5. Emission mechanism The observed line intensities and the very small apparent source diameters observed by very long base-line interferometer techniques, give maximum source brightness temperatures of the order of 10|S K and hence require a non-thermal process. Maser amplification by water molecules is consistent with the intensity requirements and line narrowing from reasonable interstellar conditions, and provides possible mechanisms for extreme time variability through changes in population inversion of the molecular energy levels, changes in amplification path length, and changes in beaming of the radiation. The mechanism for overpopulating the higher energy level relative to the lower energy level to allow amplification may be radiative, collisional, or chemical. The occurrence and temporal behaviour of H2O sources near coo! stars, strongly suggests radiative inversion of the H2O rotational levels via the 2 to 5 mq near-infra-red vibration-rotation bands. This mechanism, however, appears to be inadequate to explain emission from the vicinity of ionized hydrogen regions. The apparent coincidence of the OH and H2O masers in many objects suggests a common inversion mechanism, perhaps involving the creation and destruction of H2O molecules by radiation or collision but, as yet, no quantitative description of such a process exists. Theoretical and experimental work is still proceeding to define a clear preference for one unique mechanism. 6. Technical requirements Continuation of this research, and the important extension to the weak lines of normal water molecules, as well as research into the scientific applications of the very long base-line interferometer technique to make precise measurements of positions on the Earth, and earth motions for geodesy and geophysics, will require the highest possible measurement sensitivity and freedom from harmful interference. The observed lines are narrow (40 kHz) but are spread over a band of at least 37 MHz by Doppler shifts. Extra-galactic water lines are Doppler-shifted by greater amounts. Detailed measurements require receivers with a frequency resolution of less than 40 kHz, but tunable over many megahertz. The overall frequency band required for detailed study of the 22.235 GHz H2O line is at least 50 MHz. REFERENCES ABLIAZOV, V. A., ALEKSEEV, V. L, SOROTCHENKO, R. L„ TROITZKI, V. S„ ANTONETZ, M. A., ARISK1N, V. A., VEKSHIN, V. P„ GATELJUK, EG, DEMIN, V. M„ EFANOV, V. A., KUTUZA, B. G„ MINIATOV, B. N„ MATVEENKO, L. L, MKRETCHAN, S. M„ MOIJSEEV, LG, NIKONOV, V. N, ATANESIAN, V. A, SANAMIAN, V. A., SIZOV, A. S„ SOSNIN, V. P, FOTIEV, B P, TCHIK1N, H. N and TSHEKOTIEV, B. V. [1974] Izvestia Vuz, Series “Radiofizika". BUHL, D„ SNYDER, L. E, SCHWARTZ, P R and BARRETT, A. H. [1969] Astrophys. Journ. (Letters), 158, L97. BURKE, B.F, MATVEYENKO, L. I, MORAN, J. M, MO1SEYFV, I. G, KNOWLES, S. H„ CLARK, B. G., EFANOV, V. A., JOHNSTON, K J„ KOGAN. L. R., KOSTENKO, V. L, LO, K Y, PAPA, D. C„ PAPADOPOULOS, G. D, ROGERS, A. E. E. and SCHWARTZ, P. R. [December, 1971] Abstract, 136th Meeting AAS. BURKE, B. F, PAPA, DC, PAPADOPOULOS, G. D, SC HWARTZ, PR, KNOWLES, S. H„ SULLIVAN, W. T, MEEKS, M. L. and MORAN, J. M. [1970] Astrophys. Journ. (Letters), 160, L63. CHEUNG, AC, RANK, D. M, TOWNES, C. H„ THORNTON, D. D. and WELCH, W. J. [1969] Nature, 221, 626. CHURCHWELL, E„ WITZEL, A, HUCHTMEIER, W, PAULINY-TOTH, 1, ROLAND, J. and SIEBER, W. [1977] Astronomy and Astrophysics, 54, 969. DICKINSON, D. F„ BECH1S, K P. and BARRETT, A. H. [1973] Astrophys. Journ., 180, 831 HILLS, E, JANSSEN, M. A., THORNTON. D. D. and WELCH. W. J. [1972] Astrophys. Journ. (Letters), 175, L59. JOHNSTON, K. J, KNOWLES, S. H. and SULLIVAN, W. T. [1971a] Astrophys. Journ. (Letters), 167, L93. JOHNSTON, K.J., KNOWLES, S. H, SULLIVAN, W T„ MORAN, J. M„ BURKE, B. F„ LO, K. Y, PAPA, DC, PAPADOPOULOS, G. D„ SCHWARTZ, PR, KNIGHT, C. A , SHAPIRO, LI. and WELCH, W. J. [1971b] .Astrophys. Journ. (Letters), 166, L21. JOHNSTON, K. J, SLOANAKER, R M. and BOLOGNA, J. M. [1973] Astrophys. Journ., 182, 67. KNOWLES, S. H„ MAYER, C. H„ CHEUNG, A. C, RANK, D. M and TOWNES, C. H. [1969a] Science, 163, 1055. KNOWLES, S. H , MAYER, C. H , SULLIVAN, W T. and CHEUNG, A. C. [1969b] Science, 166, 221. MEEKS, M. L, CARTER, J. C, BARRETT, A. H , SCHWARTZ, P R„ WATERS, J. W and BROWN, W. E. [1969] Science, 165, 180. SCHWARTZ, P. R. [7 May, 1971] Dissertation, MIT. SCHWARTZ, P R. and BARRETT, A H [1970] Astrophys. Journ. (Letters), 159. LI23. SNYDER, L. E. and BUHL, D. [1969] Astrophys. Journ. (Letters), 155, L65. SULLIVAN, W. T. [19 July, 1971] Astrophys. Journ., 166, 321. Dissertation, Univ, of Maryland. SULLIVAN, W. T. [1973] Astrophys. Journ., SuppL, 222, 25, 393. TURNER, B. E, BUHL, D, CHURCHWELL, E. B, MEZGER, P G. and SNYDER, L E. [1970] Astronomy and Astrophysics, 4, 165. TURNER, B E. and RUBIN, R H [1971] Astrophys. Journ. (Letters), 170. LI 13.
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