First Two-Micron Imaging Polarimetry of β Pictoris

High-resolution K-band imaging polarimetry of the β Pic dust disk has been conducted with adaptive optics and a coronagraph using the Subaru 8.2 m telescope. Polarization of ~10% is detected out to r ~ 120 AU with a centrosymmetric vector pattern around the central star, confirming that the disk is seen as an infrared reflection nebula. We have modeled our near-infrared and previous optical polarization results in terms of dust scattering in the disk and have found that both the degrees of polarization and the radial intensity profiles are well reproduced. We argue that the observed characteristics of the disk dust are consistent with the presence of ice-filled fluffy aggregates consisting of submicron grains in the β Pic system. There is a gap around 100 AU in both the intensity and polarization profiles, which suggests a paucity of planetesimals in this region. The radial intensity profile also shows ripple-like structures, which are indicative of the presence of multiple planetesimal belts, as in the case of the M-type Vega-like star AU Mic.