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Astron. Astrophys. 318, L5-L8 (1997)

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5. Statistics of nontidal splitting and conclusions

The recent additions to the split comets have dramatically affected the orbital-period distribution of these objects. Defining as the new (or the Oort cloud) comets those having original orbits with [FORMULA] million yr, as the fairly new comets those with [FORMULA] million yr, as the old comets those with [FORMULA] yr, and as the short-period comets those with [FORMULA] yr, the 1982 (from Paper 1) and 1996 samples are compared in Table 4. These totals now favor heavily the old and the short-period comets as the objects that, at least nominally, experience nontidal splitting most often.


Table 4. Statistics of nontidally split comets and comet pairs.

This result is consistent with the conceptual model proposed in Paper 1, which can now be slightly refined by identifying most companions of the nontidally split comets as randomly jettisoned pancake-shaped fragments of the surface mantle of refractory material, with limited supplies of subsurface volatiles attached to it to account for activity. The nuclear surface of old and short-period comets is indeed believed to be heavily mantled, with only a minor fraction still active. And since a differential deceleration varies inversely as the secondary-to-principal nucleus mass ratio (Paper 1), the detected major deceleration effects imply that the companions are considerably less massive than the principal nuclei.

On the other hand, separation-velocity effects are independent of the secondary-to-principal nucleus mass ratio. Their prevalence in the motions of components of a tidally split comet indicates that the fragments are of comparable masses, none of them dominant. One can say that nuclei of the tidally split comets truly break up, while nuclei of the nontidally split comets tend to peel off instead (Paper 1).

The breakup mechanism for the nontidally split comets is unknown, but stresses built up due to rapid rotation and/or tumbling of an irregular object as well as due to high temperature gradients in the nuclear surface layer are the primary candidates. It is possible that the tidal force is not the only - and perhaps not even the decisive - cause for tidal splitting. Whereas it apparently is instrumental in cracking the nucleus, a tidal breakup may in fact likewise be completed by rotational and/or thermal forces.

Acknowledgements. This research has been carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998