Adaptive optics 0″.2 resolution infrared images of HL Tauri: Direct images of an active accretion disk around a protostar

We have obtained 0″.2 FWHM images of HL Tau at K′, H, and J utilizing the University of Hawaii Adaptive Optics System at the 3.6 m CFHT. These are the highest resolution deep images of HL Tau ever obtained in the infrared. They provide unique insight into HL Tau's circumstellar environment. An active accretion disk is directly resolved around HL Tau for the first time in the infrared. The physical characteristics of this accretion disk (R_D ∼ 150 AU, and P.A. ∼ 125°) are consistent with the inner disk discovered by submillimeter (0.8 mm) interferometry by Lay et al. (1994), and confirmed by 2.7 mm interferometry by Mundy et al. (1996). Bipolar cavities aligned with the accretion disk axis are for the first time detected in the infrared. We have monitored the upper cavity at comparable angular resolution for three epochs over the last 2 yr. The cavity appears to be expanding at up to ∼ 30 km s^-1. This cavity is estimated to have been created in an outburst in the direction of the optical jet ∼ 100 yr ago. Accurate photometry and astrometry were obtained for the nearby 0″.3 XZ Tau binary and the unresolved HL Tau star + inner disk at K′, H, and for the first time at J. The large H - K = 2.14 ± 0.11 color of the HL Tau point source indicates an extinction of A_J = 7.73 ± 0.42 (A_V ∼ 24) along the line of sight to the star. Based on this large extinction, the SED for HL Tau's unresolved central source was dereddened. A simple accretion disk + star model reproduced the newly dereddened SED. The model assumed a large infalling envelope (as observed in ¹³CO; see Hayashi et al. 1993) accreting at 5 × 10^-6 M_⊙ yr^-1 onto a stable accretion disk (R_D = 150 AU) around a young 0.7 M_⊙ pre-main-sequence (PMS) star. We find that to reproduce the observed SED, the central unresolved source in HL Tau is required to be a very young (∼ 10⁵ yr) PMS star surrounded by an active accretion disk. The large observed extinction from the inclined disk implies an estimated accretion disk mass of ∼ 0.04 M_⊙.