GAS DRAG EFFECTS ON PLANETESIMALS IN THE 2 1 RESONANCE WITH PROTO-JUPITER/

The combined effects are studied of gas drag and gravitational perturb ations by a proto-Jupiter on the orbital evolution of a swarm of plane tesimals in the-primordial asteroid belt in the 2:1 mean motion resona nce region. The gas drag in the primordial nebula causes planetesimals to spiral towards the Sun and, therefore, to cross mean motion resona nces with proto-Jupiter. The dynamics of planetesimals are numerically investigated while passing through an inner resonance in a planar mod el. It is found that eccentricities are drastically increased and the maximum value reached by each planetesimal depends only on the resonan ce argument psi at the resonance entry. The higher average eccentricit y of the swarm within the resonance borders induces a faster spirallin g rate of planetesimals and a consequent decrease of their number dens ity, in particular at the 2:1, the most relevant resonance in the aste roid belt. This phenomenon causes the formation of a gap in the swarm at the resonance location. By integrating a large number of planetesim al orbits, the gap formation process is analysed; it is found that the planetesimal number density near the resonance centre is reduced to 1 0-40% of its average value, depending on the free eccentricity assumed for the proto-Jupiter. I Relative velocities between planetesimals ar e increased by a factor of four by resonant perturbations, favouring f ragmentation at impacts; higher impact velocities and the reduced plan etesimal density slow down the planetesimal accretion process and inhi bit the formation of big bodies in the resonance region. (C) 1997 Else vier Science Ltd.