molecular cloud of low opacity places stringent conditions on optical astronomy. On the other hand, thirty times less excitation energy will produce a typical molecular rotational transition. Thus radio detection does not require a bright, hot star with an intervening molecular cloud; the background source might be an ionized hydrogen region, infra-red star, radio galaxy, or even the 3 K isotropic background radiation. As a direct consequence of the interstellar molecular radio spectroscopy of the past few years, a number of phenomena have been discovered which indicate that radio astronomers are studying a new area of chemistry. The detection of as yet unidentified molecules, such as X-ogen, emphasizes the role of a typical interstellar cloud as a unique chemical laboratory - a sample cell which would be difficult if not impossible, to duplicate in its entirety in terrestrial laboratories. With mean free paths as long as 1015 cm, low temperatures, and catalytic dust grain surfaces present, it is not unreasonable to expect future detection of interstellar molecules which cannot be observed terrestrially because of rapid collisional destruction or rapid recombination mechanisms. This new chemistry should not be considered exotic or unusual, however, as it is the common chemistry of the galaxy and man stands to gain a greater understanding of his galactic environment from it. Long, painstaking observations which often require many hours of integration are needed by radio astronomers in order to achieve initial detection of most of these lines. Many more orders of magnitude of time and effort are expended in detecting each line in other gas clouds and to obtain the signal-to-noise ratios necessary to draw conclusions of astrophysical interest from the observations. In general, bandwidths of 0.2 per cent or more of the rest line frequency are required in order to observe all components of the Doppler-shifted observed line frequency of these clouds moving along the line-of-sight within our galactic system. Wider bandwidths will of course be required for extragalactic observations. The choice of wavelength for astronomical observations naturally depends on the lines to be observed, but it may also be strongly influenced by the transmission properties of the Earth’s atmosphere, i.e., by the location of the so-called “windows” in the atmosphere. The regions of interest for ground-based observations known at present, are shown schematically in Fig. 1, based on Report 719 of Study Group 5. This Report contains the study of the attenuation in clouds at frequencies above 50 GHz. In general, the windows are defined by the resonances of molecular oxygen and water vapour. The abundance of other atmospheric constituents, for example, CO, NO, NO2, etc., is sufficiently small that their presence in a window will not significantly alter the usefulness of this window for astronomical observations. The curves in Fig. 1 show the total one-way absorption in the vertical direction for a dry atmosphere and for two representative atmospheres with water vapour concentrations of 0.5 and 7.5 g/nr at the Earth’s surface. These curves are based partly on theoretical calculations and partly on measurements. The model atmosphere adopted is one in which the water vapour concentration decreases exponentially with increasing height, the scale height being 2 km. The Annexes summarize some of the types of spectral line observations that are being conducted at observatories throughout the world. The five Annexes deal with lines of atomic hydrogen and the four molecules OH, H2O, H2CO and CO. These are not the only lines important to radioastronomy, and for historical reasons the selection does not give full weight to the millimetre wave region of the spectrum. A more complete list is given in Recommendation 314-4. However, those included in the Annexes are a reasonably representative sample showing the kinds of observation made and the variety of results obtained. The Annexes also demonstrate how some of the significant conclusions concerning the nature of the Universe can be reached on the basis of spectral line observations. REFERENCES EWEN, H. I. and PURCELL, E. M. [1951] Nature, 168, 356. HABING, H. J. [1968] Bulletin Astr. Inst. Netherlands, 19, 421. LILLEY, A. E. and PALMER, P. [1968] Astrophys. Journ. (Suppl. Ser. 144), 16, 143. SOROTCHENKO, R. L, BORODZITCH, O. S., DRAVSKIKH, Z. V. and KOLBASSOV, V. A. [1964] Proceedings of the Xllth General Assembly of the International Astronomical Union, Hamburg. STEIF, L. J., DONN, B., GLICKER, S., GENTIEU, E. P. and MENTALL, J. E. [1972] Astrophys. Journ., 171, 21. WEINREB, S„ BARRETT, A. H„ MEEKS, M. L. and HENRY, J. C. [1963] Nature, 200, 829.
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