opens up possibilities of using radio sources of very small angular size as position references for the location of the observing points on the Earth. Possible uses in measuring earth tides, continental drift, and accurate terrestrial rates of rotation, have been predicted [Gold, 1967; MacDonald, 1967], The discovery of lines due to OH is of great astrophysical significance. The extent by which the observed frequencies are displaced from their rest frequencies by Doppler effects provides direct information about the motions of the gas clouds in which the OH occurs. The anomalies in the line intensities, both in emission and absorption, throw new and unexpected light on the physical conditions in particular regions of our galaxy. The observations also give more information on the structure of our galaxy. 2. The distribution of OH in the galaxy Early survey work [Goss, 1968] showed, and later more thorough work [Turner, 1970; Turner, 1972] confirmed, that OH is very widely distributed throughout the galactic disk in a region within a few hundred parsecs * of the galactic plane. It is somewhat more closely confined to the plane than atomic hydrogen, and appears to be an excellent tracer of spiral arms. It does not appear to be closely associated with continuum sources, but rather to delineate the presence of dense neutral clouds of gas in a manner which is not possible optically or by means of 21-cm line of atomic hydrogen. This latter aspect has been of immense importance in the burgeoning studies of many other newly discovered interstellar molecules, as these molecules have been found in just these same dense clouds. In a literal sense, OH has proved to be the tracer molecule in the new field of interstellar molecular chemistry. Apart from its location in dense neutral gas clouds, OH has been discovered in some types of infra-red stars. A variety of other types of astronomical objects, such as planetary nebulae, Wolf-Rayet and T-Tauri stars, young and highly reddened stellar associations and clusters, do not yield observable OH to very low limits. It is anticipated that studies of physical conditions in these objects will shed light on the questions of formation and destruction of interstellar molecules in general, and OH in particular. Detection of OH in both absorption and emission has been made in comets, and research in this area is having important implications as to the origin and history of the solar system. 3. The properties of the absorption lines The anomalies in the intensities of the lines when observed in absorption, continue to pose important problems, as these relative intensities in the observed lines sometimes differ considerably from the expected values for a source in thermodynamic equilibrium. For example, the absorption spectrum of the supernova remnant, Cassiopeia A, was examined at the frequencies of the two satellite lines, 1612 and 1720 MHz. Whereas both the 1665 and 1667 MHz transitions show splitting into two lines, neither the 1612 nor the 1720 MHz transition shows any splitting. Furthermore, the absorption at 1612 MHz is approximately one half of that at 1665 MHz, instead of one fifth, as expected. Finally, the observations indicate a small amount of OH emission at 1720 MHz, but none is detectable at 1612 MHz. The unusual absorption features and the presence of weak emission have been independently confirmed. It is very difficult to see how any of these observations can be explained in terms of a medium in thermodynamic equilibrium, and these observations are among the many that require further explanation. Other absorption anomalies have been found in the'spectrum of the galactic centre [Robinson et al., 1964; Goldstein et al., 1964; Goss, 1968; Bolton et al., 1964b]. For example, the observed ratios of line intensities among the four lines are quite different from the ratios that are expected on the basis of thermal equilibrium. It is tempting to try to explain the observed values in terms of a medium exhibiting large attenuation, for which all ratios would approach unity, but this attempt fails. For example, isolated regions near the galactic centre show stronger absorption at 1665 than at 1667 MHz, and many regions show unequal absorption at 1612 and at 1720 MHz. It seems clear that these observations require the assumption of non-thermal distribution of the OH molecules among the internal energy states from which the radio lines are derived. The OH observations in the galactic centre pose problems other than anomalous line-intensity ratios. OH absorption is very strong and extends over a considerable frequency, or velocity range. If OH profiles for the galactic centre are interpreted in a manner similar to that described for Cassiopeia A, then one derives on OH/H abundance ratio of ~ IO-4 some 103 times the ratio in the direction of Cassiopeia A. However, this value must be used with extreme caution, because any derived values, based upon observations, must await a full understanding of the mechanism by which OH is formed, how it is distributed with respect to its internal energy levels, and how it absorbs and emits radio energy. From the frequency at which OH absorption occurs, one can determine the velocity in the line-of-sight of the OH cloud relative to the Sun; and it is found that much of the OH is moving toward the galactic centre with a velocity of 40 km/s. This cloud also contains other molecules and neutral hydrogen. On the other hand, most of the other gases associated with the galactic centre appear to be streaming away from the centre. The motions in the central region are clearly very complex. This is a curious situation and may force a revision of our ideas about the physical conditions in the galactic nucleus. It should be emphasized, though, that the observations tell us nothing about the distances to the OH and H; the concept that they are associated with the galactic centre rests strictly on interpretations of the accumulated evidence.
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