The Disk Around CoKu Tau/4: Circumbinary, not Transitional

Abstract

CoKu Tau/4 has been labeled as one of the very few known transition disk objects: disks around young stars that have their inner disks cleared of dust, arguably due to planetary formation. We report aperture-masking interferometry and adaptive optics imaging observations showing that CoKu Tau/4 is in fact a near-equal binary star of projected separation $\sim$53 mas ($\sim$8 AU). The spectral energy distribution of the disk is then naturally explained by inner truncation of the disk through gravitational interactions with the binary star system. We discuss the possibility that such ``unseen'' binary companions could cause other circumbinary disks to be labeled as transitional.