High-velocity jets in recurrent nova outbursts

Very fast bipolar mass outflows are suggested from emission-line profiles during the early stage of outbursts in the recurrent nova U Sco. The ejection velocity reaches 5000 or 6000 km s(-1) at the optical peak and gradually decreases in time. Such properties have not been reproduced so far in nova theories. We propose a jet-shaped mass outflow as a mechanism of acceleration up to several thousand kilometers per second. The mass flow is accelerated where the jet is shaped, which is deep inside the region where the spherically symmetric winds would be accelerated. The terminal jet velocity depends sensitively on the white dwarf mass but weakly on other parameters.