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The FU Orionis phenomenon

by Ivan Adamin in September-October 2013 (#2)

FU Orionis stars are pre-main-sequence eruptive variables which make up a small class of young low-mass stars appear to be a stage in the development of T Tauri stars. They gradually brighten by up to six magnitudes over several months, during which time matter is ejected, then remain almost steady or slowly decline by a magnitude or two over years. All known FU Ori stars (commonly known as fuors) are associated with reflection nebulae. The article gives a brief description of this kind of objects.

FU Orionis is somewhat 1600 light-years away and associated with a reflection nebulae in the Orion constellation. It is located about three degrees NW of Betelgeuse, and less than a degree east of the small planetary nebula NGC2022. FU Orionis is the prototype of a class of young stellar objects (YSOs), which have undergone photometric outbursts on the order of 4-6 mag in less than one year (Herbig 1966). These stars are still in the process of forming, accreting gas from the clouds they formed from. The first outburst of FU Ori was observed in 1936-37, when an ordinary undiscovered 16th magnitude star began to brighten steadily. Unlike novae bursts, which forth suddenly and then begin to fade within weeks, the FU Ori kept getting brighter and brighter for almost a year being around 10th magnitude ever since.

Typically, FU Orionis star's luminosity peaks at approximately 500 luminosities of the Sun and then appears to decay for decades. FU Orionis stars exhibit large infrared excesses, double-peaked line profiles, apparent spectral types that vary with wavelength, broad, blueshifted Balmer line absorption, and are often associated with strong mass outflows. For more details on this phenomenon please refer to Hartmann & Kenyon (1996).

The FU Orionis stars have been convincingly modeled as low-mass pre-main-sequence stars (similar to T Tauri stars) that are surrounded by luminous accretion disks. Such systems demonstrate that disk accretion in early stellar evolution is highly episodic. It seems that the phenomenon is restricted to early phases of stellar evolution. Steady accretion disk models successfully explain many observations of the FU Orionis. Photometrically FUors are characterized by violent and probable recurrent outbursts.

Since the outburst of FU Ori itself in 1936 (Wachmann 1954; Herbig 1966), only a small number of other FUor outbursts have been discovered, notably V1057 Cyg and V1515 Cyg (Herbig 1977). The light curves of these three best studied FUors show remarkable differences between each other (Vittone & Errico, 2005). In 1969, a star embedded in the "East Texas" region of the North America Nebula (NGC 7000) jumped from 16th to 10th magnitude.

The star, V1057 Cygni, attained maximum luminosity in mid-1970, and afterwards it slowly faded; it now lies three magnitudes below peak brightness. A number of objects are known that are spectroscopically similar to FUors, but for which the outburst phase has not been documented. Spectroscopically, FUors have no-emission-lines spectra with optical characteristics of G-type supergiants, yet near-IR characteristics of cooler K- or M-type giants/supergiants dominated by deep CO overtone absorption (Vittone & Errico, 2005).

Researchers led by Jean-Francois Donati of the Observatoire Midi-Pyrenees in Toulouse, France, have observed the magnetic fields in the central regions of a disc around FU Orionis. They used a new high-resolution instrument called ESPaDOnS on the Canada-France-Hawaii Telescope (CFHT) in Hawaii to detect polarised light (due to the magnetic fields) from the disc. The magnetic fields surrounding a young star have been detected for the first time. Several of fuors were also discovered in the past years, including those from the data of the Catalina Real-time Transient Survey. One more remarkable addition is the embedded protostar V1647 Orionis which erupted in January 2004.

About ten FU Orionis objects are currently known. Others almost certainly lurk undiscovered among the stars in the Herbig-Bell Catalog, provided on-line by Karen Strom at UMass.

View Issue Contents (September-October 2013)

Related Materials:
  • Herbig, G. H., Eruptive phenomena in early stellar evolution, 1977, ApJ, 217, 693
  • Herbig, G.H.; Bell, K.Robbin, Third Catalog of Emission-Line Stars of the Orion Population, 1988; Lick Observatory Bulletin, 1111, 90
  • Vittone, A.A., Errico, L., FU Orionis systems, 2005, Mem. S.A.It. Vol. 76, 320
  • Hartmann, L. & Kenyon, S. J., 1985; On the nature of FU Orionis objects, ApJ, 299, 462
  • Hartmann, Lee; Kenyon, Scott J., 1996; Ann. Rev.of Astr. and Astroph., 34, 207
  • Percy, J. R., 2007, "Understanding variable stars", Cambridge University Press, pp. 289-290
  • Discovery of possible FU-Ori and UX-Ori type objects
  • FU Orionis Stars
  • FU Orionis (Night Sky Info)
  • The Herbig-Bell Catalog
  • The Furor over FUORs

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