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 d ark cloud, spatially resolving for the first time their 10 mu m emissi on. The weak-emission T Tauri star DI Tau, tentatively identified by S krutskie 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 inf rared excess at a wavelength of 10 mu m The nearby classical T Tauri s tar DH Tau exhibits excess emission at 10 mu m consistent with predict ions 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 accretio n disk, and DI Tau is a binary system lacking such a disk. The compani on to DI Tau has a very low luminosity and is located at a projected d istance of similar to 20 AU from the primary. Assuming the system to b e coeval, we derive a mass below the hydrogen burning limit for the co mpanion. We speculate that the formation of a substellar mass companio n has led to the rapid evolution of the circumstellar disk that may ha ve surrounded DI Tau.