  We present high quality near infrared imaging polarimetry of HL Tau at 0.4 to
0.6 arcsec resolution, obtained with Subaru/CIAO and UKIRT/IRCAM. 3-D 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 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 polarisation appears to be dominated by dichroic extinction, with a
position angle inclined by ~40 degrees to the disc axis. The polarisation
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 polarisation data. The best
fit Monte Carlo model indicates a shallow near infrared extinction law. When
combined with the observed high polarisation 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 um. The best
fit 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.
