Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt..........x&link_type=abstract
PhD Dissertation, Pennsylvania Univ. Philadelphia, PA United States
Astronomy and Astrophysics
Astronomy
5
Atmospheric Models, Cometary Atmospheres, Monte Carlo Method, Radio Observation, Comets, Hydroxyl Emission, Radio Astronomy, Hydroxyl Radicals, Hydrogen, Transport Properties, Velocity Distribution, Mass Spectrometers, Giotto Mission, Water, Molecules, Anisotropy, Dynamic Models, Radio Sources (Astronomy)
Scientific paper
Isotropic and axisymmetric models of a cometary atmosphere, made up of H2O and its daughter radicals (H, OH, O and H2) have been computed using the Monte Carlo particle transport method. The approach on which the Monte Carlo models is based was checked against the cases of purely adiabatic expansion and of a vectorial model for OH, and was shown to be consistent with the results from other numerical techniques. The simulated velocity profile of water molecules in the optically thick coma (up to r = 10,000 km) is in good agreement with Giotto NMS (neutral mass spectrometer) measurements of comet Halley in Mar. 1986. The detailed fit to the measured velocity profile in the outer coma requires the inclusion of rotational cooling of water molecules in this transition region from the optically thick to optically thin regime. The axisymmetric model shows that the outward expansion of water molecules is mainly radial with negligible mean lateral velocity from the intermediate coma to the outer coma. The outflow velocity of water molecules increases with increasing local production rates. The isotropic model was also applied to comet Levy and it is shown that the simulated results can fit the 18-cm line profile by assuming an anisotropic distribution in the coma. The OH 1667 MHz transition is saturated (optically thick) from the inner coma to the intermediate coma and this results in a total reduction of about 25 percent in the retrieved gas production rate for comet Levy. The dynamic model of OH with collisions included is needed to give a better fit than the collisionless vectorial model.
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