MAJOR OUTBURST OF PERIODIC COMET HALLEY AT A HELIOCENTRIC DISTANCE OF 14 AU

Using a collection of deep CCD high-resolution images obtained in February-March 1991 at the European Southern Observatory, we investigate the properties and evolution of the major outburst experienced by Halley's comet at 14 AU from the Sun, some 5. years after perihelion. It is found that the ob-served crescent-shaped halo represents a segment of a conical surface populated by particulate ejecta that had been released from a suddenly activated source on the sunlit hemisphere of the comet's rotating nucleus. It is shown that the ejecta's total mass was at least approximately 10(12) g and that carbon monoxide was the most likely prime driver, accelerating the smallest grains (approximately 1 mum across) to a terminal velocity of approximately 45 m/s. The mass loading of the gas flow by the particulate matter (which includes water ice) is found to be enormous, its mass production rate exceeding the expected production rate of carbon monoxide by a factor of several tens. The derived particle size distribution function of the ejecta, g(a)da, appears to be relatively flat, is-proportional-to a-3.7 da for larger grains and is-proportional-to a-2.3 da for smaller grains, the dividing radius of approximately 14 mum in one particular scenario corresponding to particles populating the area of the secondary peak seen about midway from the nucleus to the halo's boundary. For the adopted rotation model, the production rate of the solids peaks shortly before the source's local noon, but the integrated contribution from the afternoon ejecta slightly exceeds that from the morning ejecta. The problem of this event's triggering mechanism is also addressed.