Flare star

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An M-type flare star stripping away the atmosphere of its planet Red Dwarf Flare Star (Artist's Illustration) (2018-46-4241).tif
An M-type flare star stripping away the atmosphere of its planet

A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to the magnetic energy stored in the stars' atmospheres. The brightness increase is across the spectrum, from X-rays to radio waves. Flare activity among late-type stars was first reported by A. van Maanen in 1945, for WX Ursae Majoris and YZ Canis Minoris. [1] However, the best-known flare star is UV Ceti, first observed to flare in 1948. Today similar flare stars are classified as UV Ceti type variable stars (using the abbreviation UV) in variable star catalogs such as the General Catalogue of Variable Stars.

Contents

Most flare stars are dim red dwarfs, although recent research indicates that less massive brown dwarfs might also be capable of flaring.[ citation needed ] The more massive RS 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 the magnetic field to become tangled. Additionally, nine stars similar to the Sun had also been seen to undergo flare events [2] prior to the flood of superflare data from the Kepler observatory. 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

The Sun is known to flare and solar flares have 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 to soft X-rays and EUV. During the impulsive phase, which lasts for three to ten minutes, a large number of electrons and sometimes also ions are accelerated to extremely high energies ranging from keV to MeV. The radiation can be seen as gyrosynchrotron radiation in the radio wavelengths and bremsstrahlung radiation in 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 lower chromosphere. 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 an accretion disk, which most of the time are protostars or pre-main sequence stars, the interactions of magnetic field between the stars and the disk can also cause flares. [7]

Nearby flare stars

A flare star with orbiting planet (artist's impression) Stellar flare hits HD 189733 b (artist's impression).jpg
A flare star with orbiting planet (artist's impression)

Flare stars are intrinsically faint, but have been found to distances of 1,000 light years from Earth. [8] On April 23, 2014, NASA's Swift satellite detected 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 largest solar flare ever recorded. [9]

Proxima Centauri

Proxima Centauri, with planet c in the foreground and the Alpha Centauri binary in the background Proxima Centauri c.png
Proxima Centauri, with planet c in the foreground and the Alpha Centauri binary in the background

The Sun's nearest stellar neighbor Proxima Centauri is a flare star that undergoes occasional increases in brightness because of magnetic activity. [10] The star's magnetic field is created by convection throughout the stellar body, and the resulting flare activity generates a total X-ray emission similar to that produced by the Sun. [11]

Wolf 359

The flare star Wolf 359 is another near neighbor (2.39 ± 0.01 parsecs). This star, also known as Gliese 406 and CN Leo, is a red dwarf of spectral class M6.5 that emits X-rays. [12] It is a UV Ceti flare star, [13] and has a relatively high flare rate.

Artist's interpretation of Wolf 359 Wolf359incelestia.jpg
Artist's interpretation of Wolf 359

The mean magnetic field has a strength of about 2.2  kG (0.2  T ), but this varies significantly on time scales as short as six hours. [14] By comparison, the magnetic field of the Sun averages 1 G (100 μT), although it can rise as high as 3 kG (0.3 T) in active sunspot regions. [15]

Barnard's Star

Size comparison between Jupiter, Barnard's star and the Sun Barnard'sStarSize en.jpg
Size comparison between Jupiter, Barnard's star and the Sun

Barnard's Star is 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 intense stellar flare, showing that Barnard's Star is a flare star. [16] [17]

EV Lacertae

Artist's conception of a flare explosion on EV Lacertae Nasa EV Lacertae 250408.jpg
Artist's conception of a flare explosion on EV Lacertae

EV Lacertae is 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 strong magnetic 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

TVLM 513-46546 is a very low mass M9 flare star, at the boundary between red dwarfs and brown dwarfs. Data from Arecibo Observatory at 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

The more massive member of the binary star 2MASS 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

The most powerful stellar flare detected, as of December 2005, may have come from the active binary II Peg. [20] Its observation by Swift suggested the presence of hard X-rays in the well-established Neupert effect as seen in solar flares.

See also

Related Research Articles

<span class="mw-page-title-main">Variable star</span> Star whose brightness fluctuates, as seen from Earth

A variable star is a star whose brightness as seen from Earth changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:

<span class="mw-page-title-main">Proxima Centauri</span> Star in the constellation Centaurus

Proxima Centauri is a small, low-mass star located 4.2465 light-years (1.3020 pc) away from the Sun in the southern constellation of Centaurus. Its Latin name means the 'nearest [star] of Centaurus'. It was discovered in 1915 by Robert Innes and is the nearest-known star to the Sun. With a quiescent apparent magnitude of 11.13, it is too faint to be seen with the unaided eye. Proxima Centauri is a member of the Alpha Centauri star system, being identified as component Alpha Centauri C, and is 2.18° to the southwest of the Alpha Centauri AB pair. It is currently 12,950 AU (0.2 ly) from AB, which it orbits with a period of about 550,000 years.

