The circumbinary discs of post-AGB stars

Many post-asymptotic giant branch (post-AGB) stars, the remnants of evolved red giants that will soon become white dwarfs, are in binary-star systems. Having gone through a phase of mass transfer and quite possibly a common-envelope event, such systems are a laboratory for binary-star astrophysics. Our best theories, however, fail to reproduce their properties: these binary stars have orbits that are often far more eccentric than tidal theory allows. Perhaps this is because many post-AGB stars have large, stable circumbinary discs? These are similar in size and mass to protoplanetary discs, and affect their parent stars' orbits significantly even over their short lifetimes. We construct a model of such circumbinary discs and apply it to the disc around IRAS 08544-4431 which was recently observed by ALMA. Our model fits the disc of IRAS 08544-4431 remarkably well and, given its computational speed, forms a platform from which we can explore whole populations of circumbinary discs in binary stars.