REFERENCES EVANS, N. J. II, CHEUNG, A. C. and SLOANAKER, R. M. [1970] Astrophys. Journ., 159, L9. FOMALONT, E. B. and WELIACHEW, L. N. [1973] Astrophys, Journ., 181, 781. GARDNER, F. F. and WHITEOAK, J. B. [1974] Nature, 247, 526. KERR, F. J. and SANDQVIST, Aa. [1970] Astrophys. Lett., 5, 59. KUTNER, M. and THADDEUS, P. [1971] Astrophys. Journ., 168, L67. KUTNER, M., THADDEUS, P., JEFFERTS, K. B„ PENZIAS, A. A. and WILSON, R. W. [1971] Astrophys. Journ., 164, L49. PALMER, P., ZUCKERMAN, B., BUHL, D. and SNYDER, L. E. [1969] Astrophys. Journ., 156, L147. SNYDER, L. E„ BUHL, D„ ZUCKERMAN, B. and PALMER, P. [1969] Phys. Rev. Lett., 22, 679. THADDEUS, P„ WILSON, R. W„ KUTNER, M„ PENZIAS, A. A. and JEFFERTS, K. B. [1971] Astrophys. Journ., 168, L59. TOWNES, C. H. and CHEUNG, A. C. [1969] Astrophys. Journ., 157 L103. TUCKER, K. D., TOMASEVICH, G. R. and THADDEUS, P. [1971] Astrophys. Journ., 169, 429. WELCH, W. J. [1970] Bulletin of the American Astronomy Society, 2, 355. WHITEOAK, J. B. and GARDNER, F. F. [1970] Astrophys. Lett., 5, 5. ZUCKERMAN, B., BUHL, D., PALMER, P. and SNYDER, L. E. [1970] Astrophys, Journ., 160, 485. ANNEX V THE CARBON MONOXIDE LINES 1. Introduction Carbon monoxide is a diatomic molecule with a simple rotation spectrum [Gilliam et al., 1950]. This molecule was first detected in interstellar space in 1970 when line emission from l2Cl6O at 115.2712 GHz, l3Cl6O at 110.2014 GHz and l2Cl8O at 109.7822 GHz was discovered [Wilson et al., 1970; Penzias et al., 1971], The most intense line, that of l2Cl6O, has been found to be readily detectable over much of the area near the plane of our galaxy, as well as in the directions of ionized hydrogen regions, interstellar dust clouds, supernova remnants and an infra-red star. This emission typically covers a considerably larger area of the sky than the optical size of the object. The corresponding l3Cl6O lines are generally present with between one third to one tenth the intensity of the more common isotope. The weaker l2Cl8O line has been detected in the strongest sources and found to be about one fifth that of the l3Cl6O line. 2. General Perhaps because it is, by a considerable margin, the most stable of the known interstellar molecules [Brewer et al., 1948], it appears, after molecular hydrogen, to be the most abundant. Its wide distribution makes it an extremely useful tool in the study of a variety of astronomical problems pertaining to the chemical composition, mass motion, excitation and density distribution of interstellar space. It permits the study of galactic motion in direct analogy with the 1420 MHz line of neutral atomic hydrogen. The much shorter wavelength of the CO line affords a large increase in resolution: 1' arc for the CO line on a 11 m diameter millimetre wave antenna compared with 10' arc resolution for the hydrogen line on a 100 m antenna. Furthermore, by observing several CO isotope lines one can obtain opacity information in the dense regions near the centre of the galaxy. The observed CO column densities are so high that much of the interstellar carbon must be in the form of CO, enabling us to investigate the total mass of hydrogen from the C/H ratio. 3. Techniques All the present work has been done with a receiver of almost two orders of magnitude less sensitive than those now used at the hydrogen-line frequency. With the advent of cryogenically cooled receivers for this work, a reduction in noise of at least a factor of ten seems likely. Such an improvement in sensitivity will undoubtedly reveal many fainter areas of CO. The most important of these will be external galaxies which may be expected to have Doppler shifts of several hundreds of MHz from the rest frequencies of the lines. The widest CO lines observed so far, those near the galactic centre, require a frequency band at least 100 MHz wide for their study. REFERENCES BREWER, L., GILLES, P. W. and JENKINS, F. A. [1948] Journ. Chern. Phys. 16, 797. GILLIAM, O. R., JOHNSON, C. M. and GORDY, W. [1950] Phys. Rev. 78, 1940. PENZIAS, A. A., JEFFERTS, K. B. and WILSON, R. W. [1971] Astrophys. Journ. 165, 229. WILSON, R. W., JEFFERTS, K. B. and PENZIAS, A. A. [1970] Astrophys. Journ., 161, L43.
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