In Search of HL Tauri

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Abstract
We present infrared polarimetric images that demonstrate that the T Tauri star HL Tau is not seen in direct light at λ ≤ 2.2 μm. The position of the intensity centroid moves monotonically southwest from V (0.555 μm) through J (1.25 μm) and H (1.65 μm) to K (2.2 μm) wave bands. Furthermore, the mean position of the J, H, and K polarimetric centroids—which marks the position of the illuminating source of extensive reflection nebulosity—is displaced ~1'' southwest of the position of the K-band intensity peak. This offset is in the same direction as the offset between the K-band intensity peak and the centroid of previously detected 3.6 cm emission. We conclude that, at optical and near-IR wavelengths, all of the observed radiation is scattered light from the circumstellar nebula. Our polarimetric images of HL Tau show a "polarization disk" at PA 134° and therefore support a model in which the projection of the equatorial plane of the protoplanetary disk is aligned at PA 134°. These results reinforce the contention that HL Tau is younger and more deeply embedded than typical T Tauri stars.