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[13.09] The Asteroidal Population in the Hecuba Gap

S. Ferraz-Mello (Observatório Nacional and IAG-USP), F. Roig, T.A. Michtchenko (IAG-USP)

We present an analysis of the dynamical behavior of the asteroidal population in the 2/1-resonance with Jupiter's motion (at a ~q 3.27 AU). They are some 120 numbered and non-numbered multi-oppositional asteroids as known on March 2000. Simulations of their motions over 100,000 years allowed us to divide them in three different classes, according to the behavior of the angle \sigma = 2\lambda_J - \lambda - \varpi: (i) \underline{resonant}, showing libration of \sigma around 0 deg with a libration amplitude of the order of 80 deg (40 objects); (ii) \underline{pericentric alternators}, for which the behavior of \sigma alternates between circulation and libration around 0 deg (72 objects); and (iii) \underline{apocentric alternators}, for which the behavior of \sigma alternates between circulation and libration around 180 deg (35 objects). Objects of classes (ii) and (iii) are related to very-low eccentricity regimes of motion. They seem to be stable over very long times (at least more than 300 Myr) and are there, very probably, since the formation of the asteroid belt. Class (i) objects are subdivided in two distinct populations: stable and unstable (20 objects in each of them). Stable resonant asteroids are clustered in a small region of the phase space with e~ 0.23 and i<3 deg (see Nesvorny et al. Icarus 130, 247, 1997). Our simulations show that they may survive there for more than 300 Myr. Unstable resonant asteroids spread irregularly over the chaotic regions of the resonance, with typical lifetimes (inside the resonance) of 10 Myr. Hypotheses on the origin of these two populations are discussed. The simulation of fictitious bodies shows that unstable resonant objects can be injected from outside the resonance, through separatrix crossing, with a rate compatible with the mean lifetime of the known population. The injection mechanism is pure dynamical diffusion due to a web of weak mean-motion resonances in the neighborhood of the resonance. The origin of the stable resonant population is puzzling. We briefly discuss the hypotheses according which these objects are either remnant of Themis family or primordially resonant. A possible role of Yarkovsky orbital drift is also discussed. Acknowledgements: FAPESP and CNPq.

The author(s) of this abstract have provided an email address for comments about the abstract: sylvio@on.br