The formation, evolution, and survivability of discs around young binary stars

Abstract

In the age of the Kepler space telescope and other exoplanet finding missions, a variety of exotic planets have been discovered. Some of these planets have been found to be in binary star systems --- systems which have historically been overlooked in planet formation models. This is due to the single star scenario being simpler to model than binaries, as well our anthropocentric bias towards single stars like our Sun. However, planet formation around binary stars in an important topic because a large fraction (50%) of stars form in binary systems.

In this thesis I investigated the physics that influences the creation, stability, and survivability of discs around binary stars with the broad understanding that the longer the lifetime of a disc (around a single or binary star) the higher the likelihood of producing planets.

The theoretical work of this thesis was conducted using the ideal magnetohydrodynamical numerical simulation program FLASH. I simulated the collapse of molecular cores until the formation of protostars and followed the early evolution of these systems. For the first theoretical project I investigated the influence that binarity had on the global evolution of a young stellar system. This included studying mechanisms such as accretion, jets and outflows, and dynamical interactions. I found that binary stars produce weaker outflows when considering the transport of mass, linear momentum, and angular momentum. For the second theoretical project I investigated the formation of discs in binary stars with the inclusion of turbulence in the initial conditions. I found that the turbulence helped to build large circumbinary discs which restructured the magnetic fields for efficient outflow launching, but too much turbulence may also disrupt this organisation of magnetic fields. Given the environment where binary stars form (turbulent molecular cores), it appears that the formation of circumbinary discs should be common place, however circumstellar discs could also be destroyed quickly in these same environments.

My observational work aimed to determine the typical survivability of discs around binary star systems. This work was carried out by using the Wide Field Spectrograph (WiFeS) on the Australian National University 2.3m Telescope to search for radial velocity variation in disc-bearing members of the 11Myr and 17Myr old star-forming regions Upper Scorpius and Upper Centaurus-Lupus. I found that the binary fraction of disc-bearing stars in these regions do not differ significantly from the field star binary fraction. I hypothesised that this is due to two competing factors: circumstellar discs are disrupted by companions and are dispersed quickly, but circumbinary discs are more common than equivalently sized discs around single stars. These results suggest that the typical lifetimes of discs in single and binary stars are comparable.

Overall, I found that in some scenarios binary stars may produce hostile environments for planet formation via the destruction of circumstellar discs, but the formation of large circumbinary discs is likely to be a common occurrence. This suggests that planet formation is as likely around binary stars as single stars. Therefore, planet formation around binary stars needs to be considered to understand overall planet formation.