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-\null rise FU Orionis outbursts in T~Tauri systems, are a natural consequence of the existence of protoplanetary protostellar companions. The \pcomp{} would be formed through gravitational instability on a dynamical timescale ($\sim 10^5\y$), and provided it was formed at less than a few \AU{}, it would be swept in to $\sim 15\text{Rsun}$ in less than ${10}^5\text{y}$. A companion of mass ${10}^{-2}$\Msun at this radius would act as a flood gate which stores material upstream and periodically releases it following the ignition of the thermal ionisation instability.