2018ApJ...869...19W

An accretion-decretion (A-D) circumstellar disk model, suitable for analysis of light and radial velocity (RV) curves, is developed for application to double contact binaries. A foundational hypothesis is that systems as different as cataclysmic variables and W Serpentis binaries-types that appear to have next to nothing in common other than being highly evolved, share the morphological trait of double contact and the related evolutionary trait of having A-D disks. The development is built upon a globally self-gravitating equipotential disk model and allows disk semi-transparency by attenuation of internal disk light and the light of both binary components. Tidal stretching of the disk with consequent brightness variation, as in the "ellipticity" effect for ordinary binaries, is a natural consequence of the disk's tidally distended structure. Light/velocity curve fitting for β Lyrae and CI Aquilae explores the idea that accretion and decretion can co-exist in statistical equilibrium. The basic similarity between CI Aql's pre- and post-eruption light curves-in both form and overall brightness, establishes that its disk was not seriously affected by the outburst of early 2000. Model computations show that in principle the Rossiter-McLaughlin RV disturbance should be very large for disks and a good diagnostic of orbital inclination, although absorption line disk velocities have not yet been measured for β Lyr or CI Aql.