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

TL;DR

This study uses high-resolution millimeter polarimetric observations to show that large-scale circumstellar disks generally exhibit very low polarization fractions, challenging previous claims of higher polarization levels and informing magnetic field structure understanding.

Contribution

The paper provides new high-resolution polarimetric data on three disks, demonstrating that millimeter polarization fractions are typically below 0.5%, refining previous estimates and highlighting the need for smaller-scale observations.

Findings

No significant polarization detected at large disk scales.

Polarization fractions are generally below 0.5%.

High-resolution observations are needed to probe smaller-scale magnetic structures.

Abstract

Millimeter-wavelength polarization measurements offer a promising method for probing the geometry of magnetic fields in circumstellar disks. Single dish observations and theoretical work have hinted that magnetic field geometries might be predominantly toroidal, and that disks should exhibit millimeter polarization fractions of 2-3%. While subsequent work has not confirmed these high polarization fractions, either the wavelength of observation or the target sources differed from the original observations. Here we present new polarimetric observations of three nearby circumstellar disks at 2" resolution with the Submillimeter Array (SMA) and the Combined Array for Research in Millimeter Astronomy (CARMA). We reobserve GM Aur and DG Tau, the systems in which millimeter polarization detections have been claimed. Despite higher resolution and sensitivity at wavelengths similar to the previous observations, the new observations do not show significant polarization. We also add observations of a new HAeBe system, MWC 480. These observations demonstrate that a very low (<0.5%) polarization fraction is probably common at large (>100 AU) scales in bright circumstellar disks. We suggest that high-resolution observations may be worthwhile to probe magnetic field structure on linear distances smaller than the disk scale height, as well as in regions closer to the star that may have larger MRI-induced magnetic field strengths.