The Dynamics and Energetics of Circumbinary Discs

The student project will entail investigations of circumbinary orbits around binary star systems in the Galaxy of varying mass ratio and eccentricity. These investigations will take place via computational calculation and via direct imaging of time series of spectroscopic data from the Global Jet Watch observatories. This is a fruitful and important area of study for the following four reasons:

- PLANETS: The first circumbinary planet (Kepler 16b) was discovered in September 2011 (three weeks after my previous Doctoral student and I published a paper on 3D N-body simulations that had demonstrated beyond doubt the existence of stable circumbinary orbits if the definition of stable is sufficiently liberated as to include precession in these orbital planes. We discovered three remarkably stable families of orbits, described in that work, and in the last few years Kepler and other missions have discovered further circumbinary planets. It remains to be explored and understood how the gas can cool and coalesce on a timescale appropriate to planet formation
- ECCENTRICITY: It remains a mystery why the eccentricity of some binary star systems persists long after they should have circularised. It has been suggested that a body in a circumbinary orbit can not only preserve but drive this eccentricity, but it remains to be explored how this can take place when that third body is smeared out into a ring or disc, and furthermore, when this entity is precessing.
- MASS-LOSS: in the only circumbinary disc well-studied to date, that of the Galactic microquasar SS433, the amount of mass-loss from the circumbinary disc is very comparable with that lost via its accretion disc wind. In order to properly account for how mass-transfer and mass-loss takes place in such binary systems, and more broadly how binary evolution proceeds, it is essential to understand what part is played by mass lost from the matter on circumbinary orbits
- ACCRETION: Krolik (Johns Hopkins) has suggested that circumbinary discs can actually be the means by which binaries can accrete material from their surroundings.

Given the importance of these four areas separately, this suggests that this is an opportune time to investigate them collectively: via computational modelling and via direct long-term times series spectroscopy that I have been collecting for this student over the last 1+ year from my Global Jet Watch observatories.