The Disk around Coku Tau / 4 : Circumbinary , Not Transitional

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 ∼53 mas (∼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. The lifetime of disks around young stars sets the timescale for giant planet formation. Most young stars have disks that are optically thick at all wavelengths or have no observable disk at all, while a small subset of young stars are in an intermediate state, thought to be in " transition " (Skrutskie et al. 1990). Many of these intermediate objects have optically thick " cold disks " as seen by large excesses at ∼30 µm, but much smaller or non-existent near-and mid-infrared excesses (Brown et al. 2007). One model of this transition process is one where the inner disk is cleared of material, leaving an inner hole and low accretion rates, followed ∼ 10 5 years later by a clearing of the outer disk. This inner hole could be caused by natural processes of grain growth and/or photo-evaporation (Alexander et al. 2006), or truncation of the disk due to dynamical interactions with companions (Lin & Papaloizou 1979). Although the most exciting interpretation for the development of gaps is arguably truncation due to planetary formation, it is also possible to dynamically truncate disks due to stellar companions (e.g Jensen & Mathieu 1997; Beust & Dutrey 2005; White & Hillenbrand 2005). An example of this is CS Cha, which was announced as a transitional disk (Espaillat et al. 2007) shortly after its discovery as a cir-cumbinary disk (Guenther et al. 2007). Some authors (e.g. Furlan et al. 2007) have considered the term " transitional " to be an observational term that means the spectral signature of a cleared inner disk rather than an evolutionary term that implicitly applies to single stars. We use the term " cold disk " for this purpose. CoKu Tau/4 is a weak-lined M1.5 T Tauri star (H-Tau/4 was discovered to have …