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RE: CoKu Tau/4
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Title: Wall emission in circumbinary disks: the case of CoKu Tau/4
Authors: Nagel, Erick; D'Alessio, Paola; Calvet, Nuria; Espaillat, Catherine; Sargent, Ben; Hernandez, Jesus; Forrest, William J.

A few years ago, the mid-IR spectrum of a Weak Line T Tauri Star, CoKu Tau/4, was explained as emission from the inner wall of a circumstellar disk, with the inner disk truncated at ~10 AU. Based on the SED shape and the assumption that it was produced by a single star and its disk, CoKu Tau/4 was classified as a prototypical transitional disk, with a clean inner hole possibly carved out by a planet, some other orbiting body, or by photodissociation. However, recently it has been discovered that CoKu Tau/4 is a close binary system. This implies that the observed mid-IR SED is probably produced by the circumbinary disk. The aim of the present paper is to model the SED of CoKu Tau/4 as arising from the inner wall of a circumbinary disk, with parameters constrained by what is known about the central stars and by a dynamical model for the interaction between these stars and their surrounding disk. In order to fit the Spitzer IRS SED, the binary orbit should be almost circular, implying a small mid-IR variability (10%) related to the variable distances of the stars to the inner wall of the circumbinary disk. Our models suggest that the inner wall of CoKu Tau/4 is located at 1.7a, where a is the semi-major axis of the binary system (a~8AU). A small amount of optically thin dust in the hole (<0.01 lunar masses) helps to improve the fit to the 10microns silicate band. Also, we find that water ice should be absent or have a very small abundance (a dust to gas mass ratio <5.6X10^{-5}). In general, for a binary system with eccentricity e>0, the model predicts mid-IR variability with periods similar to orbital timescales, assuming that thermal equilibrium is reached instantaneously.

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Title: The Disk Around CoKu Tau/4: Circumbinary, not Transitional
Authors: M.J. Ireland, A.L. Kraus
(Version v2)

CoKu Tau/4 has been labelled 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 labelled as transitional.

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