  Aims. We investigate the feasibility of planetesimal growth in circumbinary
protoplanetary disks around the observed systems Kepler- 16 and Kepler-34 under
the gravitational influence of a precessing eccentric gas disk. Methods. We
embed the results of our previous hydrodynamical simulations of protoplanetary
disks around binaries into an N-body code to perform 3D, high-resolution,
inter-particle gravity-enabled simulations of planetesimal growth and dynamics
that include the gravitational force imparted by the gas. Results. Including
the full, precessing asymmetric gas disk generates high eccentricity orbits for
planetesimals orbiting at the edge of the circumbinary cavity, where the gas
surface density and eccentricity have their largest values. The gas disk is
able to efficiently align planetesimal pericenters in some regions leading to
phased, non-interacting orbits. Outside of these areas eccentric planetesimal
orbits become misaligned and overlap leading to crossing orbits and high
relative velocities during planetesimal collisions. This can lead to an
increase in the number of erosive collisions that far outweighs the number of
collisions that result in growth. Gravitational focusing from the static
axisymmetric gas disk is weak and does not significantly alter collision
outcomes from the gas free case. Conclusions. Due to asymmetries in the gas
disk, planetesimals are strongly perturbed onto highly eccentric orbits. Where
planetesimals orbits are not well aligned, orbit crossings lead to an increase
in the number of erosive collisions. This makes it difficult for sustained
planetesimal accretion to occur at the location of Kepler-16b and Kepler-34b
and we therefore rule out in-situ growth. This adds further support to our
initial suggestions that most circumbinary planets should form further out in
the disk and migrate inwards.
