Orbital Evolution of Algol Binaries with a Circumbinary Disk

Abstract

It is generally thought that conservative mass transfer in Algol binaries causes their orbits to be wider, in which the less massive star overflows its Roche-lobe. The observed decrease in the orbital periods of some Algol binaries suggests orbital angular momentum loss during the binary evolution, and the magnetic braking mechanism is often invoked to explain the observed orbital shrinkage. Here we suggest an alternative explanation, assuming that a small fraction of the transferred mass forms a circumbinary disk, which extracts orbital angular momentum from the binary through tidal torques. We also perform numerical calculations of the evolution of Algol binaries with typical initial masses and orbital periods. The results indicate that, for reasonable input parameters, the circumbinary disk can significantly influence the orbital evolution, and cause the orbit to shrink on a sufficiently long timescale. Rapid mass transfer in Algol binaries with low mass ratios can also be accounted for in this scenario.