FU Orionis star

FU Orionis stars or FUORs eruptive variable pre-main sequence with a brightness increase of more than 5 mag. Within a few hundred days and a descent to rest brightness within decades

Properties

The FU Orionis stars show a steep increase in brightness over a period of 100 days to one year. The brightness in the visible increases by at least 5 to like it and then falls very slowly again. A return to quiet brightness has not yet been observed, and the period of the outbreak is likely to last longer than 100 years. The brightness during an outbreak is subject to slight variations. The FUORs were originally classified as extremely slow novae, but this interpretation is considered outdated. Prior to the outbreak of the stars show a low variability of about one magnitudine.

The spectrum of a FUORs is that of a supergiant with a spectral class A to G in the optical spectral range with a surface temperature of up to 7000 K, while the spectrum appears rather than that of a Red Riesens with a temperature of 3000 K in the near infrared. In the ultraviolet, the spectrum is earlier than in the optical. These different spectral types can not be interpreted as a consequence of rapid rotation of the star, as in some Be stars, since the rotation speed for the required flattening would tear the star. All FUORs show a substantial infrared excess. From the blue-shifted emission lines a stellar wind with a speed of some 100 km / s can be derived with a mass loss rate to 10-5 solar masses per year. Characteristic of FU Orionis stars are the P Cygni profiles of the hydrogen and sodium, as well as absorption bands of carbon monoxide in the near infrared. Before the outbreak FUORs show the spectrum of a T Tauri star. The stars are very young and are always located within star-forming regions. Your young age is enhanced by a high proportion of lithium in their spectra, which has not yet been destroyed by thermonuclear reactions. Almost all FUORs are embedded in a reflection nebula. In addition, part of the FUORs shows connection to Herbig -Haro objects, optical jets and molecular outflows. Furthermore, they show signs of flickering and periodic variations in the shape of the spectral lines. Observed pseudo unstable periods 2-9 days in the light curves of FUORs are probably the orbital periods of the inner edge of the accretion disks Inhomognitäten. The amplitudes of the brightness variations and changes in the color index BV are correlated.

Known FUORs are next to the prototype FU Orionis and V1057 Cygni V1515 Cygni.

Cause of the outbreak

The cause of the prolonged eruption were suspected:

  • Resolution of a circumstellar resolution
  • Structural changes in the stellar interior
  • Incidence of a large object on the star
  • Near-surface nuclear reactions
  • Decay of a magnetic field below a critical value
  • Release of thermal radiation

All these hypotheses are considered obsolete, and the FU Orionis stage is now interpreted as an illumination of the accretion disk around the Vorhauptreihenstern. Accordingly, the accretion disk is in a bistable state as in the dwarf novae. In the resting phase, the disc collects more incident matter from the environment of the star, as she passes on to the T Tauri star. Due to thermal instability, the viscosity changes within the accretion disk and this internal friction leads to both a flash of the disc and to a sharp rise in the accretion rate onto the central star. During a Fuor eruption up to 0.01 solar masses can be transferred to the star, where the mass accretion rate can reach 10-5 solar masses per year. In this case, the luminosity of the accretion disk of the star exceed by a factor of 1000, so that only radiation from the disk can be detected. FU Orionis stars show during the outbreak brightness changes with period lengths on the order of days. This is interpreted as the rotation period of the star, which accretes along magnetic field lines matter from the circumstellar disk onto its surface. Through the rotation of the hot Akkretionsflecken are periodically visible and modulate the light curve. In addition, in FUORs also known as Flickering non-periodic brightness change has been observed.

For statistical arguments has been deduced that all T Tauri stars the Fuor - phase 10 to 20 times through, and probably a significant portion of the accreted mass of a young star is thereby recorded low mass. The average distance between the outbreaks is likely to be from 5000 to 50,000 years.

The outbreaks in the FU Orionis phase are likely the cause of the bolometric luminosities large spread of T Tauri stars in the Hertzsprung -Russell diagram. After the increased incidence of the central star has taken copious amounts of matter in a short period and is then not yet returned to its thermal equilibrium. In addition, the star has absorbed large amounts of thermal energy released during the impact of the material on the surface. The Vorhauptreihenstern responded with an expansion of its radius. Under an increased radiation of the star returns to its equilibrium back with a twitch radius until the next Fuor phase.

The eruptions of FUORs be associated with the formation of chondrites in the protoplanetary disks. During a Akkretionereignisses a shock wave passes through the glass and heats up the matter on how the analysis of these meteorites suggests.

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