AAS 207th Meeting, 8-12 January 2006
Session 84 Planets in Binary Star Systems
Special Session, Tuesday, 10:00-11:30am, January 10, 2006, Maryland

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[84.03] Gas Giant Protoplanets Formed by Disk Instability in Binary Star Systems

A. P. Boss (DTM - Carnegie Institution)

Roughly half of nearby primary stars are members of binary or multiple systems, so the question of whether or not they can support the formation of planetary systems similar to our own is an important one in the search for life outside our solar system. Previous theoretical work has suggested that binary star systems might not be able to permit the formation of gas giant planets, because of the heating associated with shock fronts driven in the stars' protoplanetary disks by tidal forces during the periodic close encounters between the two stars. As a result, the disks could become too hot for icy bodies to exist, thereby preventing giant planet formation by the core accretion mechanism, and too hot for giant planets to form by the disk instability mechanism. However, gas giant planets have been discovered in orbit around a number of stars that are members of binary or triple star systems, with binary semimajor axes ranging from ~ 12 to over 1000 AU. We present here a suite of three dimensional radiative gravitational hydrodynamics models suggesting that binary stars may be quite capable of forming planetary systems similar to our own. One difference between the new and previous calculations is the inclusion of artificial viscosity in the previous work, leading to significant conversion of disk kinetic energy into thermal energy in shock fronts and elsewhere. New models are presented showing how vigorous artificial viscosity can help to suppress clump formation. The new models with binary companions do not employ any explicit artificial viscosity, and also include the third (vertical) dimension in the hydrodynamic calculations, allowing for transient phases of convective cooling. The new calculations of the evolution of initially marginally gravitationally stable disks show that the presence of a binary star companion may actually help to trigger the formation of dense clumps that could become giant planets. Earth-like planets would form much later in the inner disk regions by the traditional collisional accumulation of progressively larger, solid bodies. We also show that in models without binary companions, which begin their evolution as gravitationally stable disks, the disks evolve to form dense rings, which then break-up into self-gravitating clumps. These latter models suggest that the evolution of any self-gravitating disk with sufficient mass to form gas giant planets is likely to lead to a period of disk instability, even in the absence of a trigger such as a binary star companion.

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