NUCLEAR DUST EMISSION PATTERN OF COMET WILSON-1987-VII

Four photographic exposures of the dust coma of Comet Wilson 1987 VII were obtained at heliocentric distances from 3.0 to 2.7 AU preperiheli on. Digitally processed using the Ring Masking algorithm, brightness m aps revealed a strong asymmetry in dust emission from the nucleus of t he comet. Such a phenomenon was reported previously by both visible-li ght and IR observers (Debi Prasad et al. 1992, Icarus 95, 211-221; Cam pins et al. 1989, Icarus 80, 289-302). The possibility of reproducing the directional distribution of the dust production rate from the 2-D brightness profile was studied. The method of directional deconvolutio n based on the maximum likelihood approach was developed. Maps of the directional distribution of dust emission rate are presented. The most striking feature is the occurrence of a large, stable region of enhan ced dust production located almost perpendicular to the comet-Sun dire ction. Its nucleocentric longitude was unchanged during the 1-month ob servations (October 1986) but the region shifted toward the ''north po le.'' The possibilities of forming of such a stable active area locate d near the nucleus terminator are discussed. Furthermore, such paramet ers as the dust ejection velocity, the power index of the exponential dust size distribution, and the power index in the exponential depende nce of the grain velocity upon the 1 - mu parameter were obtained. The dust ejection velocity (for 1 - mu = 1.0) rose from 0.04 km sec-1 at 3.0 AU to 0.09 km sec-1 at 2.7 AU. The power index in the dust size di stribution was (with one exception) close to 3, which is in good agree ment with the results of dust tail analysis by Cremonese and Fulle (19 90, Astron. J. 100, 1285-1292). The dependence of grain ejection veloc ity upon 1 - mu parameter is described by a power index ranging from 0 .17 to 0.32, which is a factor of two less than that predicted by the dusty gas hydrodynamical simulation of the cometary coma. (C) 1994 Aca demic Press, Inc.