The transitional pre-main-sequence object DI Tauri: Evidence for a substellar companion and rapid disk evolution

We report mid-IR observations of two young stars found in the Taurus dark cloud, spatially resolving for the first time their 10 mu m emission. The weak-emission T Tauri star DI Tau, tentatively identified by Skrutskie et al. on the basis of 12 mu m IRAS data as an object in the process of dissipating its circumstellar disk, is found to have no infrared excess at a wavelength of 10 mu m The nearby classical T Tauri star DH Tau exhibits excess emission at 10 mu m consistent with predictions based on circumstellar disk models. While both objects appear to have the same stellar mass, age, and rotation rate, they differ in two fundamental respects: DH Tau is a single star with an active accretion disk, and DI Tau is a binary system lacking such a disk. The companion to DI Tau has a very low luminosity and is located at a projected distance of similar to 20 AU from the primary. Assuming the system to be coeval, we derive a mass below the hydrogen burning limit for the companion. We speculate that the formation of a substellar mass companion has led to the rapid evolution of the circumstellar disk that may have surrounded DI Tau.