A SEARCH FOR CIRCULAR-POLARIZATION IN CATACLYSMIC VARIABLES

The AM Her-type of mass-transfer binary displays strong circular polarization in its optical flux, due to high harmonic cyclotron emission from the vicinity of the accretion shock. Circular polarization is the best direct evidence for magnetic white dwarf primaries in these cataclysmic variable (CV) systems, indicating high field strengths of B greater-than-or-similar-to 10(7) G. For more than a decade, we have carried out an optical and infrared polarimetric observing program at Steward Observatory to search for AM Her systems and to detect circularly polarized light in CV systems in which a magnetic white dwarf may be present. This survey and similar work by our collaborators has led to the discovery of over half of the 19 polarized, high-field CV systems. For the remaining survey objects, which include most of the so-called intermediate polars and members from all other CV classes, our results place strict limits on the number densities and field strengths of high field systems. In particular, we argue that the lack of AM Her variables with higher field strengths than have been observed (B > 6 x 10(7) G) and asynchronous CV systems with moderate fields (B > 5 x 10(6) G) is not due to selection effects or to the survey sensitivity but must reflect a difference in the period evolution of magnetic systems with respect to nonmagnetic CVs. We suggest that asynchronous diskless magnetic CVs may accelerate their evolution through a "propeller-like" expulsion of material and associated angular momentum.