Matthew R. Bate
Ph.D. Thesis, University of Cambridge, U.K. (1995)
In this thesis, a new method for following fragmentation calculations beyond the formation of protostars is presented. The numerical technique of Smoothed Particle Hydrodynamics (SPH) is modified to incorporate sink particles. In standard SPH, high-density regions of gas are represented by many particles in a small volume of space. Short time steps are required to evolve these particles correctly, and this results in fragmentation calculations being halted soon after protostars are formed due to their high densities. In the new method, the many gas particles representing each protostar are replaced by a single, non-gaseous particle, with appropriate boundary conditions. These sink particles contain all the mass of the particles they replace, and accrete any infalling mass. This enables a collapse calculation to be followed to an arbitrary time after the protostars are formed, and, thus, for the final results to be determined.
Using this method, the effects of accretion on a forming binary system are studied. A binary is placed within a gaseous cloud from which it accretes. The effects of this accretion on the mass ratio and orbital separation of the system are studied. This study is performed first in the ballistic case, where the gas is modelled by non-interacting particles, and then by gaseous accretion using SPH with pressure and viscous forces. The effects of the accretion are found to depend primarily on the mass ratio of the binary and the angular momentum of the infalling cloud. For the accretion of low-angular-momentum material, the binary's separation deceases, and its mass ratio is lowered. For high-angular-momentum accretion, the binary's separation increases, and its mass ratio is forced toward unity. The magnitude of the effects depends on the mass ratio. The formation of circumstellar and circumbinary discs is also studied. It is found that circumstellar discs are formed around only the primary for low-angular-momentum material, around both protostars for intermediate-angular-momentum material, and around both protostars for the highest-angular-momentum material with a circumbinary disc also present. Finally, conclusions are drawn on how accretion may help to explain the mass-ratio distributions and the presence of discs that are observed for binary systems.
Contents
Chapters 1-5 and 8 are available as gzipped postscript files.
1 Introduction (includes Abstract)
2 Smoothed Particle Hydrodynamics
4 Modelling Accretion on to Protobinary Systems
5 The Effects of Ballistic Accretion on a Protobinary System
6 The Effects of Gaseous Accretion on a Protobinary System
7 Protobinary Evolution Under Massive Accretion
8 Conclusions and Observational Implications (includes Bibliography)