DYNAMICAL STRUCTURE OF THE 2 1 COMMENSURABILITY WITH JUPITER AND THE ORIGIN OF THE RESONANT ASTEROIDS/

Using a semi-analytic approach, we provide a three-dimensional picture of the secular dynamics in the 2/1 mean-motion commensurability, mapp ing the most chaotic regions and the two islands of quasi-regular moti on. Five asteroids have recently been discovered in one of these islan ds and it has been conjectured that they were injected by the Themis-f amily formation event, which occurred close to the 2/1 resonance borde r. We show that this scenario is viable from the orbital and size dist ribution point of view and that it would also explain why no asteroids have been observed in the second island. Finally we discuss the possi bility that the 2/1 resonance is the dominating source of CI and CM me teorites, as was suggested on spectro-scopical bases. We find that the efficiency of the 2/1 resonance is far too small for it to be such a dominating source. (C) 1998 Academic Press.