diameters giving apparent brightness temperatures greater than IO12 K; and by polarization of the radiation. The first celestial H2O sources were found in, or near, galactic ionized hydrogen clouds and positions near OH emission sources and in some cases infra-red sources. Subsequently, the line emission of water molecules was found in a number of infra-red stars [Knowles et al., 1969a; Turner eta!., 1970; Schwartz and Barrett, 1970; Schwartz, 1971; Dickinson et al., 1973], and in external galaxies [Churchwell et al., 1977]. The discovery of the intense non-thermal radio radiation from water molecules in space, is of great astrophysical significance and practical importance. It provides a new means for investigating the distribution of molecules in the Universe, the motions of the gas clouds, and the physical conditions in space which support the maser amplification process. These intense, very small radio sources in the sky give a new potential for applications as known radio beacons, and with the very long base-line interferometry a technique for accurate position determination on earth over great distances. Thermal emission or absorption by water molecules in space has not yet been detected and it is important to extend the research to weaker signal levels to determine the distribution of water molecules in the Universe. 2. Spectral properties Spectrometry with high resolution shows the H2O source spectra to be complex composites of intense, narrow lines which are Doppler-shifted in frequency by different amounts depending on the velocities in the line-of-sight of the emitting gas clouds, with respect to the Earth. The broadest known spectrum, in the source W49, shows lines spread over a range of line-of-sight velocities of at least ± 240 km/s corresponding to a frequency spread of 37 MHz. Individual lines have total widths at the half intensity point as small as 40 kHz (corresponding to about 0.5 km/s in the line-of-sight velocity), the range of line widths extends up to three times this value, and lines at different Doppler shifts blend together to produce broad spectral features. Individual line intensities range up to a flux density of approximately 10-21 W/m2 • Hz and vary with time. In some cases, order of magnitude intensity variations are observed on a time scale of weeks, while some lines vary little over time periods of a year or more. An increase in line intensity is, in many cases, accompanied by maser narrowing of line width. Small variations of line frequency with time are observed, and are probably the result of intensity variations in unresolved line blends. The intensity variations of individual lines are largely unrelated, indicating separate emission regions, and neither order nor periodicity has been found in most sources. Exceptions are the H2O sources associated with long period variable stars, where the H2O emission varies periodically in phase with the stellar light variation. 3. Polarization properties Linear polarization of a magnitude up to approximately 50% has been observed in some lines, but the linear polarization of the majority of the observed lines is less than 10%. The degree of polarization observed in the Orion Nebula varies with time in the same sense as the line intensity. Circular polarization has not been detected. 4. Sizes and positions of H2O sources The sizes and positions of the H2O sources are currently being investigated by the technique of very long base-line interferometry. The intense source, W49, has been most intensively studied at increasingly longer base lines up to 7375 km (547 million wave lengths) where the fringe spacing is 0.00038 arc seconds. At this base line, the strongest emission source is partially resolved; this result is consistent with a uniformly bright source of diameter 0.0003 arc seconds. In comparison, the sizes of OH sources in the same vicinity are about 0.05 arc seconds. The H2O sources in the Orion region, which is about 30 times nearer Earth than the W49 region, are in some cases partially resolved at a base line ten times shorter. At shorter base lines, the relative positions of the sources, each of which corresponds to an individual line in the W49 spectrum, have been measured. They are found to scatter over an area of sky about 1.5 arc seconds in diameter. There is no obvious correlation between the positions and the line-of-sight velocities of the lines which would indicate systematic motions of the water molecule clouds. The absolute positions of the most intense lines were measured to 1" x 10" arc accuracy and there is agreement, within this range of uncertainty, with the position of the OH source. The dimensions of the space occupied by the Orion and W49 sources are roughly equal, despite a difference of over three orders of magnitude in their intrinsic intensities. Accurate interferometer positions have been measured for several of the intense H2O sources [Hills et al., 1972] confirming the close association of H2O and OH sources, and in some cases, close association in position with infra-red sources and compact ionized hydrogen regions. These experiments demonstrate that H2O sources can be used for interferometry over the maximum distances allowable on Earth for astrophysical, geodetic, and geophysical investigations.
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