Wikipedia

Flare star

Aflare staris avariable starthat can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous tosolar flaresin that they are due tothe magnetic energystored in thestars' atmospheres. The brightness increase is across thespectrum,fromX-raystoradio waves.Flare activity amonglate-type starswas first reported byA. van Maanenin 1945, forWX Ursae MajorisandYZ Canis Minoris.[1]However, the best-known flare star isUV Ceti,first observed to flare in 1948. Today similar flare stars are classified asUV Ceti typevariable stars(using the abbreviationUV) in variable star catalogs such as theGeneral Catalogue of Variable Stars.

An M-type flare star stripping away the atmosphere of its planet

Most flare stars are dimred dwarfs,although recent research indicates that less massivebrown dwarfsmight also be capable of flaring.[citation needed]The more massiveRS Canum Venaticorum variables(RS CVn) are also known to flare, but it is understood that these flares are induced by a companion star in a binary system which causes themagnetic fieldto become tangled. Additionally, nine stars similar to theSunhad also been seen to undergo flare events[2]prior to the flood ofsuperflaredata from theKeplerobservatory. It has been proposed that the mechanism for this is similar to that of the RS CVn variables in that the flares are being induced by a companion, namely an unseen Jupiter-like planet in a close orbit.[3]

Stellar Flare Model

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The Sun is known to flare andsolar flareshave been extensively studied over all the spectrum. Even though the Sun on average shows less variability and weaker flares compared with other stars that are similar to the Sun in spectral type, rotation period and age, it is generally thought that other stellar flares and the solar flares share the same or similar processes.[4]Thus the solar flare model has been used as the framework for understanding other stellar flares.

The general idea is that flares are generated through the reconnection of the magnetic field lines in the corona.[5]There are several phases for the flare: preflare phase, impulsive phase, flash phase and decay phase. Those phases have different timescales and different emissions across the spectrum. During the preflare phase, which usually lasts for a few minutes, the coronal plasmas slowly heats up to temperatures of tens of millions Kelvin. This phase is mostly visible tosoft X-raysandEUV.During the impulsive phase, which lasts for three to ten minutes, a large number of electrons and sometimes alsoionsare accelerated to extremely high energies ranging from keV to MeV. The radiation can be seen asgyrosynchrotron radiationin the radio wavelengths andbremsstrahlung radiationin the hard X-rays wavelengths. This is the phase where most of the energy is released.[6]The later flash phase is defined by the rapid increase in Hα emissions. The free streaming particles travel along the magnetic lines, propagating energy from the corona to the lowerchromosphere.The material in the chromosphere is then heated up and expands to the corona. Emission in the flash phase is primarily due to thermal radiation from the heated stellar atmosphere. As the material reaches the corona, the intensive release of energy slows down and cooling starts. During the decay phase which lasts for one to several hours, the corona returns back to its original state.

This is the model for how isolated star generates flares but this is not the only way. Interactions between a star and the companion or sometimes the environment can also produce flares. In binary systems such as RS Canum Venaticorum variable stars (RS CVn), flares can be produced through the interactions between the magnetic fields of the two bodies in the systems. For stars that have anaccretion disk,which most of the time areprotostarsor pre-main sequence stars, the interactions of magnetic field between the stars and the disk can also cause flares.[7]

Nearby flare stars

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A flare star with orbiting planet (artist's impression)

Flare stars are intrinsically faint, but have been found to distances of 1,000light yearsfrom Earth.[8]On April 23, 2014,NASA'sSwift satellitedetected the strongest, hottest, and longest-lasting sequence of stellar flares ever seen from a nearby red dwarf,DG Canum Venaticorum.The initial blast from this record-setting series of explosions was as much as 10,000 times more powerful than the largestsolar flareever recorded.[9]

Proxima Centauri

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Proxima Centauri, withplanet cin the foreground and theAlpha Centauri binaryin the background

The Sun's nearest stellar neighborProxima Centauriis a flare star that undergoes occasional increases in brightness because of magnetic activity.[10]The star'smagnetic fieldis created byconvectionthroughout the stellar body, and the resulting flare activity generates a totalX-rayemission similar to that produced by the Sun.[11]

Wolf 359

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The flare starWolf 359is another near neighbor (2.39 ± 0.01 parsecs). This star, also known as Gliese 406 and CN Leo, is ared dwarfofspectral classM6.5 that emits X-rays.[12]It is aUV Cetiflare star,[13]and has a relatively high flare rate.

