Kreutz Sungrazers - Dynamical History and Evolution

Dynamical History and Evolution

A study by Brian Marsden in 1967 was the first attempt to trace back the orbital history of the group to identify the progenitor comet. All known members of the group up until 1965 had almost identical orbital inclinations at about 144°, as well as very similar values for the longitude of perihelion at 280–282°, with a couple of outlying points probably due to uncertain orbital calculations. A greater range of values existed for the argument of perihelion and longitude of the ascending node.

Marsden found that the Kreutz Sungrazers could be split into two groups, with slightly different orbital elements, implying that the family resulted from fragmentations at more than one perihelion. Tracing back the orbits of Ikeya–Seki and the Great Comet of 1882, Marsden found that at their previous perihelion passage, the difference between their orbital elements was of the same order of magnitude as the difference between the elements of the fragments of Ikeya–Seki after it broke up. This meant it was realistic to presume that they were two parts of the same comet which had broken up one orbit ago. By far the best candidate for the progenitor comet was that seen in 1106 (Great Comet of 1106): Ikeya–Seki's derived orbital period gave a previous perihelion almost exactly at the right time, and while the Great Comet of 1882's derived orbit implied a previous perihelion a few decades later, it would only require a small error in the orbital elements to bring it into agreement.

The sungrazing comets of 1668, 1689, 1702 and 1945 seem to be closely related to those of 1882 and 1965, although their orbits are not well enough determined to establish whether they broke off from the parent comet in 1106, or the previous perihelion passage before that, some time in the 3–5th centuries AD. This subgroup of comets is known as Subgroup II. Comet White–Ortiz–Bolelli, which was seen in 1970, is more closely related to this group than Subgroup I, but appears to have broken off during the previous orbit to the other fragments.

The sungrazing comets observed in 1843 (Great Comet of 1843) and 1963 (Comet Pereyra) seem to be closely related and belong to the subgroup I, although when their orbits are traced back to one previous perihelion, the differences between the orbital elements are still rather large, probably implying that they broke apart from each other one revolution before that. They may not be related to the comet of 1106, but rather a comet that returned about 50 years before that. Subgroup I also includes comets seen in 1695, 1880 (Great Southern Comet of 1880) and in 1887 (Great Southern Comet of 1887), as well as the vast majority of comets detected by SOHO mission (see below).

The distinction between the two sub-groups is thought to imply that they result from two separate parent comets, which themselves were once part of a 'grandparent' comet which fragmented several orbits previously. One possible candidate for the grandparent is a comet observed by Aristotle and Ephorus in 371 BC. Ephorus claimed to have seen this comet break into two. However modern astronomers are skeptical of the claims of Ephorus, because they were not confirmed by other sources. Instead comets that arrived between 3rd and 5th centuries AD (comets of 214, 426 and 467) are considered as possible progenerators of the Kreutz family. The original comet must certainly have been very large indeed, perhaps as large as 100 km across (for comparison, the nucleus of Comet Hale–Bopp was about 40 km across).

Although its orbit is rather different from those of the main two groups, it is possible that the comet of 1680 is also related to the Kreutz Sungrazers via a fragmentation many orbits ago.

The Kreutz Sungrazers are probably not a unique phenomenon. Studies have shown that for comets with high orbital inclinations and perihelion distances of less than about 2 AU, the cumulative effect of gravitational perturbations tends to result in sungrazing orbits. One study has estimated that Comet Hale–Bopp has about a 15% chance of eventually becoming a sun-grazing comet.