Observational Aspects of Cometary Disintegration

Abstract

Questions of cometary lifetimes and disintegration rates are important to current theories concerning the origin of comets. Data from recent objective observations are not sufficient for a statistically meaningful discussion of disintegration phenomena. It is necessary to make use of older visual observations, which are often rather vague and subject to personal biases. In spite of the difficulties involved in such a study, it is possible to retrieve useful information from these observations. It has been my goal in this paper to thoroughly utilize the observational material relevant to a meaningful discussion of cometary origins. First, the sources and techniques of the study are described. A discussion of the various sorts of activity is included. Persistence, significance, and conditions of occurrence are discussed. Next, theories of cometary origin are summarized. Important parts of the Oort theory are outlined, with emphasis on points relating to cometary disintegration and the observable characteristics. The calculations of cometary parameters by Oort and Whipple are reviewed, and the important consequences are discussed. Lyttleton's attack on the Oort cloud hypothesis is briefly described and criticized. If the Oort theory is correct, "new" comets on their first close approach to the sun from the comet cloud should show a high disintegration rate or appreciable fading. An attempt is made to correlate activity and disintegration with cometary "age," using reciprocal (original) semi -major axes (1 /a) and spectral continuum strength as indicators of "age." No correlation of any sort is apparent. It appears that activity and disintegration- related phenomena are not correlated with these age criteria. Since outright division and related catastrophic disintegration phenomena apparently do not account for the excessive loss rate among young comets, as required by the Oort theory, fading may be called into play. The mechanisms of brightness variation and the results of previous studies are summarized. Everhart's study of discovery probability functions is described, and the results of this study are utilized to determine the amount of fading required to render "new" comets (on their first passage through the observability region in the inner solar system) undiscoverable on the second and subsequent perihelion passages. A total fading of one magnitude is needed to cause 55% of the "new" comets to disappear (become undiscoverable) at the next perihelion passage. This figure is approximate, and is based on some uncertain assumptions concerning the average brightnesses and other characteristics of various groups of comets. Using two different methods, I have attempted to uncover observational evidence of fading among "new" comets. First, absolute magnitudes are correlated with age, again using 1/a as the criterion. No correlation is found. Next, a number of long -period comets are investigated to determine the exact period over which they were observed. The mean absolute magnitude of the comets observed only before perihelion passage is compared with the mean absolute magnitude of those observed only after, with the expectation of demonstrating fading after perihelion. The opposite effect is found. Some mechanism of residual activity apparently causes these objects to become brighter after perihelion passage. This result is not in very good agreement with some previous studies. We must conclude that there is no direct observational evidence of appreciable fading among new comets. The data obtained in this study have been applied to several subsidiary investigations. An attempt is made to correlate cometary activity and disintegration with sunspot numbers. No significant correlation is found. An investigation is made into some results predicted by Harwit on the basis of his theory of cometary splitting due to collisions in the ecliptic plane. According to Harwit, the incidence of splitting should be higher for retrograde comets than for objects in direct orbits. I find no pronounced correlation of splitting with orbital inclination among long -period comets. Finally, a brief study is made of the absolute magnitudes of split comets. The results verify Whipple and Stefanik's contention that split comets tend to be bright, but I would attribute this, effect to observational selection. The data on splitting among faint comets are probably very incomplete.