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Astron. Astrophys. 325, 1237-1248 (1997)

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Injection of large grains into orbits around comet nuclei

M. Fulle

Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, I-34131 Trieste, Italy

Received 21 August 1996 / Accepted 16 January 1997


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 km3. This last estimate is the only one which depends on quantities known only poorly, as the dust to gas ratio and the boulder bulk density and scattering efficiency.

Key words: comets: general – comets: individual: 1P/Halley, 46P/Wirtanen


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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998