The Structure and Evolution of Circumbinary Disks in Cataclysmic Variable Systems

We investigate the structure and evolution of a geometrically thin, viscous, Keplerian circumbinary (CB) disk, using detailed models of their radiative/convective vertical structure. We use a simplified description for the evolution of the cataclysmic binary and focus on cases where the circumbinary disk causes accelerated mass transfer (≳10-8 M☉ yr-1). The inner edge of the disk is assumed to be determined by the tidal truncation radius, and the mass input rate into the disk is assumed to be a small fraction (10-5 to 0.01) of the mass transfer rate. Under the action of the viscous stresses in the disk the matter drifts outward with the optically thick region extending to several AU. The inner part of the disk is cool with maximum effective temperatures ≲3000 K, while the outermost parts of the disk are ≲30 K and optically thin. We calculate the effects of thermal instability on a sufficiently massive CB disk. It leads to outbursts reminiscent of those in thermally unstable accretion disks, with the instability remaining confined to the inner regions of the CB disk. However, for most of the evolutionary sequences, the surface densities required to trigger instability are not reached. The spectral energy distributions from circumbinary disks are calculated, and the prospects for the detection of such disks in the infrared and submillimeter wavelength regions are discussed.