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  • 1
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    Gas phase chemistry in comets (1976)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: The significance of gas phase reactions in determining the nuclear structure of comets is discussed. The sublimation of parent molecules such as H2O, CH4, CO2, and NH3 from the surface of the nucleus and their subsequent photodissociation and ionization in forming observed cometary molecular species are elaborated.
    Keywords: ASTROPHYSICS
    Type: NASA. Goddard Space Flight Center The Study of Comets, Part 2; p 753-762
    Format: application/pdf
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  • 2
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    The effect of solar-cycle ultraviolet flux variations on cometary gas (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The effects on comet gas of variations in the solar ultraviolet flux during solar cycle 21 are discussed. The photoionization, photodissociation, and resonance fluorescence and scattering rates of individual atoms and molecules increase by factors ranging up to 4, leading to potential order of magnitude variations in emission-line fluxes between minimum and maximum solar activity. These effects are illustrated for H2O and CO. Recent observations of comet Bradfield (1979) are discussed, and it is suggested that comets appearing near solar maximum be extensively observed to provide information on the response of cometary gas to solar flux variations, which can be used to discriminate between cometary models.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 241
    Format: text
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  • 3
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    Sodium D-line emission in Comet West /1975n/ and the sodium source in comets (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: A spectrum of the sodium D-line emission from Comet West (1975n) taken at heliocentric distance 1.4 AU is analyzed by comparing the D-line brightness to that in the underlying continuum. Using the observations of this comet by Ney and Merrill, it is found that the dust grains which dominate the visible and infrared continua are too cool to provide the observed sodium atoms through evaporation of sodium metal or a sodium compound from grain surfaces. Though sodium metal may evaporate from a small-grain component, it is suggested that molecules embedded in the volatile nuclear matrix are a more plausible source of sodium. The relationship between this source and the interstellar sodium abundance is discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 240
    Format: text
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  • 4
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    Molecular diagnostics of interstellar shocks (1980)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: The chemistry of molecules in shocked regions of the interstellar gas is considered and calculations are carried out for a region subjected to a shock at a velocity of 8 km/sec. Substantial enhancements are predicted in the concentrations of the molecules H2S, SO, and SiO compared to those anticipated in cold interstellar clouds.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 236
    Format: text
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  • 5
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    An analysis of the coma of comet Bennett 1970 II (1978)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Brightness profiles for emission features of H2O(+) in comet Bennett 1970 II are analyzed, taking into account the role of chemical reactions in the coma. By comparing the rates of transport processes derived from the brightness profile with known chemical rate constants, upper limits on the abundances and production rates of H2O, CH4, NH3, and other possible coma constituents are found. The derived upper limit on the H2O production rate inside 10 to the 4th power km is less than the observed OH production rate averaged over the coma of this comet. It is concluded that the brightness profiles of H2O(+) and OH in comet Bennett 1970 II which are presently available are inconsistent with production of OH primarily by photodissociation of H2O molecules sublimating from the nucleus. The existence of an extended source of H2O is not ruled out.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 225
    Format: text
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  • 6
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    Ultraviolet absorption studies of H2O and other species in comet comae with satellite telescope-spectrometers (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: It is noted that ultraviolet absorption by H2O and other species in the comae of comets could be detected by studying, with satellite telescope-spectrometers, the occultation of hot stars by comets. Observations of this type could produce the first detection of H2O, the fundamental parent molecule in comet comae, and give measures of molecular level populations. The capabilities of the High Resolution Spectrograph on Space Telescope, the first instrument suitable for such observations, are discussed. A Haser model is used to estimate the molecular column densities and to predict equivalent widths for lines of H2O, OH, CO, C, and O as functions of time and angular distance from a comet with a high H2O production rate. The minimum detectable equivalent widths are determined, thereby giving the maximum angular separation from such a comet at which H2O, OH, and CO could be studied. Estimated equivalent widths for CO, OH, and the resonance lines of C and O suggest that these species may also be detected.
    Keywords: ASTROPHYSICS
    Format: text
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