A possible influence of comets on the growth of heavy elements abundance suggested several years ago by Tinsley and Cameron is shortly reviewed. Such an effect would be significant if the formation of comets produce cometary material equivalent to 0.01 - 0.02 M per one solar mass. In the view of current hypothesis of the formation and structure of cometary nuclei the role of comets should be considered as one of the possible moderators of the Z value variation during the chemical evolution of our Galaxy.
Aus den photographischen und visuellen Kometenhelligkeiten werden die photometrischen Parameter m0 und n berechnet. Es zeigt sichm dass die absolute Helligkeit der durchschnittlichen Grösse m0 der meisten Kometen gleich sind. Der Exponent n ist aber verhältnissmassig klein, nesonders für die visuellen Beobachtungen. Der Farbenindex änderte sich mit wachsendem Abstand des Kometen von der Sonne.
The history and dynamical evolution of the Kreutz group of sungrazing comets can be investigated by modeling the observed temporal and spatial distributions of the group's members, more than 250 of which were known by the end of 2000. This paper presents the results of an early phase of this modeling effort, which is based on (i) a long-standing consensus that all comets belonging to the Kreutz group are fragments of one original, massive comet; and (ii) a breakthrough in the understanding of the sungrazers' pairs and tight clusters detected in large numbers with the coronagraphs onboard SOHO. All these objects are products of progressive fragmentation that typically begins with a tidally-triggered (or tidally-assisted) breakup near perihelion, in the immediate proximity of the Sun. However, instead of terminating at that point - as postulated in all previous hypotheses - the process is shown to continue in a cascading fashion throughout the orbit about the Sun.
The theory of cooling of cometary dust by a gas outflowing from the nucleus is developed and used for explaining the anomalous distribution of sodium atom emission in the head of Comet Mrkos 1957 d. The results obtained indicate the universal character of the phenomenon of anomalous distribution of metal atom emissions,
including the Na I lines, in the heads of bright comets and the possibility of condensation of a cometary gas (Hg^2O, CO^2 etc.) in the nearnuclear region.
The present paper is an evaluation of photoelectric observations of teh eomts 1956h amd 1957d, made at the observatory of the University in Brno. The observations were carried out both in integral light and in selected spectral ranges, corresponding to the international colour system. the colour index of the comet 1956h indicates a sudden drop on May 2nd, which has been verified also by other observations. This drop is explicable only by an increased emission of gases into the coma. The mean colour index is 0.58, which corresponds to the colour index of stars of the spectral class GO. The mean colour index of part of the tail is - if we disregard the drop on May 2nd - +0·78. The photometric parameters, derived from flawless observations are m0υ = 6·55. nυ = 2·03 and m0B = 7·15, nB = 2·30. One good observation of the comet 1957d yielded the colout index of 0·15.
Photoelectric measurements of light-changes in the comet
Arend-Roland were made from April 22nd to May 6th 1957. From the measurements the absolute magnitude of the comet for the spectral range of 4000 A, m0 = 6.73. n = 3.48 was established. The curve of the light-changes was compared with the incidence of chromospherical flares in this period. Traces of a possible connection of the alterations in the comet´s brightness with changes in the solar activity were recorded.
Isophotes of comet Arend-Roland were constructed from 20 photographs gained by objective Tessar 1 : 4,5, f = 18 cm. The photographs were exposed in the visible spectral region in integral light, and with orange and blue filters on panchromatic emulsion of Agfa ISS plates (21/10 Din). The plates were measured on recording microdensitometer. The extreme measurable extent of the normal tail was 7° and of the anomal tail 1,5°.
15 sets of isophotae of the comet Arend-Roland are presented. They were obtained from photographs taken with the double astrograph of the Ondřejov Observatory. The method of reductions is briefly described. A list of 16 other plates of good or medium quality, not reduced here, is addied. The striking features of the comet´s appearance during the days April 27 to May 2 are quite briefly discussed. The isophotae describe the appearance of the comet on blue and panchromatic plates to a distance of 2°-3° from the head.
All known groups of fireballs contain bodies which may reach the Earths surface as meteorites and are of planetary origin. Since some fireballs have cometary orbits in the Solar System, then comets are probably of planetary origin. The possible explosion
of a large planet is discussed again. This time on the basis of gravitationally decelerated expansion of an originally superdense embryo.