An edge-on circumstellar disk in the young binary system HK Tauri

Hubble Space Telescope (HST) images of HK Tauri reveal that the companion star in this 2." 4 (340 AU) pre-main-sequence binary system is an entirely nebulous object at visual wavelengths. HK Tau/c appears as two elongated reflection nebulosities separated by a dark lane. Near-infrared adaptive optics observations made at the Canada-France-Hawaii Telescope show a similar morphology and no directly visible star at lambda less than or equal to 2.2 mu m. HK Tau/c is strikingly similar to scattered light models of an optically thick circumstellar disk seen close to edge-on and to the HST images of KH 30. HK Tau/c is therefore the first disk to be clearly resolved around an individual star in a young binary system. The disk properties have been constrained by fitting model reflection nebulae to the HST images. The disk has a radius of 105 AU, an inclination of about 5 degrees, a scale height of 3.8 AU at r = 50 AU, and is flared. The absence of a point source in the near-IR requires A(v) > 50 mag toward the unseen central star. The thickness of the dark lane establishes a disk mass near 10(-4) M. (similar to 0.1 M-Jupiter) of dust and gas, if the dust grains have interstellar properties and remain fully mixed vertically. With the observed disk radius equal to one-third of the projected separation of the binary, there is a strong possibility that tidal truncation of the circumsecondary disk has occurred in this system.