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(gzipped) PostScript## Injection of large grains into orbits around comet nuclei
Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, I-34131 Trieste, Italy
Starting from the nucleus surface, the motion of dust boulders of diameters
larger than the solar radiation pressure resonance size (Richter & Keller
1995) is investigated by numerical integration of the motion equation for
times larger than the comet orbital period. The motion equation takes into
account the comet nucleus gravity force, the solar gravity force, the solar
radiation pressure force and the gas drag force acting on the test particle.
The ejection anisotropy and the gas loss rate are assumed to be power laws of
the solar zenithal angle cosine and of the sun comet distance, respectively.
We search for the ejection cometocentric coordinate values corresponding to a
final motion bound around the comet nucleus, and, for each starting coordinate,
we investigate when the gas drag is strong enough to lift the grain from the
nucleus surface, thus determining the ejection time of the grain. Then, we
compute the fraction of grains in bound orbits in the hypothesis of a nucleus
which corotates with its orbital motion. The convolution of this fraction with
the dust number loss rate and the dust size distribution provides an estimate
of the total dust mass orbiting around the comet nucleus in a stable debris
cloud, and its size and mass distributions. The model is applied to comets
1P/Halley and 46P/Wirtanen, the targets of the GIOTTO and ROSETTA missions,
respectively. For both comets, the debris cloud mass is insensitive to a dust
size distribution power index between -3.3 and -3.7 (Fulle et al. 1995). For
1P/Halley, the mass fraction in the cloud is between 1.3% and 1.7%, the mean
radius of the cloud is 500 km, and the boulder sizes range from 0.1 m to 4 m.
For 46P/Wirtanen, the mass fraction in the cloud is between 0.07% and 0.1%,
the cloud mean radius is 200 km, and the boulder sizes range from 0.1 m to
0.3 m. For both comets, the mean number density in the cloud is close to one
boulder per 100 km
© European Southern Observatory (ESO) 1997 Online publication: September 9, 1997 |