Origin of the Marsden and Kracht groups of sunskirting comets. I. Association with comet 96P/Machholz and its interplanetary complex

Of the three major groups of comets approaching the Sun to between 6 and 12 solar radii and discovered with the coronagraphs on board SOHO, we investigate the Marsden and Kracht groups. We call these comets "sunskirters" to distinguish them from the Kreutz system sungrazers. Our objective is to understand the origin, history, and orbital evolution of the two groups. The tendency for their members to arrive at perihelion in pairs or clusters is a result of their recent fragmentation. As fragments of more massive precursor objects, the Marsden- and Kracht-group comets are mostly less than 10 yr old. Although the two groups and several meteoroid swarms, such as the Daytime Arietids and Southern delta Aquarids, appear as separate populations of a complex associated with comet 96P/Machholz, our orbit integrations suggest that we deal with a single, essentially continuous population that extends from the comet's orbit for more than 160 degrees in the longitude of the node. First-generation fragments of their common progenitor with comet 96P, which were the initial direct ancestors of this population, are called the first precursors. Nearly 60,000 orbit integration runs are made in our search for their birth scenarios. We find that these objects separated from the progenitor comet before AD 950 and, as sources of continuing activity, pursued an orbital evolution very different from that of 96P. All first precursors of this low-inclination population experienced a sequence of encounters with Jupiter within 0.5 AU, starting in AD 1059 or earlier and continuing for centuries. In the process, they split into smaller pieces in a fashion reminiscent of "cascading" fragmentation of the Kreutz system. The secular planetary (mainly Jovian) perturbations control the motions of both 96P and the low-inclination population, but the dynamical evolution of the latter has been markedly accelerated by Jupiter during close encounters, so that the population's present-day orbital changes are similar to those the comet will undergo centuries from now. Precursors to the Southern delta Aquarids of the 1950s passed through the Marsden-group stage around 1700 and through the Kracht-group stage in the 1780s. The Daytime Arietids appear to be related most directly to the Marsden- group comets, which can closely approach Earth around June 12, the time of the stream's peak activity.