Circumstellar disks in binary star systems

Models for γ Cephei and α Centauri

Institut für Astronomie & Astrophysik, Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
e-mail: Tobias_Mueller@twam.info

Received: 4 October 2011
Accepted: 2 December 2011

Abstract

Context. As of today, over 50 planetary systems have been discovered in binary star systems, some of which have binary separations that are smaller than 20 AU. In these systems the gravitational forces from the binary have a strong influence on the evolution of the protoplanetary disk and hence the planet formation process.

Aims. We study the evolution of viscous and radiative circumstellar disks under the influence of a companion star. We focus on the eccentric γ Cephei and α Centauri system as examples and compare disk quantities such as disk eccentricity and precession rate to previous isothermal simulations.

Methods. We performed two-dimensional hydrodynamical simulations of the binary star systems under the assumption of coplanarity of the disk, host star and binary companion. We used the grid-based, staggered mesh code FARGO with an additional energy equation to which we added radiative cooling based on opacity tables.

Results. The eccentric binary companion perturbs the disk around the primary star periodically. Upon passing periastron, spirals arms are induced that wind from the outer disk towards the star. In isothermal simulations this results in disk eccentricities up to e_disk ≈ 0.2, but in more realistic radiative models we obtain much smaller eccentricities of about e_disk ≈ 0.04−0.06 with no real precession. Models with varying viscosity and disk mass indicate that disks with less mass have lower temperatures and higher disk eccentricity.

Conclusions. The fairly high disk eccentricities, as indicated in previous isothermal disk simulations, implied a more difficult planet formation in the γ Cephei system caused by the enhanced collision velocities of planetesimals. We have shown that under more realistic conditions with radiative cooling the disk becomes less eccentric and thus planet formation may be made easier. However, we estimate that the viscosity in the disk has to very low, with α ≲ 0.001, because otherwise the disk’s lifetime will be too short to allow planet formation to occur along the core instability scenario. We estimate that the periodic heating of the disk in eccentric binaries will be observable in the mid-IR regime.