Artist's interpretation of Wolf 359

The mean magnetic field has a strength of about2.2kG(0.2T), but this varies significantly on time scales as short as six hours.[14]By comparison, the magnetic field of theSunaverages1 G(100 μT), although it can rise as high as3 kG(0.3 T) in activesunspotregions.[15]

Barnard's Star

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Size comparison betweenJupiter,Barnard's Star and theSun

Barnard's Staris the fourth nearest star to the Sun. Given its age, at 7–12 billion years of age, Barnard's Star is considerably older than the Sun. It was long assumed to be quiescent in terms of stellar activity. However, in 1998, astronomers observed an intensestellar flare,showing that Barnard's Star is a flare star.[16][17]

EV Lacertae

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Artist's conception of a flare explosion on EV Lacertae

EV Lacertaeis located 16.5 light-years away, and is the nearest star in its constellation. It is a young star, about 300 million years old, and has a strongmagnetic field.In 2008, it produced a record-setting flare that was thousands of times more powerful than the largest observed solar flare.[18]

TVLM513-46546

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TVLM 513-46546is a very low mass M9 flare star, at the boundary between red dwarfs andbrown dwarfs.Data fromArecibo Observatoryat radio wavelengths determined that the star flares every 7054 s with a precision of one one-hundredth of a second.[19]

2MASS J18352154-3123385 A

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The more massive member of the binary star2MASS J1835,an M6.5 star, has strong X-ray activity indicative of a flare star, although it has never been directly observed to flare.

Record-setting flares

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The most powerful stellar flare detected, as of December 2005, may have come from the active binaryII Peg.[20]Its observation bySwiftsuggested the presence of hard X-rays in the well-establishedNeupert effectas seen insolar flares.

See also

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  • Solar flare– Eruption of electromagnetic radiation
  • Superflare– Strong explosion observed on stars
  • Variable star– Star whose brightness fluctuates, as seen from Earth

