Eruptive phenomena in early stellar evolution.

Available optical material on FU Ori, V1057 Cyg, and V1515 Cyg is reviewed, and some tentative ideas are offered concerning the nature and frequency of the FU Ori phenomenon. The more recent photometric behavior of FU Ori and evidence for its association with interstellar matter are described, with attention given to the star's IR excess, photographic/B light curve, and radial velocities. The early history of V1057 is examined along with its spectral classification, radial velocities, line structure, colors, dimensions, IR excesses, and associated reflection nebulae. The discovery of V1515 Cyg is discussed, spectral characteristics and statistics of the three known FU Ori variables are evaluated, and it is suggested that the mean time between successive outbursts of the FU Ori type in an individual T Tauri star is roughly 10,000 years. Some effects of one or more FU Ori-type eruptions in the young sun on matter in the inner solar nebula are considered together with several proposed explanations for the cause of such outbursts. It is concluded that no convincing explanation of the FU Ori phenomenon is as yet available.