<span class="mw-page-title-main">Wolf 359</span> Red dwarf in the constellation Leo

Wolf 359 is a red dwarf star located in the constellation Leo, near the ecliptic. At a distance of 7.86 light-years from Earth, it has an apparent magnitude of 13.54 and can only be seen with a large telescope. Wolf 359 is one of the nearest stars to the Sun; only the Alpha Centauri system, Barnard's Star, and the brown dwarfs Luhman 16 and WISE 0855−0714 are known to be closer. Its proximity to Earth has led to its mention in several works of fiction.

Ross 154 is a star in the southern zodiac constellation of Sagittarius. It has an apparent visual magnitude of 10.44, making it much too faint to be seen with the naked eye. At a minimum, viewing Ross 154 requires a telescope with an aperture of 6.5 cm (3 in) under ideal conditions. The distance to this star can be estimated from parallax measurements, which places it at 9.71 light-years away from Earth. It is the nearest star in the southern constellation Sagittarius, and one of the nearest stars to the Sun.

Gliese 65, also known as Luyten 726-8, is a binary star system that is one of Earth's nearest neighbors, at 8.8 light-years from Earth in the constellation Cetus. The two component stars are both flare stars with the variable star designations BL Ceti and UV Ceti.

<span class="mw-page-title-main">RS Canum Venaticorum variable</span>

An RS Canum Venaticorum variable is a type of variable star. The variable type consists of close binary stars having active chromospheres which can cause large stellar spots. These spots are believed to cause variations in their observed luminosity. Systems can exhibit variations on timescales of years due to variation in the spot surface coverage fraction, as well as periodic variations which are, in general, close to the orbital period of the binary system. Some systems exhibit variations in luminosity due to their being eclipsing binaries. Typical brightness fluctuation is around 0.2 magnitudes. They take their name from the star RS Canum Venaticorum.

<span class="mw-page-title-main">AD Leonis</span> M-type star in the constellation Leo

AD Leonis (Gliese 388) is a red dwarf star. It is located relatively near the Sun, at a distance of 16.2 light-years, in the constellation Leo. AD Leonis is a main sequence star with a spectral classification of M3.5V. It is a flare star that undergoes random increases in luminosity.

Kappa<sup>1</sup> Ceti Variable yellow dwarf star in the constellation Cetus

Kappa1 Ceti, Latinized from κ1 Ceti, is a variable yellow dwarf star approximately 30 light-years away in the equatorial constellation of Cetus.

<span class="mw-page-title-main">Stellar magnetic field</span> Magnetic field generated by the convective motion of conductive plasma inside a star

A stellar magnetic field is a magnetic field generated by the motion of conductive plasma inside a star. This motion is created through convection, which is a form of energy transport involving the physical movement of material. A localized magnetic field exerts a force on the plasma, effectively increasing the pressure without a comparable gain in density. As a result, the magnetized region rises relative to the remainder of the plasma, until it reaches the star's photosphere. This creates starspots on the surface, and the related phenomenon of coronal loops.

<span class="mw-page-title-main">HD 17925</span> Star in the constellation Eridanus

HD 17925 is a variable star in the equatorial constellation of Eridanus. It has the Gould designation 32 G. Eridani and the variable star designation EP Eri. The star has a yellow-orange hue and is dimly visible to the naked eye in good seeing conditions with an apparent visual magnitude that varies from 6.03 down to 6.08. It is located nearby at a distance of 34 light years from the Sun based on parallax, and is drifting further away with a radial velocity of +18 km/s. It is a likely member of the Local Association of nearby, co-moving stars. The spectrum shows a strong abundance of lithium, indicating that it is young star. This likely makes its point of origin the nearby Scorpio–Centaurus Complex.

<span class="mw-page-title-main">II Pegasi</span> Star in the constellation Pegasus

II Pegasi is a binary star system in the constellation of Pegasus with an apparent magnitude of 7.4 and a distance of 130 light-years. It is a very active RS Canum Venaticorum variable, a close binary system with active starspots.

<span class="mw-page-title-main">EQ Virginis</span> Star in the constellation Virgo

EQ Virginis is a single variable star in the equatorial constellation of Virgo. It has a baseline visual apparent magnitude of 9.36, but is a flare star that undergoes sporadic bursts of brightening. The star is located at a distance of 67 light-years from the Sun based on parallax measurements, but is drifting closer with a radial velocity of −23 km/s. It is a member of the IC 2391 moving group of stars, which is between 30 and 50 million years old.

An AM Canum Venaticorum star, is a rare type of cataclysmic variable star named after their type star, AM Canum Venaticorum. In these hot blue binary variables, a white dwarf accretes hydrogen-poor matter from a compact companion star.

