Sungrazing Comets Research Articles

Disappearance and disintegration of comets

Observational evidence on the disappearance of comets is classified and tested for signatures of rapidly advancing decay. Of the several categories of phenomena, emphasis is on the dissipating comets, a group of objects that are observed to expand rapidly in dimensions, to fade, and eventually to disappear on the sky background on a time scale of several days to weeks. The tail can become the brightest part of the object and survive the head. The process appears to be triggered by an eruptive dust-emission event several weeks before the ultimate demise. Analogy is drawn between the physical changes experienced by the dissipating comets and by the secondary nuclei of the split comets. It is suggested that the nuclei of the dissipating comets are strongly nonspherical, structurally heterogeneous, and very poorly cemented. Detailed descriptions of the physical behavior of six dissipating comets (1897 III, 1913 VI, 1926 III, 1954 II, 1957 IX, and 1974 XV) are presented and both similarities and differences between these and the headless sungrazing comet 1887 I are summarized. Limited evidence on P/Westphal, the only known short-period dissipating comet, suggests the presence of persistent brightness fluctuations coupled with generally elevated activity one revolution prior to the dissipating event. Potential roles of nongravitational forces (causing the nucleus to precess) and brightness fluctuations in the process of comet dissipation are considered and a few comets exemplifying these phenomena are singled out. It is concluded that the dissipating comets could indeed be completely disintegrating into particulate debris while under observation and that the shattered mass can be made comparable with the expected masses of subkilometer-sized comets if the mass-distribution function of the fragments is relatively flat.

The path and surviving tail of a comet that fell into the sun

A satisfactory orbital solution for Comet Howard-Koomen-Michels 1979 XI is found on the assumption that the comet's line of apsides coincided with that of the Kreutz sungrazing comet group. The derived perihelion distance then shows that this is the first known case of a comet falling into the sun. A dust tail that survived the comet is studied as a particle flow phenomenon controlled by no force other than solar gravity and solar radiation pressure. The tail's outline is interpreted in terms of an onset of dust production, a peak repulsive force on the particles, and a circumsolar dustfree zone due to particle sublimation. It is shown that the surviving debris consisted mostly of absorbing, submicron size particles in hyperbolic trajectories convex to the sun and curving toward the earth. The tail width may be a product of the interaction of charged dust in the tail with a complicated structure of the coronal magnetic field.

Observations of a Comet on Collision Course with the Sun

A brilliant new comet (1979 XI: Howard-Koomen-Michels) was discovered in data from the Naval Research Laboratory's orbiting SOLWIND coronagraph. An extensive sequence of pictures, telemetered from the P78-1 satellite, shows the coma, accompanied by a bright and well-developed tail, passing through the coronagraph's field of view at afew million kilometers from the sun. Preliminary orbital calculations based on the observed motion of the comet's head and morphology of the tail indicate that this previously unreported object is a sungrazing comet and may be one of the group of Kreutz sungrazers. It appears from the data that the perihelion distance was less than 1 solar radius, so that the cometary nucleus encountered dense regions of the sun's atmosphere, was completely vaporized, and did not reappear after the time of closest approach to the sun. After this time, however, cometary debris, scattered into the ambient solar wind, caused a brightening of the corona over one solar hemisphere and to heliocentric distances of 5 to 10 solar radii.

Substances of cometary grains estimated from evaporation and radiation pressure mechanisms

The shape and intensity distribution of tails for several large comets are estimated on the basis of grain properties in the solar radiation field. The following results are obtained: (1) The ratio of the maximum radiation pressure force to the gravitaional force acting on dust grains in cometary tails is found to be less than 2.5. This means that grains such as graphite particles in the size range 0.02–0.2 μm do not exist in them, because such particles would allow forces greater than 2.5 (2) Tail substances supplied near the time of perihelion passage for the Sun-grazing comet Ikeya-Seki (1965 VIII) and Comet Seki-Lines (1962 III) were composed of particular grains which had values of radiation pressure ratio less than 1.0. Therefore, it is concluded that the material was composed of silicate grains only, since iron grains had sublimated and there were no graphite particles.

Nongravitational perturbations of long-period comets

The model of Marsden et al. (1973) is used to investigate the effect of nongravitational forces resulting from water ice sublimation on the orbits of long-period comets. The orbits of hypothetical comets with perihelion distances of 0.005 to 4 AU are integrated numerically along initially parabolic trajectories through one perihelion passage for nongravitational forces of 10 millionths to 200 millionths the solar attraction at 1 AU and lag angles of 0, 5, and 10 deg. The results indicate that the nongravitational perturbations are possibly of equal or greater importance than planetary perturbations for comets with small perihelia and that nongravitational forces could have brought sun-grazing comets to their current orbits in two or three returns. It is concluded that nongravitational forces are likely to be found only for those long-period comets that have large nongravitational accelerations to begin with and are bright enough to be discovered early and tracked over a very long arc of their orbits.

The reality of comet groups and pairs

Although the common genetic origin of the Kreutz family of Sun-grazing comets has generally been accepted, there remains uncertainty with regard to genetic identity among other groups of comets whose orbital elements are nearly alike. Porter has listed a number of such grouds and Öpik has made a statistical study of the orbits of 472 comets with aphelion distance beyond Saturn. He lists 97 groups that show similarities among their three angular elements. He calculates an overall probability of some 10 −39 that these similarities could have occurred by chance, and thus concludes that 60% or more of such comets fall into genetic groups containing from two to seven members. This paper explores the statistical reality of Öpik's groups utilizing the Monte Carlo method of statistics as well as ordinary probability theory. The conclusion is reached that except for a few pairs the similarity among orbital elements within the groups is no greater than random expectation.

The sungrazing comet group

view Abstract Citations (100) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The sungrazing comet group Marsden, B. G. Abstract The principal effects of planetary perturbations on the orbit of a sungrazing comet are derived, both from a simplified general theory and by direct numerical integration of typical trajectories. New orbit determinations are made for the three most recently observed members of the Kreutz group: comets 1965 VIII, 1963 V, and 1945 VII. For comet 1965 VIII the results are obtained in terms of both general relativity theory and the Brans-Dicke theory, but the observations are insufficient to enable one to decide which theory is correct. Some remarks are made about the less certain members of the comet group, and evidence is provided for dividing the comets into two distinct subgroups. Studies on the past motions of comets 1882 II and 1965 VIII reveal it as virtually certain that these comets separated from each other at their previous perihelion passage, and it seems possible that they were then observed as the comet of 1106. Not all the members of each subgroup can be explained so simply, but it is probable that the subsidiary separations took place not more than two revolutions ago. The two subgroups can presumably be explained by a similar split some 10 to 20 revolutions ago, although a few other possible explanations are touched upon in the final section. An ephemeris is provided for use in searching for further members of the group. Publication: The Astronomical Journal Pub Date: November 1967 DOI: 10.1086/110396 Bibcode: 1967AJ.....72.1170M full text sources ADS |