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Millimeter-wavelength polarization measurements offer a promising method forprobing the geometry of magnetic fields in circumstellar disks. Single dishobservations and theoretical work have hinted that magnetic field geometriesmight be predominantly toroidal, and that disks should exhibit millimeterpolarization fractions of 2-3%. While subsequent work has not confirmed thesehigh polarization fractions, either the wavelength of observation or the targetsources differed from the original observations. Here we present newpolarimetric observations of three nearby circumstellar disks at 2" resolutionwith the Submillimeter Array (SMA) and the Combined Array for Research inMillimeter Astronomy (CARMA). We reobserve GM Aur and DG Tau, the systems inwhich millimeter polarization detections have been claimed. Despite higherresolution and sensitivity at wavelengths similar to the previous observations,the new observations do not show significant polarization. We also addobservations of a new HAeBe system, MWC 480. These observations demonstratethat a very low (<0.5%) polarization fraction is probably common at large (>100AU) scales in bright circumstellar disks. We suggest that high-resolutionobservations may be worthwhile to probe magnetic field structure on lineardistances smaller than the disk scale height, as well as in regions closer tothe star that may have larger MRI-induced magnetic field strengths.

Interferometric Upper Limits on Millimeter Polarization of the Disks around DG Tau, GM Aur, and MWC 480