Physics
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt.........5s&link_type=abstract
Thesis (PHD). UNIVERSITY OF CALIFORNIA, SAN DIEGO , Source DAI-B 59/01, p. 260, Jul 1998, 119 pages.
Physics
Dust Grains
Scientific paper
Significant constraints are placed on the physical and chemical properties of cometary dust grains by analyzing mid-infrared imaging data obtained with the UCSD 'Golden Gopher' infrared camera at the 1.5-meter telescope at Mt. Lemmon Observatory. Dust grains are modeled as Mie spheres composed of either amorphous olivine (a silicate glass) or an organic residue mixture, and the results show that the optical characteristics of the two materials are quite different. The equilibrium temperatures of organic grains are much higher than for olivine grains (for the same size grains), and their respective mid-infrared emission properties are rather distinct. The results of these calculations are applied to multi-wavelength imaging of comets C/1996 B2 Hyakutake and C/1995 O1 Hale-Bopp, and it is found that the observed thermal emission from these comets can be duplicated by emission from a mixture of olivine and organic grains. For Hyakutake, the best fit to the data is achieved using 1 μm olivine grains and 7 μm organic grains, with an olivine mass fraction of 10%. For Hale-Bopp the predominant grain radii are 1 μm for olivine and 2.5 μm for organics, with a much higher olivine mass fraction of 40%. Dynamical simulations are performed using these grains in order to interpret 11.7 μm images of comet 109P/Swift-Tuttle taken over a three week period in November 1992. The nucleus of the comet is found to be in a simple rotational state (at least over short time-scales), undergoing prograde rotation with an obliquity of 45o± 10o relative to its orbital plane. At perihelion, the angle between the rotation axis and the Sun is 115o± 10o, so that the 'Southern' hemisphere of the comet is generally sunward. Assuming a spherical nucleus rotating with a period of 67.5 hours, three major emission regions on the surface are identified from the recurring jet pattern. The observed dust morphology is best simulated using a grain size distribution of the form f(a)~ a-β for grain radii in the range 0.6 < a < 10/ μm with β = 2.5 ± 0.5, and using jets whose opening angles are between 50o and 80o.
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