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L.-M. Lara (Centro de Astrobiologia, Torrejon de Ardoz, Spain), L. Kolokolova (University of Florida, Gainesville), R. Schulz (ESA, ESTEC, Noordwijk, The Netherland), J.A. Stuwe (Astronomisches Institut, Ruhr-Universitat Bochum, Germany), G.P. Tozzi (Osservatorio Astrofisico di Arcetri, Firenze, Italy)
Spectrophotometric CCD observations of comet Tabur (C/19967 Q1) allowed for the first time to accomplish a detailed multi-color photometry of cometary continuum in the near-nucleus region. Thus, the change of spectral properties of the cometary dust with the distance from the nucleus was obtained. We have developed a theoretical technique, based on the approach suggested by Kolokolova et al.(Icarus, v.126, p. 351, 1997), that enables to reveal the size distribution of the dust and the optical properties of its material from the brightness data in six continuum wavebands. Based on this technique we have found that in the near-nucleus region of comet Tabur the dust can be described by a power-law size distribution with the power around 2.75 - 3.05, the lower limit of particle radii 0.05 - 0.25 micron, and the upper limit of the order of several centimeters. The change of the dust properties with the distance from the nucleus in the Sun-tail direction was also studied. The color trends showed that with cometocentric distance particles became smaller but kept the power of the size distribution constant. The change of their size was accompanied by the change of their composition, that can be presented as evaporation of icy or organic grains in a mixture of grains of various composition, or, more likely, as sublimation of mantles (icy or organic) from silicate/carbon cores. On September 13, 1996 an ejection in the Sun direction of smaller and of different composition particles was seen from the color scans. Particles of the ejecta had a short lifetime and showed light-scattering properties that are typical either for very transparent (icy?) particles or for particles with the refractive index higher (dark organics?) than the one for the surrounding coma.