High-resolution imaging polarimetry of HL Tau and magnetic field structure

We present high-quality near-infrared imaging polarimetry of HL Tau at 0.4-0.6 arcsec resolution, obtained with Subaru/CIAO and UKIRT/IRCAM. Three-dimensional Monte Carlo modelling with aligned oblate grains is used to probe the structure of the circumstellar envelope and the magnetic field, as well as the dust properties. At the J band, the source shows a centrosymmetric pattern dominated by scattered light. In the H and K bands, the central source becomes visible and its polarization appears to be dominated by dichroic extinction, with a position angle inclined by approximate to40degrees to the disc axis. The polarization pattern of the environs on scales up to 200 au is consistent with the same dichroic extinction signature superimposed on the centrosymmetric scattering pattern. These data can be modelled with a magnetic field which is twisted on scales from tens to hundreds of au, or alternatively by a field which is globally misaligned with the disc axis. A unique solution to the field structure will require spatially resolved circular polarization data. The best-fitting Monte Carlo model indicates a shallow near-infrared extinction law. When combined with the observed high polarization and non-negligible albedo, these constraints can be fitted with a grain model involving dirty water ice mantles in which the largest particles have radii slightly in excess of 1 mum. The best-fitting model has an envelope structure which is slightly flattened on scales up to several hundred au. Both lobes of the bipolar outflow cavity contain a substantial optical depth of dust (not just within the cavity walls). Curved, approximately parabolic, cavity walls fit the data better than a conical cavity. The small inner accretion disc observed at millimetre wavelengths is not seen at this spatial resolution.