Evolution of Circumstellar Disks in Binary Star Systems

Abstract

The discoveries of several exoplanets in close binary star systems by the radial velocity technique have motivated new research into improving our understanding of planet formation in stellar binaries. The largest problem confronting our current theory of planet formation in binaries is the fragmentation barrier to planetesimal growth. In this stage of planet formation, the collisional growth of planetesimals into planet cores requires very low impact velocities, and is consequently highly sensitive to gravitational perturbations by the stellar companion. These perturbations generally drive eccentricity of planetesimal orbits and drastically raise impact velocities, causing collisional destruction rather than growth. Recent studies incorporating the gas drag and the gravity of an eccentric protoplanetary disk show that, in sufficiently massive and weakly-eccentric circumstellar protoplanetary disks, these effects may reduce relative planetesimal eccentricities and impact velocities enough to allow planet formation in close binaries. This process places tight constraints on the disk characteristics, however: the disk needs to be large enough in mass, and have low eccentricity and good alignment with the binary apsidal line for planet formation to occur. The focus of this paper is a parameter study where we examine the dependence of disk behavior – e.g., disk size or eccentricity – on several disk system parameters. We find that the disk eccentricity generally settles at a low value around 0.05 in the steady state in systems like γ Cep, with the exceptions of models with low binary eccentricity or small aspect ratio. The disk experiences initial oscillations in free eccentricity which damp over hundreds of binary orbits. The final disk orientation is usually misaligned by around 180◦ with the binary apsidal line.