Low-mass companions to T Tauri stars: A mechanism for rapid-rise FU Orionis outbursts

We show that outside-in disc instabilities, which can produce rapid-rise FU Orionis outbursts in T Tauri systems, are a natural consequence of the existence of protoplanetary/protostellar companions. The protoplanetary/protostellar companion would be formed through gravitational instability on a dynamical time-scale (similar to 10(5) yr), and, provided that it was formed at less than a few au, it would be swept in to similar to 15 R. in less than 10(5) yr. A companion of mass 10(-2) M. at this radius would act as a flood gate which stores material upstream and periodically releases it following the ignition of the thermal ionization instability.