Discrete levels of beginning height of meteors in streams

Abstract

Of the 26 streams of meteors classified according to Ceplecha's discrete levels of beginning height, 13 are associated with known comets. Comet Biela produced in the Andromedids a double-peaked distribution (Classes A and Ci). Apparently no known comets produce a stream of Class B. Consideration of Whipple and Stefanik's model of an icy conglomerate nucleus with radioactive heating and redistribution of ice leads to association of Ceplecha's Class C with the residue of the ice-impregnated surface of a cometary nucleus after sublimation of the ices, and of Ceplecha's Class A with the core of a cometary nucleus. Class B meteoroids are then to be associated with less dense cores of smaller cometary nuclei that have lost their surfaces and are too small to have been observed. Furthermore, the density of Class A meteoroids (1.2 g cm"1 ) is so close to that of Type I carbonaceous chondrites (2 g cm"1 ) as to suggest that the latter come from old cores of very large nuclei of comets, an idea originally proposed by McCrosky and Ceplecha. It is suggested that two inert objects that look like asteroids may yet remain from the two pieces observed at the last return of P/Comet Biela. The recovery of Comet 1930 VI, Schwassmann-Wachmann 3, at its return in 1979 is urged since it is the only available comet producing a shower (T Herculids) of Class A. A search for an asteroidal object or a very small comet in the orbit of the Geminids is also urged as the best chance of finding an object that produces meteoroids of Class B. Further study of the distribution of radiants and velocities of meteors in June and early July in Scorpius, Sagittarius, Ophiuchus, and Serpens is required to sort out the true structure there, if indeed one exists.