High-resolution near-infrared polarimetry of a circumstellar disk around UXTauA

We present H-band polarimetric imagery of UXTauA taken with HiCIAO/AO188 on the Subaru Telescope. UXTauA has been classified as a pre-transitional disk object, with a gap structure separating its inner and outer disks. Our imagery taken with the 0. ''15 (21AU) radius coronagraphic mask has revealed a strongly polarized circumstellar disk surrounding UXTauA, which extends to 120AU, at a spatial resolution of 0. ''1 (14AU). It is inclined by 46 ± 2, since the west side is nearest. Although SED modeling and sub-millimeter imagery have suggested the presence of a gap in the disk, with the inner edge of the outer disk estimated to be located at 25-30AU, we detect no evidence of a gap at the limit of our inner working angle (23AU) at the near-infrared wavelength. We attribute the observed strong polarization (up to 66%) to light scattering by dust grains in the disk. However, neither polarization models of the circumstellar disk based on Rayleigh-scattering nor Mie-scattering approximations were consistent with the observed azimuthal profile of the polarization degrees of the disk. Instead, a geometric optics model of the disk with nonspherical grains with radii of 30μm is consistent with the observed profile. We suggest that the dust grains have experienced frequent collisional coagulations, and have grown in the circumstellar disk of UXTauA.