lines because of massdependent effects on the molecular energy levels. Early searches for the OI8H lines were successful [Rogers and Barrett, 1966; Wilson and Barrett, 1970]. Although weak as expected, both main lines (at 1637 and 1639 MHz) were detected in the galactic centre, and yielded an abundance ratio OI8H/OI6H of 1/390, which is consistent with the terrestrial ratio for Ol8/O'6 of 1/490 within the uncertainties in the observations. Laboratory measurements of the 18 cm lines in the ground states of OI8H and OI?H have been made [Ball et al., 1970]. Because of the nuclear spin of the O17 nucleus, there are many more hyperfine transitions within the ground state of OI7H than in the other two isotopes. However, the line strengths of only two of these 0|7H lines are strong enough to warrant astronomical searches with present equipment. These two lines, at 1624.4 and 1626.2 MHz have been found in the direction towards the galactic centre. 6. Positions and sizes of OH emission sources The positions of a number of the OH sources have been determined to an accuracy of better than one second of arc [Rogers et al., 1967], The results have shown that an emission source can be a number of very small bright regions separated by angular distances of a few seconds of arc. The angular sizes of the regions themselves have been studied with interferometers of ever increasing base lines and angular resolutions. First results come with spacings between the antennae of 127 km, or 7 x 105 wavelengths [Davies et al., 1967], and these showed that one emitting region was less than 0.05 seconds of arc in size.
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