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Detections of planets in eccentric, close (separations of ~20 AU) binarysystems such as \alpha Cen or \gamma Cep provide an important test of planetformation theories. Gravitational perturbations from the companion are expectedto excite high planetesimal eccentricities resulting in destruction, ratherthan growth, of objects with sizes of up to several hundred km in collisions ofsimilar-size bodies. It was recently suggested that gravity of a massiveaxisymmetric gaseous disk in which planetesimals are embedded drives rapidprecession of their orbits, suppressing eccentricity excitation. However, disksin binaries are themselves expected to be eccentric, leading to additionalplanetesimal excitation. Here we develop secular theory of eccentricityevolution for planetesimals perturbed by the gravity of an ellipticalprotoplanetary disk (neglecting gas drag) and the companion. For the first timewe derive an expression for the disturbing function due to an eccentric disk,which can be used for a variety of other astrophysical problems. We obtainexplicit analytical solutions for planetesimal eccentricity evolution anddelineate four different regimes of dynamical excitation. We show that insystems with massive (>10^{-2}M_Sun) disks, planetesimal eccentricity isusually determined by the gravity of the eccentric disk alone, and iscomparable to the disk eccentricity. As a result, the latter imposes a lowerlimit on collisional velocities of solids, making their growth problematic.This fragmentation barrier can be removed if the gaseous disk rapidly precessesor if its own self-gravity is efficient at lowering disk eccentricity.

Planet formation in binaries: dynamics of planetesimals perturbed by the eccentric protoplanetary disk and the secondary