Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988apj...327.1044c&link_type=abstract
Astrophysical Journal v.327, p.1044
Astronomy and Astrophysics
Astronomy
44
Comets, Particle Acceleration, Comets, Monte Carlo Methods, Particles, Trajectories, Neutral Particles, Gases, Models, Calculations, Numerical Methods, Distribution, Energy, Atmosphere, Comae, Parameters, Morphology, Lyman-Alpha Radiation, Water, Photodissociation, Hydroxyl Radical, Radiation, Pressure, Flux, Kohoutek, Photochemistry, Heating, Thermalization, Paramters, Hydrogen, Composition, Acceleration, Production Rate
Scientific paper
The Monte Carlo particle-trajectory model (MCPTM) developed in Paper 1 is applied to explain the observed morphology of the spatially extended Lyα comae of comets. The physical processes and assumptions used in the model as they relate to the photodissociation of H2O and OH and the solar radiation pressure acceleration are presented herein. For this first application, the rocket and Skylab images of the Lyα coma of comet Kohoutek were chosen for study. The self-consistent modeling analysis of these data consisted of two parts. The first part entailed using a steady state spherically symmetric inner coma MCPTM coupled with a simple gas-dynamic model to calculate the physical development of the coma, i.e., the dependence of coma temperature and outflow speed on radial distance to the center of the nucleus, as a function of the (time) heliocentric distance of the comet. The inner coma MCPTM was used to calculate correctly the photo-chemical heating of the coma due to the partial collisional thermalization of the hot hydrogen atoms produced in the photodissociation of water molecules. In the second part of the analysis the results from the first part were used in a fully time-dependent and three-dimensional extended coma MCPTM which includes the explicit calculation of partial thermalization of the H atoms by multiple collisions with coma molecules. The same physical model yielded very good matches between the modeled Lycα isophotes and those observed in both of the two very different images of comet Kohoutek. The production rate was varied in time as implied by the shape of the visual light curve. All other physical parameters were varied only according to their naturally expected heliocentric distance and velocity dependencies. The complete physical description of the inner coma provided by the coupled gas-dynamic/MCPTM calculation was needed to obtain a good fit to the data. The correct inner coma description is important since it provides not only the initial conditions for the photodissociated H atoms but also (and most importantly) the collisional targets for the H atoms produced in the innermost regions of the coma. Simplistic descriptions for the coma (single speed and perfectly radial molecular motion) do not yield realistic isophote contours. The implications of the model results as they apply to other comets, species, and a variety of conditions are also discussed.
Combi Michael R.
Smyth William H.
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