5 Are dynamically "new'' comets different from others?

Less than a year after his "defining'' work on the cloud of comets, Oort made an attempt (in collaboration with M. Schmidt, Oort & Schmidt 1951) to find any physical differences between "new'' and "old'' comets according to their dynamical definition, on the basis of the original semimajor axes. Their main conclusions were: 1) There are too many "new'' orbits in comparison to the steady-state model prediction, which implies that they deteriorate faster than others, 2) "new'' comets with large perihelion distances (larger than 1 AU) are characterized by strong continuous spectra (mainly reflective) and 3) the brightness of "new'' comets varies more slowly with the heliocentric distance. After this pioneering work many authors studied various physical properties of comets versus with their "dynamical age'' (see for example: Kresák 1977; Whipple 1992a; A'Hearn et al. 1995). According to widespread opinion, the dynamically "new'' comets are generally more active and brighter at larger heliocentric distances and they have statistically greater perihelion distances. Having a set of previous aphelion and perihelion distances, we decided to look for any correlations between these values and various physical parameters of elliptical, long-period comets.

Our first attempt was to obtain a set of "absolute magnitudes'' (H₁₀) of all long-period comets and to check for a correlation with past motion characteristics. We were not able to find the appropriate values for all comets in our sample but on the basis of the papers by Vsekhsvyatskii (1958, 1967, 1979), Andrienko & Karpenko (1987), Hughes (1987), Kosai & Nakamura (1991) and Yoshida's Internet cometary catalogue (Yoshida 1999), we collected a large set of H₁₀ values. A complete list of all values we used may be found in the file: http://main.amu.edu.pl/~dybol/DH/mag_all.dat. We assumed, as a first and very simplified approach, that the absolute magnitude parameter H₁₀ can be treated as a rough measure of the cometary activity. In Fig. 2 one can find a distribution of H₁₀ against the previous aphelion distance. The dashed line is the least squares straight line approximation to all points. A small decrease of H₁₀ (increase in brightness) with increasing the previous aphelion distance is clearly visible. The effect is not very strong, but it remains visible when dealing with different subsets of cometary data (different orbit quality classes etc.). A similar effect is evident when looking for a correlation between the absolute magnitude and the previous perihelion distance.

Figure 2: The correlation between cometary absolute magnitude H10 and the previous aphelion distance.

Next we looked for any correlation between dynamical history of a comet and physical characteristics, expressed in terms of the volatility and activity indices, developed recently by Whipple (1992b,c), but we found that these correlations are rather weak.

Figure 3: The dependence of the observed perihelion distance on the previous perihelion distance. We included here all elliptical orbits, which crossed the PPL.

Finally, we tried to check one of the theses of the Kresák's paper (Kresák 1977) about a prevalence of orbits of large perihelion distance among the "new'' comets. For this purpose we prepared a plot, presented here as Fig. 3. It illustrates the dependence of the observed perihelion distance (vertical axis) on the previous perihelion distance (horizontal axis). The vertical, dashed line in the middle of the plot corresponds to our threshold value of 15 AU for the previous perihelion distance, defining all comets to the left of this line as dynamically "old'' and to the right as dynamically "new''. One can easily see the prominent feature in the left part of the plot: this straight line grouping of dots corresponds to those "old'' comets, which do not change their perihelion distance during the last revolution before being observed. All other dots represent the comets undergoing changes in perihelion distance in that interval. Interpretation of this picture from the point of view of Kresák's paper seems to be difficult. It is clearly visible only that at small perihelion distances (less than 1 AU) we observe mainly "old'' comets from the straight line groupings (i.e. having small semimajor axes).