Adaptive Optics 0.̋2 Resolution Infrared Images of HL Tauri: Direct Images of an Active Accretion Disk around a Protostar

Abstract

We have obtained FWHM images of HL Tau at K@, H, and J utilizing the University of Hawaii 0A .2 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 TauÏs circumstellar environment. An active accretion disk is directly resolved around HL Tau for the Ðrst time in the infrared. The physical characteristics of this accretion disk AU, and P.A. D 125¡) are consistent with the inner disk (R D D 150 discovered by submillimeter (0.8 mm) interferometry by Lay et al. (1994), and conÐrmed by 2.7 mm inter-ferometry by Mundy et al. (1996). Bipolar cavities aligned with the accretion disk axis are for the Ðrst 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 D30 km s~1. This cavity is estimated to have been created in an outburst in the direction of the optical jet D100 yr ago. Accurate photometry and astrometry were obtained for the nearby XZ Tau binary and the unre-0A .3 solved HL Tau star ] inner disk at K@, H, and for the Ðrst time at J. The large H[K \ 2.14 ^ 0.11 color of the HL Tau point source indicates an extinction of along the line of A J \ 7.73 ^ 0.42 (A V D 24) sight to the star. Based on this large extinction, the SED for HL TauÏs 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 yr~1 onto a stable accretion disk AU) around a young 0.7 preÈmain-sequence (PMS) M _ (R D \ 150 M _ star. We Ðnd that to reproduce the observed SED, the central unresolved source in HL Tau is required to be a very young (D105 yr) PMS star surrounded by an active accretion disk. The large observed extinction from the inclined disk implies an estimated accretion disk mass of D0.04 M _. Subject headings : circumstellar matter È infrared : stars È stars : individual (HL Tauri) È stars : preÈmain-sequence