Building tatooine: Suppression of the direct secular excitation in kepler circumbinary planet formation

摘要

Circumbinary planetary systems recently discovered by Kepler represent an important testbed for planet formation theories. Planetesimal growth in disks around binaries has been expected to be inhibited interior to ～10 AU by secular excitation of high relative velocities between planetesimals, leading to their collisional destruction (rather than agglomeration). Here we show that gravity of an axisymmetric gaseous circumbinary disk in which planets form drives fast precession of both the planetesimal and binary orbits, resulting in strong suppression of planetesimal eccentricities beyond 2-3 AU and making possible the growth of 1-10~2 km objects in this region. The precise location of the boundary of the accretion-friendly region depends on the size of the inner disk cavity cleared by the binary torques and on the disk mass (even 0.01 M_⊙ disk strongly suppresses planetesimal excitation), among other things, but this zone does not extend to present orbits of Kepler circumbinary planets. The precession of the orbit of the central binary, enhanced by the mass concentration that is naturally present at the inner edge of a circumbinary disk, plays a key role in this suppression, which is a feature specific to the circumbinary planet formation.