An Edge-On Circumstellar Disk in the Young Binary System HK Tauri

Abstract

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 λ<=2.2 μm. 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 HH 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°, 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_solar (~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.

Using the NASA/ESA Hubble Space Telescope.

Based on observations made with the Canada-France-Hawaii Telescope, operated by the NRC of Canada, the CNRS de France, and the University of Hawaii.