Rep. 223-4 SECTION 2E: RADIOASTRONOMY AND RADAR ASTRONOMY Recommendations and Reports REPORT 223-4 LINE FREQUENCIES, ARISING FROM NATURAL PHENOMENA, OF INTEREST TO RADIOASTRONOMY AND RELATED SCIENCES (Question 5-1/2) (1963 - 1966 - 1970 - 1974 - 1978) The amount of scientific information obtained from radioastronomical observations has greatly increased in recent years. Technology improvements in receivers and antennae have been partly responsible for this increase. Astronomical observations have now been made from ground based observatories using radio techniques, at wavelengths shorter than 1 mm (300 GHz), employing cooled bolometric detectors or various solid state devices. Thus from the point of view of availability of equipment, the entire radio spectrum can now be employed for astronomical measurements. Another reason for the rapid growth of interest, particularly in that part of the spectrum from 100 MHz to 300 GHz, has been the discovery of line radiation from a number of extra-terrestrial substances of astrophysical importance. First, there are many discrete spectral lines called “recombination lines” that arise when atoms of hydrogen, helium, carbon, etc. gain or lose energy as their electrons jump from one orbit to another under different conditions of atomic excitation. The first of these lines was discovered in space in 1964 by radio astronomers in the U.S.S.R. [Sorotchenko, et al., 1964], These lines are numerous and spread throughout the radio spectrum [Lilley and Palmer, 1968]. Through careful observation of the strengths and shapes of these lines, radio astronomers are able to determine the physical conditions such as temperature, density and the extent of ordered or random motions of any celestial object in which recombination lines are found. Second, there have been discoveries of emission or absorption lines arising in interstellar space due to neutral atomic hydrogen and many inorganic and organic molecules. Following the first observation of the hyperfine spin-flip transition of hydrogen in 1951 [Ewen and Purcell, 1951], it was not until 1963 that the first molecular line (OH) was detected in the radio spectrum [Weinreb et al., 1963], and not until 1968 that other molecules were observed. However, since then many more interstellar molecules have been identified, the majority by means of spectral lines at millimetre wavelengths. Table I lists, in order of increasing frequency, the lines of interstellar organic and inorganic molecules reported by radio astronomers. The first column in each table gives the frequency; the second column lists the molecular formula and name, and the third column indicates whether the spectral line has been observed in absorption (A) or emission (E). A double asterisk in this column indicates that the line is afforded at least footnote recognition by the World Administrative Radio Conference for Space Telecommunications, Geneva, 1971. It is realized that only some of these lines can be afforded protection in the Radio Regulations by allocations, and the astrophysically most important lines are listed in Report 224-4 and Recommendation 314-4. The formation mechanism for interstellar molecules is not well understood. For typical interstellar cloud densities ranging between one and one million molecules per cubic centimetre and temperatures between 10 K and 100 K, the time between molecular collisions ranges from one thousand years to one third of a day. Thus typical conditions in interstellar clouds do not enhance collisional formation of polyatomic molecules although it is possible to explain the presence of CH, CH+ and CN by invoking radiative association. Also, recent laboratory determinations of lifetime against photo dissociation [Steif et al., 1972], indicate that, given an ambient ultraviolet radiation field of 4 x 10”17 ergs cm-3 A 1 [Habing, 1968], it takes only ten years for unshielded interstellar carbonyl sulphide to be destroyed; thirty years for formaldehyde; forty years for ammonia; and one thousand years for carbon monoxide. Certainly, under these conditions an unshielded polyatomic molecule could not survive the harsh interstellar environment. Even though molecular formation is not clearly understood, there are three reasons which appear to explain the remarkable observational results which radio astronomers are now obtaining. First, all the new molecules appear to be associated with regions of obscuration or interstellar dust. Experimentally, it has been found.that molecular lifetime with respect to photo dissociation increases tremendously with each increment of visual extinction. Thus once formed, a molecule may exist many orders of magnitude longer inside a dust cloud than it can in unshielded space before ultra-violet photo dissociation occurs. Second, radio waves are not significantly attenuated by dust particles which are of sub-micron size; hence, radio signals can penetrate clouds which completely attenuate optical waves. Third, the excitation requirements are low for radio detection of molecules. For example, optical astronomers require a background source of excitation of at least 15 000 K in order to observe optical absorption lines; thus a very special geometry of a very hot star with an intervening
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