Secondary fragmentation of the Solar and Heliospheric Observatory sungrazing comets at very large heliocentric distance

The temporal distribution of the Kreutz group of sungrazing comets has been known to have an episodic character on timescales from weeks to tens of years. With the large number of minor members of this group being nowadays discovered in images taken with the Solar and Heliospheric Observatory coronagraphs, it has become apparent that the distribution of these faint comets is episodic on a much shorter timescale, with objects arriving in pairs during a small fraction of a day. It is shown that the rate of these pairs is much too high for a random sample. Their existence is readily explained as a result of secondary, low-velocity, nontidal fragmentation episodes, which occur virtually spontaneously at very large heliocentric distances and involve the products of near-perihelion splitting of progenitor fragments during their previous return to the Sun. In fact, the pairs are merely extreme manifestations of larger clusters of such subnuclei, with a complex hierarchy of fragments. Each cluster is an outcome of a sequence of nontidal fragmentation events, which begins-after the initial tidal breakup-at some point along the outbound leg of the orbit and then continues episodically to and past aphelion. A similar scenario of posttidal progressive disintegration was firmly established for comet Shoemaker-Levy 9, based on extensive observations of its secondary and tertiary nuclei during many months preceding the comet's collision with Jupiter. Also, there are similarities with the mechanism proposed recently for the formation of striations in the dust tail of comet Hale-Bopp, and a logical extension of this process is the evolution of comet dust trails.