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• W51996013 abstract "The wavelength band between 60 and 120 ym is particularly important for observations of the interstellar medium, for it is here that the cores of galactic molecular clouds radiate most of their energy. Both continuum radiation from dust grains and line radiation from ions, atoms, and molecules are important cooling mechanisms for material warmed b the intense UV radiation from newly formed sfars. Velocity gradients within the clouds are so small that the resolving power of the spectrometer should exceed 106, if the intrinsic line shapes (and the information they contain) are not to be degraded.. Heterodyne receivers capable of such resolution have so far only been operated at wavelengths >118 ym (2.5 THz) [1]. Of particular interest to astronomers are observations of the fine structure line from neutral oxygen (0 I) at 63 jim (4.75 THz). This transition is the major cooling line for the warm (T 200 K) and dense (n > 10cm -3 ) gas in the cloud core. Of course this radiation cannot penetrate the earth's atmosphere, so all observations must be done above the tropopause with aircraft or space-borne instruments. To date only grating and Fabry-Perot spectrometers have been successfully used for observations of the oxygen line. albeit with far less spectral resolution than that possible with heterodyne techniques. We can estimate some of the line parameters for the oxygen line by comparison with another fine structure line emitted by these clouds: the 158inn line of ionized carbon (C II). This longer wavelength line has been observed with a laser heterodyne spectrometer aboard NASA's Kuiper Airborne Observatory (K AO) for over 7 years now [2]. Heterodyne observations at 3 MHz (0.5 km sI ) spectral resolution have shown that the C emission may in some sources be optically thick, in which case its function as a major cooling mechanism is somewhat impaired. For the conditions expected in cloud cores, emission. from the 63 ,um line should be even more optically thick (saturated), but no direct spectroscopic evidence exists. Up to now most theoretical analyses of excitation temperatures in photodissociated gas have been based simply on the observed intensity ratio of unresolved 158 ym carbon and 63 ym oxygen lines, which are assumed to be optically thin. Therefore the derived excitation temperatures (and element abundances) could be erroneous if either line is optically thick. A better way to determine the gas excitation temperature would be to measure the peak brightness temperatures of resolved 0 I and C II lines. If a case for optically thick emission can be made, then the peak temperature yields the excitation temperature directly." @default.
• W51996013 created "2016-06-24" @default.
• W51996013 creator A5041623688 @default.
• W51996013 date "1995-03-01" @default.
• W51996013 modified "2023-09-26" @default.
• W51996013 title "A Practical Schottky Mixer for 5 THz" @default.
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