Dust Emission for Comets Shoemaker–Levy 1991a1 and McNaught–Russell 1993v

Abstract

We present CCD photometric results for the dust comae of the dynamically new 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 McNaught–Russell 1993v obtained at a heliocentric distance close to 1.0 AU post-perihelion. Maps of the directional distribution of the dust emission rate from these cometary nuclei were obtained using the directional deconvolution method (Waniak 1994,Icarus111, 237–245). For Comet Shoemaker–Levy the prominent region of enhanced dust production was situated between the solar terminator and the nucleocentric meridian opposite the subsolar point. Activity in this region on the night side of the nucleus may be explained both by the heating of the nucleus' surface by scattered visible and reemitted infrared radiation, which is produced by the dust coma, or by non-solar radiation sources of energy, such as chemical reactions or phase transitions. During the period of observations the dust emission rate for this region decreased in comparison with that of another region of enhanced dust production situated on the subsolar hemisphere. For Comet McNaught–Russell two active 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 was 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 4π solid angle) dust emission rate for Comet Shoemaker–Levy changed asr−2.3hfor the observed range of heliocentric distancerh. For both comets, the ejection velocity of submicron dust particles was of the order of 0.1 km sec−1and the power-law size distribution of dust particles (a−n) had a mean value of exponentnequal to 2.9. The power-law dependence of the ejection velocity upon the β parameter (v∼ βk) was specified by a mean value ofkclose to 0.18.