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

Abstract

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 proto-planetary/protostellar companions. The protoplanetary/protostellar companion would be formed through gravitational instability on a dynamical time-scale ( ∼ 10⁵ yr), and, provided that it was formed at less than a few au, it would be swept in to ∼ 15 R_⊙ in less than 10⁵ 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.