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Astron. Astrophys. 336, 1056-1064 (1998)

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6. Conclusions

The collision probabilities and collision velocities of the asteroid belt have been derived and analysed with emphasis on the Hilda asteroids. Good agreement is found (where comparisons are possible) between results obtained in this study and the results of earlier studies of main-belt and Trojan asteroids by Bottke et al. (1994) and Marzari et al. (1996). The analysis of the collision properties of Hilda asteroids have given the following new results:

i) Out of ten collisions experienced by a Hilda asteroid, approximately seven of these are with main-belt objects, and the remaining three collisions are about equally distributed between objects in the Cybele, Hilda, and Trojan groups.

ii) The mean collision probabilities are lower for Hilda asteroids than for any other group of asteroids (i.e., the main-belt, Cybele and Trojan groups), indicating that the Hilda asteroids are less collisionally evolved than other groups of asteroids. Since not all Trojan asteroids with [FORMULA] 50 km are discovered at this time, the frequency of collisions between Hilda and Trojan asteroids is underestimated. However, this will not alter the conclusion above because of the small relative importance of HT collisions.

iii) The range of collision probabilities for individual objects in the Hilda group is about a factor of six. This is larger than the range found in the main-belt and Cybele groups, and is comparable to the spread in collision probabilities among the Trojan asteroids.

iv) There is a strong correlation of collision probability with eccentricity for Hilda asteroids, giving rise to the large range in collision probabilities among Hilda objects. This is due to the possibility of Hilda objects to encounter a larger fraction of the objects in the main-belt when the eccentricities of their orbits are larger. There is also a preference for Hilda asteroids with high collision probability to have higher mean collision velocities. This is due to that larger eccentricities results in both higher velocities and higher collision probabilities.

v) Two of the Hilda objects, 334 Chicago and 1256 Normannia, have very low collision probabilities and collision velocities, indicating that they may be among the least collisionally evolved objects of the 909 asteroids in the sample.

vi) The mean collision velocity of Hilda asteroids is 4.6 [FORMULA], which is about 0.5 [FORMULA] lower than the mean velocity in the main-belt. The mean velocity for collisions among Hilda asteroids is only 3.1 [FORMULA], (confirming the result by Marzari et al. 1996). However, these collisions contribute less than 10% to the collision probabilities of Hilda objects. A low velocity tail (down to [FORMULA] 0.3 [FORMULA]) from mutual Hilda collisions is found, indicating the occurrence of very `soft' collisions between Hilda asteroids.

vii) The non-isotropic collision geometries among asteroids found in earlier investigations, with large velocity components perpendicular to the ecliptic plane, are confirmed. However, the collision geometries for Hilda objects are more isotropic than for the other groups of asteroids, due to a small inclinations of the Hilda type orbits.

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© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998