Dust emission for Comets Shoemaker-Levy 1991a1 and McNaught-Russell 1993v

We present CCD photometric results for the dust comae of the dynamically ne w Comet Shoemaker-Levy 1991a1, carried out at heliocentric distances from 1 .2 to 0.8 AU pre-perihelion, and the high-eccentricity, long-period Comet M cNaught-Russell 1993v obtained at a heliocentric distance close to 1.0 AU p ost-perihelion. Maps of the directional distribution of the dust emission r ate from these cometary nuclei were obtained using the directional deconvol ution method (Waniak 1994, Icarus 111, 237-245). For Comet Shoemaker-Levy t he prominent region of enhanced dust production was situated between the so lar terminator and the nucleocentric meridian opposite the subsolar point. Activity in this region on the night side of the nucleus may be explained b oth by the heating of the nucleus' surface by scattered visible and reemitt ed infrared radiation, which is produced by the dust coma, or by non-solar radiation sources of energy, such as chemical reactions or phase transition s. During the period of observations the dust emission rate for this region decreased in comparison with that of another region of enhanced dust produ ction situated on the subsolar hemisphere. For Comet McNaught-Russell tno a ctive regions were also visible, although the subsolar region was much more active than that on the night side of the nucleus. For both comets, dust w as emitted from the entire surface of the nucleus at a level no lower than 30% of the maximum value for the active regions. The total (integrated over a 4 pi solid angle) dust emission rate for Comet Shoemaker-Levy changed as r(h)(-2.3) for the observed range of heliocentric distance r(h). For both comets, the ejection velocity of submicron dust particles was of the order of 0.1 km sec(-1) and the power-law size distribution of dust particles (a( -n)) had a mean value of exponent n equal to 2.9. The power-law dependence of the ejection velocity upon the beta parameter (nu similar to beta(k)) wa s specified by a mean value of k close to 0.18. (C) 1998 Academic Press.