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M. Fulchignoni, M. Birlan, M.A. Barucci (Observatoire de Paris-Meudon, France)
Recently, we have discussed the possible subdivision in few groups of a sample of 22 TNO and Centaurs for which the BVRIJ photometry were available (Barucci et al., 2001, A&A, 371,1150).
We obtained this results using the multivariate statistics adopted to define the current asteroid taxonomy, namely the Principal Components Analysis and the G-mode method (Tholen & Barucci, 1989, in ASTEROIDS II).
How these methods work with a very small statistical sample as the TNO/Centaurs one?
Theoretically, the number of degrees of freedom of the sample is correct. In fact it is 88 in our case and have to be larger then 50 to cope with the requirements of the G-mode. Does the random sampling of the small number of members of a large population contain enough information to reveal some structure in the population?
We extracted several samples of 22 asteroids out of a data-base of 86 objects of known taxonomic type for which BVRIJ photometry is available from ECAS (Zellner et al. 1985, ICARUS 61, 355), SMASS II (S.W. Bus, 1999, PhD Thesis, MIT), and the Bell et al. Atlas of the asteroid infrared spectra.
The objects constituting the first sample were selected in order to give a good representation of the major asteroid taxonomic classes (at least three samples each class): C,S,D,A, and G. Both methods were able to distinguish all these groups confirming the validity of the adopted methods. The S class is hard to individuate as a consequence of the choice of I and J variables, which imply a lack of information on the absorption band at 1 micron.
The other samples were obtained by random choice of the objects. Not all the major groups were well represented (less than three samples per groups), but the general trend of the asteroid taxonomy has been always obtained.
We conclude that the quoted grouping of TNO/Centaurs is representative of some physico-chemical structure of the outer solar system small body population.
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The author(s) of this abstract have provided an email address for comments about the abstract: Marcello.Fulchignoni@obspm.fr