Orbital Evolution of Algol Binaries with a Circumbinary Disk

Abstract

It is generally thought that conservative mass transfer in Algolbinaries causes their orbits to be wider, so that the less massive staroverflows its Roche lobe. The observed decrease in the orbital periodsof some Algol binaries suggests orbital angular momentum loss during thebinary evolution, and the magnetic braking mechanism is often invoked toexplain the observed orbital shrinkage. Here we suggest an alternativeexplanation, in which a small fraction of the transferred mass forms acircumbinary disk, which extracts orbital angular momentum from thebinary through tidal torques. We also perform numerical calculations ofthe evolution of Algol binaries with typical initial masses and orbitalperiods. The results indicate that, for reasonable input parameters, thecircumbinary disk can significantly influence the orbital evolution, andcause the orbit to shrink on a sufficiently long timescale. Rapid masstransfer in Algol binaries with low mass ratios can also be accountedfor in this scenario.