THE EJECTION OF T-TAURI STARS FROM MOLECULAR CLOUDS AND THE FATE OF CIRCUMSTELLAR DISCS

We investigate the evolution of circumstellar discs around T Tauri sta rs that are ejected from small stellar clusters within molecular cloud s. In particular, we study how the interaction that leads to ejection may hasten the transition between classical and weak-lined T Tauri sta tus. In our models, ejections of T Tauri stars at velocities of 3-10 k m s(-1) truncate the accretion disc at radii between 1 and 10 au, redu cing the viscous evolution time of the disc so that accretion rapidly ceases. The observational appearance of the resulting systems is then dependent on the presence or absence of a stellar magnetic field. For non-magnetic stars we find that a near-infrared excess should persist as a result of the reprocessing of stellar radiation, but that this is greatly diminished for magnetic T Tauri stars by the presence of a ma gnetosphere extending to corotation. In either case, there is a period when ejected stars should appear as non-accreting systems with detect able circumstellar material at wavelengths of 5 mu m and beyond. We di scuss the implications of these results for models in which ejected st ars contribute to the halo of pre-main-sequence objects discovered fro m ROSAT observations of star-forming regions and the All-Sky Survey.