FU Orionis star

from Wikipedia, the free encyclopedia

FU Orionis stars or FUORs are eruptive, variable, pre- main sequence stars with an increase in brightness of more than 5 mag within a few hundred days and a decrease to rest brightness within decades. The star class is named after its prototype FU Orionis .

properties

spectrum

The spectrum of a FUOR is that of a super giant with a spectral class A to G in the optical spectral range and a surface temperature of up to 7000 K , while in the near infrared the spectrum appears more like that of a red giant with a temperature of 3000 K. The spectrum is earlier in the ultraviolet than in the optical. These different spectral classes - unlike some Be stars - cannot be interpreted as a result of a rapid rotation of the star, since the speed of rotation for the necessary flattening would tear the star apart.

All FUORs show a pronounced infrared excess . From the blue-shifted emission lines , a stellar wind with a speed of several 100 km / s can be derived with a mass loss rate of around 10 −5 solar masses per year. The P-Cygni profiles of hydrogen and sodium as well as absorption bands of carbon monoxide in the near infrared are characteristic of FU Orionis stars . Before the eruption, FUORs show the spectrum of a T Tauri star .

Light curve

The FU Orionis stars show a steep increase in brightness over a period of 100 days to a year. The brightness in the visual increases by at least 5 mag and then falls again very slowly. A complete return to calm brightness has not yet been observed, the period of the eruption is therefore likely to last longer than 100 years. During an outbreak, the brightness fluctuates only slightly. The FUORs were originally classified as extremely slow novae , but this interpretation is considered obsolete. Before they erupt, they show a low variability of around one magnitude.

Others

FUOR stars are very young and are always located within star formation areas . Their young age is underlined 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, FUORs show signs of flickering and periodic variations in the shape of the spectral lines. Observed unstable pseudoperiods of 2 to 9 days in the light curves are probably the orbital periods of inhomogeneities at the inner edge of the accretion discs. The amplitudes of the brightness fluctuations and the changes in the color index BV are correlated with one another.

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists 11 stars with the abbreviation FU as well as another 14 suspected FU Orionis stars. This type is very rare and less than 0.05% of all stars in this catalog belong to the class of FU Orionis stars.

Cause of the outbreak

Artist's impression of a FU Orionis star with an accretion disk

The FU Orionis stage is interpreted today as a lighting up of the accretion disk around the pre-main sequence star. According to this, the accretion disk is in a bistable state ( disc instability model ) as in the dwarf novae . In the resting phase, the disk collects more incident matter from the star's surroundings than it passes on to the T-Tauri star.

Due to a thermal instability, the viscosity in the accretion disk changes, and this internal friction leads to both the disk lighting up and a sharply increasing accretion rate to the central star: during an eruption, up to 0.01 solar masses can be transferred to the star , whereby the mass accretion rate from the surrounding area can only reach 10 −5 solar masses per year. The luminosity of the accretion disk can exceed that of the star by a factor of 1000, so that only radiation from the disk is detected.

From statistical arguments it has been deduced that all T-Tauri stars go through the FUOR phase 10 to 20 times, probably a considerable part of the accreted mass of a young star of low mass is taken up. The average interval between eruptions is likely to be between 5,000 and 50,000 years.

During the eruption, FU Orionis stars show changes in brightness with periods on the order of days. This is interpreted as the period of rotation of the star, which accretes matter from the circumstellar disk onto its surface along magnetic field lines . The rotation makes the hot accretion spots periodically visible and modulates the light curve.

The outbursts in the FU-Orionis phase are probably the cause of the large spread of the bolometric magnitudes of T-Tauri stars in the Hertzsprung-Russell diagram . After the increased incidence, the central star absorbed large amounts of matter in a short period of time and has not yet returned to its thermal equilibrium afterwards . In addition, the star has absorbed large amounts of thermal energy that was released when the matter hit the surface. The pre-main sequence star reacts to this by expanding its radius. With increased radiation and with a decreasing radius, it then returns to its equilibrium until the next FUOR phase.

The eruptions of FUORs are associated with the formation of chondrites in protoplanetary disks . As the analysis of these meteorites suggests, a shock wave runs through the disk during an accretion event and heats up the matter.

Outdated theories

The cause of the long-lasting eruption was previously suspected:

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

All of these hypotheses are now considered obsolete.

Examples

  • FU Orionis
  • Other well-known FUORs are V1057 Cygni and V1515 Cygni

Individual evidence

  1. ^ Cuno Hoffmeister , G. Richter, W. Wenzel: Veränderliche Sterne . JA Barth Verlag, Leipzig 1990, ISBN 3-335-00224-5 .
  2. ↑ Tigran Yu. Magakian et al .: V2494 Cyg: A unique FU Ori type object in the Cygnus OB7 complex . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1209.5033v1 .
  3. ^ John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
  4. ↑ Evgeni Semkov, Stoyanka Peneva: The new Fuor star HBC 722 - one year after the outburst . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1112.3918 .
  5. ^ Joel D. Green et al .: Variability at the Edge: Optical Near / IR Rapid Cadence Monitoring of Newly Outbursting FU Orionis Object HBC 722 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1212.2610 .
  6. ↑ Stacie L. Powell, Mike Irwin, Jerome Bouvier, Cathie J. Clarke: The Periodic Spectroscopic Variability of FU Orionis . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1209.0981 .
  7. ↑ Michal Siwak et al .: Photometric variability in FU Ori and Z CMa as observed by MOST . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1303.2568v1 .
  8. ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved August 4, 2019 .
  9. ↑ Lee Hartmann: Accretion Processes in Star Formation (Cambridge Astrophysics) . Cambridge University Press, Cambridge 2001, ISBN 0-521-78520-0 .
  10. ↑ Sergei Nayakshin, Giuseppe Lodato: FU Ori outbursts and the planet-disc mass exchange . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1110.6316 .
  11. ^ William J. Fischer et al.: Multiwavelength Observations of V2775 Ori, an Outbursting Protostar in L 1641: Exploring the Edge of the FU Orionis Regime . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.2466 .
  12. ↑ Alexander Scholz, Dirk Froebrich, Kenneth Wood: A systematic survey for eruptive young stellar objects using mid-infrared photometry . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.3152 .