SYNCHRONIZATION OF THE MAGNETIC NOVA-V1500 CYGNI

The outburst of Nova Cyg 1975, which disrupted the spin/orbit synchronism of the AM Herculis-type host binary (V1500 Cyg), created an ideal system in which to study the coupling processes which act between the stellar components of strongly magnetic binaries. Extensive polarimetric monitoring of the system has now provided a direct measurement of the magnitude of these torques in the form of a detection of spin-down of the magnetic primary star. Two-color circular polarimetry of V1500 Cyg indicates a polar magnetic field strength consistent with those measured in the synchronized AM Her variables (B(p) greater-than-or-similar-to 25 MG). However, the symmetry and long-term repeatability of the polarized flux curves suggest that accretion occurs onto large regions of the white dwarf surface, and thus that the streams are likely being fed by an abbreviated disk or accretion "ring." The presence of a disk in V1500 Cyg and its absence in AM Hers of similar period may be due to its unusually massive primary, which implies a larger system and substantially reduced magnetic stresses at the radius where disk formation occurs. The locking torque in V1500 Cyg is sufficiently strong to return the magnetic primary star to rotational synchronism with the orbital period in only approximately 150 yr. In the context of synchronization by MHD dissipation, the interaction is so strong as to require either that the primary star have virtually the Chandrasekhar mass (i.e., be tiny) or that the companion also be magnetic (mu-2 greater-than-or-similar-to 2 x 10(34) G cm3). The dissipation of rotational energy, equivalent to a few percent of a solar luminosity, may cause ohmic heating of the companion and/or the acceleration of particles in the binary magnetosphere. If the normal evolution of a strongly magnetic cataclysmic variable includes a phase similar to that observed in V1500 Cyg, secular synchronization of the binary is very rapid and unlikely to be detected except in large samples of objects.