On the nature of FU Orionis objects.

We argue that rapid accretion onto pre-main sequence stars causes the outbursts of FU Orionis objects. In our model, emission from the hot, optically thick accretion disk dominates the system light at maximum. The radial temperature gradient of the disk produces an M-type spectrum in the infrared, a G-type spectrum at optical wavelengths, and an excess below 4000 Å, as observed. The absence of detectable radial velocity variations leads us to suggest that the accreted material comes from a surrounding protostellar nebula, rather than a companion star. A crucial prediction of the disk model is that weak absorption lines should be double-peaked. We report high-resolution spectra of V1057 Cyg which show double-peaked profiles in red photospheric lines. Herbig and Petrov have observed a similar effect in FU On, indicating that doubled profiles are characteristic of FU Orionis objects. In principle, the double absorption might be produced by a spectroscopic binary system, but the orbital period of such a system must be short, and no evidence for velocity-crossing has yet been observed. We further propose that the observed massive winds of FU Orionis objects arise from the luminous disk surface. The large occultation of redshifted material implied by this picture is in agreement with the meager redshifted emission observed in the Hα P Cygni profiles of these objects. Bipolar outflows have been associated with a number of pre-main sequence stars; we point out that ejection from a disk can naturally and efficiently produce winds with the appropriate geometry.