AM Canum Venaticorum is a hydrogen-deficient cataclysmic variable binary star in the constellation of Canes Venatici. It is the type star of its class of variables, the AM Canum Venaticorum stars. The system consists of a white dwarf gaining matter via an accretion disk from a semi-degenerate or white dwarf companion.

<span class="mw-page-title-main">EV Lacertae</span> Star in the constellation Lacerta

EV Lacertae is a faint red dwarf star 16.5 light years away in the constellation Lacerta. It is the nearest star to the Sun in that region of the sky, although with an apparent magnitude of 10, it is only barely visible with binoculars. EV Lacertae is spectral type M3.5 flare star that emits X-rays.

<span class="mw-page-title-main">Gliese 752</span> Binary star system in the constellation Aquila

Gliese 752 is a binary star system in the Aquila constellation. This system is relatively nearby, at a distance of 19.3 light-years.

Superflares are very strong explosions observed on stars with energies up to ten thousand times that of typical solar flares. The stars in this class satisfy conditions which should make them solar analogues, and would be expected to be stable over very long time scales. The original nine candidates were detected by a variety of methods. No systematic study was possible until the launch of the Kepler space telescope, which monitored a very large number of solar-type stars with very high accuracy for an extended period. This showed that a small proportion of stars had violent outbursts. In many cases there were multiple events on the same star. Younger stars were more likely to flare than old ones, but strong events were seen on stars as old as the Sun.

<span class="mw-page-title-main">V1005 Orionis</span> Young flame star in the constellation of Orion

V1005 Orionis is a young flare star in the equatorial constellation of Orion. It has the identifier GJ 182 in the Gliese–Jahreiß catalogue; V1005 Ori is its variable star designation. This star is too faint to be visible to the naked eye, having a mean apparent visual magnitude of 10.1. It is located at a distance of 79.6 light years from the Sun and is drifting further away with a radial velocity of 19.2 km/s. The star is a possible member of the IC 2391 supercluster.

<span class="mw-page-title-main">UV Piscium</span> Binary star system in the Pisces constellation

UV Piscium is a binary star system in the constellation of Pisces. With a peak apparent visual magnitude of 8.98, it is too faint to be visible to the naked eye. This is an eclipsing binary system that decreases to magnitude 10.05 during the primary eclipse, then to magnitude 9.54 with the secondary eclipse. It is located at a distance of 232 light years from the Sun based on parallax measurements, and is receding with a radial velocity of 6.5 km/s. The position of this star near the ecliptic means it is subject to lunar occultation.

AR Piscium is a binary star system in the northern constellation of Canes Venatici, abbreviated AR Psc. It has the Henry Draper Catalogue identifier HD 8357; AR Piscium is its variable star designation. The pair have a combined apparent visual magnitude that fluctuates around 7.24, which is too faint to be readily visible to the naked eye. Parallax measurements place it at a distance of 148 light years from the Sun. The motion of this star through the Milky Way suggests it is a member of the intermediate disc population.

References

  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. S2CID   10586370.
  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. S2CID   15709625.
  4. ↑ Aschwanden, Markus J.; Stern, Robert A.; Gudel, Manuel (2008). "Scaling Laws of Solar and Stellar Flares". Astrophysical Journal. 672: 659-673. doi:10.1086/523926.
  5. ↑ Benz, Arnold O. (2017). "Flare Observations". Living Reviews in Solar Physics. 14. doi:10.1007/s41116-016-0004-3.
  6. ↑ Benz, Arnold O.; Gudel, Manuel (2010). "Physical Processes in Magnetically Driven Flares on the Sun, Stars, and Young Stellar Objects". \araa. 48: 241-287. doi:10.1146/annurev-astro-082708-101757.
  7. ↑ Feigelson, Eric D.; Montmerle, Thierry (1999). "High-Energy Processes in Young Stellar Objects". \araa. 37: 363-408. 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. S2CID   116948759.
  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. S2CID   118537213.
  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. S2CID   122101052.
  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. S2CID   17926213.
  15. ↑ "Calling Dr. Frankenstein! : Interactive Binaries Show Signs of Induced Hyperactivity". National Optical Astronomy Observatory. 7 January 2007. Archived from the original on 2019-06-22. Retrieved 2006-05-24.
  16. ↑ Croswell, Ken (November 2005). "A Flare for Barnard's Star". Astronomy Magazine. Kalmbach Publishing Co. Retrieved 2006-08-10.
  17. ↑ "V2500 Oph". The International Variable Star Index. Retrieved 18 November 2015.
  18. ↑ "Pipsqueak Star Unleashes Monster Flare". NASA. Retrieved December 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. S2CID   119114679.
  20. ↑ Osten, Rachel; Drake, Steve; Tueller, Jack; Cameron, Brian; "Swift Observations of Stellar Flares", Swift Team Meeting, 1 May 2007
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