2008A&A...492..731M

We present the JHK-band high-resolution polarimetric images of the low-mass proto-stellar object HL Tau using the adaptive optics-equipped CIAO instrument on the Subaru telescope. Our polarization images show a butterfly-shaped polarization disk with an ∼0.9"x3.0" extension. In the nebula, where polarization vectors are centro-symmetrically aligned, the polarization is as high as P_J∼30%, P_H∼42%, and P_K∼55%. On the other hand, low polarizations of P<3% in the J, H, and K bands and a low color excess ratio of E_J–H/E_H–K=1.1 compared to the standard cloud value of 1.75 are detected towards the central star. We estimated the upper limit of the grain sizes amax to be 0.4µm in the nebula and >0.7µm in the line of sight towards the central star. Our high-resolution polarimetric data, which spatially resolves the polarization disk, provides us with important information about grain growth in the region close to the central star.