Dynamics of planetesimals due to gas drag from an eccentric precessing disc

Abstract

We analyse the dynamics of individual kilometre-size planetesimals in circumstellar orbits of a tight binary system. We include both the gravitational perturbations of the secondary star and a non-linear gas drag stemming from an eccentric gas disc with a finite precession rate. We consider several precession rates and eccentricities for the gas, and compare the results with a static disc in circular orbit.The disc precession introduces three main differences with respect to the classical static case. (i) The equilibrium secular solutions generated by the gas drag are no longer fixed points in the averaged system but limit cycles with frequency equal to the precession rate of the gas. The amplitude of the cycle is inversely dependent on the body size, reaching negligible values for ~50-km-size planetesimals. (ii) The maximum final eccentricity attainable by small bodies is restricted to the interval between the gas eccentricity and the forced eccentricity, and apsidal alignment is no longer guaranteed for planetesimals strongly coupled with the gas. (iii) The characteristic time-scales of orbital decay and secular evolution decrease significantly with increasing precession rates, with values up to two orders of magnitude smaller than for static discs.Finally, we apply this analysis to the γ-Cephei system and estimate impact velocities for different-size bodies and values of the gas eccentricity. For high disc eccentricities, we find that the disc precession decreases the velocity dispersion between different-size planetesimals, thus contributing to accretional collisions in the outer parts of the disc. The opposite occurs for almost circular gas discs, where precession generates an increase in the relative velocities. © 2010 The Authors. Journal compilation © 2010 RAS.