Class Of Comets Research Articles

Chemical and physical aging of comets

AbstractAging effects for comet nuclei encompass both chemical and physical changes to the nucleus which may be manifested as a change in the level of and type of activity that the nucleus experiences. Recent observations have shown the importance of the amorphous to crystalline water ice phase transition in comet nuclei for controlling activity at large distances, and for altering the porosity and volatile distribution in the interior of comets. Evidence for secular changes in activity levels or differences between dynamical classes of comets which have spent different amounts of time in the inner solar system must be decoupled from expected primordial differences between comets. Long-term observational programs by several groups have demonstrated that the effects of aging are detectable. This paper will discuss the expected primordial comet differences, the expected physical and chemical effects of aging, the techniques for observing differences in activity levels, and will present summaries of results of several observing programs.

Are there two populations of comets based on polarimetric properties of dust particles?

AbstractA critical analysis of attempts to classify comets on the basis of their polarimetric properties is given. It was shown that a degree of polarization depends strongly on the gas-to-dust ratio of comets and bandpass of filters. At present, there are no rigorous polarimetric evidences for two taxonomic classes of comets.

Origin and dynamics of comets and star formation

Several important consequences that would arise for the solar system's comet population if the Sun formed in an embedded star cluster are outlined. Owing to the higher stellar number density and larger tidal fields in this environment, formation of the comet cloud would proceed differently from that envisaged in a usually assumed isolated solar nebula scenario. In particular, early planetesimal scattering by the giant planets may lead to a substantial population of free-floating intracluster comets. Furthermore, the potentially important influence the primordial binaries may have had for cometary dynamics is emphasized. Long-period binaries are likely to accelerate planetesimal growth, and short-period binaries increase the capture cross-section of intracluster comets. Some of the intracluster comets might have been captured by the Sun and remained bound to it until after the cluster dispersed. If captured comets were in stable orbits over the remaining age of the solar system, it is expected they will constitute a second class of comets in addition to the comets scattered outwards by the planets.

Comet Taxonomy and Evolution

Measurements of physical and dynamical properties of comets have yielded considerable evidence for variations in the origin and evolution of these primordial objects. Given these findings, one might reasonably expect to observe differences in the chemical composition as well. This paper reviews our current state of knowledge of the similarities and differences in the composition of comets, and summarizes the recent findings by A'Hearn et al. (1994) for two taxonomic classes of comets and evidence of compositional variation due to origin and/or evolution.

Monochromatic brightness variations of comets

A class of comets, of which Comet Kohoutek (1973f) is typical, show total as well as monochromatic brightness asymmetries about perihelion. They are fainter after perihelion than before at the same heliocentric distance. A model of the cometary nucleus consisting of a growing non-volatile dust mantle surrounding a volatile icy core is used to discuss this phenomenon.

Our Astronomical Column

THE PERIODICAL COMET OF TEMPEL-SWIFT (1869-1880).—This object is one of the most interesting of the somewhat numerous class of comets which at aphelion pass just outside the orbic of Jupiter and perform their revolutions in periods ranging from about 5 to 9 years. First seen by Tempel in 1869 November 27, the character of the orbit was not determined until its independent discovery by Lewis Swift in 1880 October 10. It was then found to be moving in an orbit of short period for the elements deduced by Bruhns, for the apparition of 1869 verv closely resembled those obtained by Chandler for the return of 1880, and the latter pointed out the true character of the orbit early in 1880 November. Messrs. Schulhof and Bossert, of Paris, also published elements indicating a periodic time of 5½ years.

Our Astronomical Column

PHYSICAL APPEARANCE OF PERIODIC COMETS.—Comets possess no personal characteristic appearance; hut Mr. Barnard, writing to the Astronomical Journal No. 246, suggests that it may be possible to arrange those of short period according to their physical peculiarities. To the first class he would assign those comets which are large, round, and very gradually brighter in the middle, with no special condensation, and of a very diffused nature. They have no nucleus or tail, and are so decidedly periodic that, trusting to this peculiarity, Mr. Barnard predicted that the comet discovered by Swift in November 1889, and D'Arrest's comet at its return last year, were of short period. The most distinctive members of this class of comets are D'Arrest's, Swift's 1880, Brooks's 1886, and Swift's 1889. There are few nebulæ that resemble this class. A much larger and less exclusive class contain comets which are comparatively small, and which have an indefinite central brightness or nucleus. Many of the parabolic comets resemble these, and there are hundreds of nebulæ exactly like them in telescopic appearance. To this class are assigned comets Faye; Wolf, 1884 III.; Finlay, 1886 VII.; Brooks, 1889 V.; Spitaler, 18go. It is possible that the peculiarities of these two distinct classes of short-period comets may furnish some information as to their relative ages.