References

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  1. ^Joy, Alfred H.(February 1954). "Variable Stars of Low Luminosity".Publications of the Astronomical Society of the Pacific.66(388): 5.Bibcode:1954PASP...66....5J.doi:10.1086/126639.
  2. ^Schaefer, Bradley E.; King, Jeremy R.; Deliyannis, Constantine P. (February 2000). "Superflares on Ordinary Solar-Type Stars".The Astrophysical Journal.529(2): 1026.arXiv:astro-ph/9909188.Bibcode:2000ApJ...529.1026S.doi:10.1086/308325.S2CID10586370.
  3. ^Rubenstein, Eric; Schaefer, Bradley E. (February 2000). "Are Superflares on Solar Analogues Caused by Extrasolar Planets?".The Astrophysical Journal.529(2): 1031.arXiv:astro-ph/9909187.Bibcode:2000ApJ...529.1031R.doi:10.1086/308326.S2CID15709625.
  4. ^Aschwanden, Markus J.; Stern, Robert A.; Gudel, Manuel (2008)."Scaling Laws of Solar and Stellar Flares".Astrophysical Journal.672(1): 659-673.arXiv:0710.2563.Bibcode:2008ApJ...672..659A.doi:10.1086/523926.
  5. ^Benz, Arnold O. (2017)."Flare Observations".Living Reviews in Solar Physics.14(1): 2.Bibcode:2017LRSP...14....2B.doi:10.1007/s41116-016-0004-3.hdl:20.500.11850/377258.
  6. ^Benz, Arnold O.; Gudel, Manuel (2010)."Physical Processes in Magnetically Driven Flares on the Sun, Stars, and Young Stellar Objects".Annual Review of Astronomy and Astrophysics.48:241-287.Bibcode:2010ARA&A..48..241B.doi:10.1146/annurev-astro-082708-101757.
  7. ^Feigelson, Eric D.; Montmerle, Thierry (1999)."High-Energy Processes in Young Stellar Objects".Annual Review of Astronomy and Astrophysics.37:363-408.Bibcode:1999ARA&A..37..363F.doi:10.1146/annurev.astro.37.1.363.
  8. ^Kulkarni, Shrinivas R.; Rau, Arne (2006). "The Nature of the Deep Lens Survey Fast Transients".Astrophysical Journal.644(1): L63.arXiv:astro-ph/0604343.Bibcode:2006ApJ...644L..63K.doi:10.1086/505423.S2CID116948759.
  9. ^NASA/Goddard Space Flight Center,"NASA's Swift mission observes mega flares from nearby red dwarf star",ScienceDaily,30 September 2014
  10. ^Christian, Damian J.; Mathioudakis, Michail; Bloomfield, D. Shaun; Dupuis, Jean; Keenan, Francis P. (2004). "A Detailed Study of Opacity in the Upper Atmosphere of Proxima Centauri".Astrophysical Journal.612(2):1140–6.Bibcode:2004ApJ...612.1140C.doi:10.1086/422803.hdl:10211.3/172067.
  11. ^Wood, Brian E.; Linsky, Jeffrey L.; Müller, Hans-Reinhard; Zank, Gary P. (2001). "Observational Estimates for the Mass-Loss Rates of α Centauri and Proxima Centauri Using Hubble Space Telescope Lyα Spectra".Astrophysical Journal.547(1):L49 –L52.arXiv:astro-ph/0011153.Bibcode:2001ApJ...547L..49W.doi:10.1086/318888.S2CID118537213.
  12. ^Schmitt, Juergen H. M. M.; Fleming, Thomas A.; Giampapa, Mark S. (September 1995)."The X-Ray View of the Low-Mass Stars in the Solar Neighborhood".Astrophysical Journal.450(9):392–400.Bibcode:1995ApJ...450..392S.doi:10.1086/176149.
  13. ^Gershberg, Roald E.; Shakhovskaia, Nadezhda I. (1983). "Characteristics of activity energetics of the UV Cet-type flare stars".Astrophysics and Space Science.95(2):235–53.Bibcode:1983Ap&SS..95..235G.doi:10.1007/BF00653631.S2CID122101052.
  14. ^Reiners, Ansgar; et al. (2007)."Rapid magnetic flux variability on the flare star CN Leonis"(PDF).Astronomy and Astrophysics.466(2):L13 –L16.arXiv:astro-ph/0703172.Bibcode:2007A&A...466L..13R.doi:10.1051/0004-6361:20077095.S2CID17926213.
  15. ^"Calling Dr. Frankenstein!: Interactive Binaries Show Signs of Induced Hyperactivity".National Optical Astronomy Observatory.7 January 2007. Archived fromthe originalon 2019-06-22.Retrieved2006-05-24.
  16. ^Croswell, Ken (November 2005)."A Flare for Barnard's Star".Astronomy Magazine.Kalmbach Publishing Co.Retrieved2006-08-10.
  17. ^"V2500 Oph".The International Variable Star Index.Retrieved18 November2015.
  18. ^"Pipsqueak Star Unleashes Monster Flare".NASA.RetrievedDecember 28,2023.
  19. ^Wolszczan, A.; Route, M. (2014). "Timing Analysis of the Periodic Radio and Optical Brightness Variations of the Ultracool Dwarf, TVLM 513-46546".The Astrophysical Journal.788(1): 23.arXiv:1404.4682.Bibcode:2014ApJ...788...23W.doi:10.1088/0004-637X/788/1/23.S2CID119114679.
  20. ^Osten, Rachel; Drake, Steve; Tueller, Jack; Cameron, Brian;"Swift Observations of Stellar Flares",Swift Team Meeting, 1 May 2007
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