The OH radical in comets - Observation and analysis of the hyperfine microwave transitions at 1667 MHz and 1665 MHz

Details The OH radical in comets - Observation and analysis of the hyperfine microwave transitions at 1667 MHz and 1665 MHz The OH radical in comets - Observation and analysis of the hyperfine microwave transitions at 1667 MHz and 1665 MHz

Jun 1981

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981a%26a....99..320d&link_type=abstract

Astronomy and Astrophysics, vol. 99, no. 2, June 1981, p. 320-340.

Astronomy and Astrophysics

Astronomy

80

Abundance, Comets, Hydroxyl Radicals, Hyperfine Structure, Microwave Spectra, Radio Astronomy, Brightness Temperature, Comet Nuclei, Hydroxyl Emission, Line Spectra, Molecular Clouds, Transition Probabilities, Anisotropy, Telescopic Observations, Kobayashe-Berger-Milon Comet, Comae, Scalelength, Comets, Hydroxyl Ion, Microwaves, Observations, Spectrum, Analysis, Radiation, Emissions, Distance, Procedure, Ultraviolet, Data, Wavelengths, Radio Astronomy, Brightness, Asymmetry, Production Rate, West Comet, Enc

Scientific paper

Observations: the main lines of OH at 1667 MHz and 1665 MHz were searched in five comets: Kobayashi-Berger-Milon (KBM) (1975, IX), West (1976, VI), Encke (1977, XI), Kohler (1977, XIV), - and Bradfield (1978, VII). Both transitions were observed in C/West and Kohler: the 1667/1665 line ratio is compatible with the LTE value of 1.8. At a heliocentric distance (rh) of 1 AU the 1667 MHz profiles have a line width (FWHP) of 3 km s-1, probably decreasing with increasing . The line profile is generally asymmetrical with respect to the nucleus radial velocity but there is no significant radiation beyond 2 km s-1.
The OH cloud is extended:1 667 MHz emission (or absorption) is detectable at 3'.5 from the nuclei of both comets and up to 7' in C/Kohler where a strong East-West asymmetry was found.
C/KBM and Bradfield were also detected but the signal is an order of magnitude fainter than in C/West and Kohler. Periodic comet Encke was never detected despite lengthy integration times.
Analysis: the UV pumping model developed by Biraud et al. (1974) to explain the observations of C/Kohoutek (1973, XII) was improved by including the υ = 1 levels of the Χ2π and A2Σ+ states and is confirmed by the present data: C/KBM, West, Kohler and Bradfield underwent a reversal of the 1667 MHz line, close to the heliocentric radial velocity (υh) expected from the model. In the case of C/Kohler a quantitative verification of the model predictions was possible since both the heliocentric and geocentric distances remained constant (rh = Δ = 1 AU) during one month while Vh increased from -10 to +10 km s-1. All published 1667 MHz data were interpreted using Haser's model (1957) for the OH coma. The scalelengths of the OH radical (LOH) and its parent molecule (Lp) were determined in C/Kohoutek, West, and Kohler by comparing the brightness temperatures measured at the nucleus and offset positions with the Nançay radio telescope and/or at the nucleus only with the Nançay and another radio telescope. With a typical value of 1 ± 0.25 105 km for Lp (the scalelength of H2O) one finds LOH = 6+3-1.5 105 km at rh = 1 AU. The lifetime of the OH radical is then 4+2-1 105 at 1 AU. The model fitting is better if one assumes that the OH population inversion is quenched by collisions in the inner coma. The production rate of the parent molecule (Qp) of OH was computed for comets Kohoutek, West, P/d'Arrest (1976, XI), P/Encke (upper limit), Kohler, Bradfield and Meier (1978, XXI) after its first detection in May 1978 (Crovisier et al., 1978). For new comets there is a remarkable proportionality between Q0p (Qp reduced to 1 AU assuming a 2 dependence) and B0 (the concomitant visual brightness assuming a rh-2 dependence) and B0 (the concomitant visual brightness reduced to a rh-4 dependence).
Finally the 1667 MHz profile asymmetry and/or the East-West asymmetry observed in C/West and Kohler strongly suggest the existence of the Greenstein effect while other effects like anisotropic outgassing are probably present. Key words: comets - OH radical - 18 cm radiation

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