A BINARY ORIGIN FOR FU ORIONIS STARS

We explore the possibility that the FU Orionis phenomenon is due to the binary nature of the stellar source. In our model, tidal effects from a companion induce enhanced accretion rates. In an eccentric system the effects are greatest at closest approach. Additionally, mass transfer from the primary's to the secondary's disk at closest approach can provide the necessary matter that is accreted. Accretion rates by this process can exceed 1 x 10(-4) M. yr-1 and lead to significant increases in the masses of the components. The outbursts are periodic, occurring at each closest approach, in good agreement with the observed statistics. The binary nature of the exciting source is expected to be at least partially obscured by the differential reddening expected due to the accretion processes in an unequal mass binary system. The system is thus composed by an optically visible secondary with an infrared companion as the primary. The detection of an infrared companion to the FUor star Z CMa lends credence to our model.