Imaging photometry and color of Comet Shoemaker-Levy 9

An investigation of brightness and color of Comet SL9 was carried out, when the comet had a phase angle less than 1 [degrees]. The color of the subnuclei was the same within error limits and was close to solar color. The g-r color of the dust increases along the tails of the subnuclei until they merge in the common dust envelope. Similarly in the common dust envelope there seems to be a trend of increased reddening with increased distance from the chain of subnuclei. The brightness peaks at minimum phase angle. Comparison with other observers indicates a phase curve similar to the phase dependence observed in C-type asteroids. The comet did not change its appearance when it passed minimum phase angle. This means that there is no evidence for fresh dust leaving the subnuclei in the form of syndyne tails or ongoing fragmentation producing micrometer-sized particles in the common dust envelope. We conclude that most of the dust present in the image was emitted either at breakup of the parent nucleus or a little later. As in this case the grains must be large as compared to the visual wavelength we can interpret the color as color of a macroscopic body. The fact that it is close to the solar color is consistent with the color of C-type asteroids. The color trend, if real, could then be due to separation of brighter and redder grains from the dark carbonaceous component by radiation pressure.