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Session 10 - Interstellar Medium and Star Formation.
Display session, Monday, June 08
Atlas Ballroom,
Hubble Space Telescope images of HK Tauri reveal that the companion star in this 2.4 arcsec (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 wavelenghts less than 2.2 microns. 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 (Burrows et al. 1996, Ap.J. 473 437). HK Tau/c is therefore the first circumstellar disk to be clearly resolved 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, inclination of about 5 degrees, scale height of 3.8 AU at r= 50 AU, and is flared. The absence of a point source in the near-IR requires Av > 50 mag toward the unseen central star. The thickness of the dark lane establishes a disk mass near 0.0001 solar masses of dust and gas, if the dust grains have interstellar properties and remain fully mixed vertically. With the observed disk radius equal to only 1/3 the projected separation of the binary, there is a strong possibility that tidal truncation of the circumsecondary disk has occurred in this system.
