FU Orionis eruptions and early stellar evolution
Flare Stars in Star Clusters, Associations and the Solar Vicinity
The FU Orionis phenomenon has attracted increasing attention in recent years, and is now accepted as a crucial element in the early evolution of low mass stars. The general characteristics of FUors are outlined and individual members of the class are discussed. The discovery of a new FUor, BBW 76, is presented, together with a discussion of photometric and spectroscopic observations of the star. The evidence for circumstellar disks around Τ Tauri stars is briefly outlined, and the FUor
... d the FUor phenomenon is discussed in the context of a disk accretion model. A large increase in the accretion rate through a circumstellar disk makes the disk self-luminous with a luminosity two or more orders larger than that of the star. Massive cool winds rise from FUors, and it is conceivable that they are related to the initiation of Herbig-Haro flows. The FUor phenomenon appears to be repetitive, and newborn low-mass stars may be cycling between the FUor state and the Τ Tauri state. 1 The FUor phenomenon The story of FU Orionis eruptions began in 1936, when a faint red variable star, FU Orionis, located in the dark cloud B35 in the λ Ori region, brightened by 6 magnitudes in a period possibly as short as four months. Early photometry has been discussed by Wachmann (1939, 1954) and a detailed photometric and spectroscopic analysis is presented by Herbig (1966, 1977). There are now five such objects known, dubbed FUors by Ambartsumian (1971), which all have in common that they were observed to brighten considerably, although on rather different time scales. In their bright states, these stars share a number of spectroscopic properties, notably F-G type supergiant spectra with abnormally strong Balmer absorption lines, strong Li I 6707 lines, pronounced Ρ Cygni profiles at Ha and several other lines, and broad lines suggesting rapid rotation. They are all located in regions of recent star formation, and are associated with reflection nebulae. After 229 L. V. Mirzoyan et al. (eds.), Flare Stars in Star Clusters, Associations and the Solar Vicinity, 229-251.