Flare star

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. The first known flare stars (V1396 Cygni and AT Microscopii) were discovered in 1924. 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.

Most flare stars are dim red dwarfs, although recent research indicates that less massive brown dwarfs might also be capable of flaring. 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[1] 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.[2]

Nearby flare stars

Flare stars are intrinsically faint, but have been found to distances of 1,000 light years from Earth.[3] 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. 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.[4]

Proxima Centauri

The Sun's nearest stellar neighbor Proxima Centauri is a flare star that undergoes occasional increases in brightness because of magnetic activity.[5] 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.[6]

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.[7] It is a UV Ceti flare star,[8] and has a relatively high flare rate.

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.[9] 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.[10]

Barnard's Star

Barnard's Star is the second nearest star system to Earth. 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.[11][12]

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[13].

2MASS JJ18352154-3123385 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.[14] Its observation by Swift suggested the presence of hard X-rays in the well-established Neupert effect as seen in solar flares.

See also

References

  1. ^ 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.
  2. ^ 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.
  3. ^ 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.
  4. ^ NASA/Goddard Space Flight Center, "NASA's Swift mission observes mega flares from nearby red dwarf star", ScienceDaily, 30 September 2014
  5. ^ 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.
  6. ^ 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.
  7. ^ 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.
  8. ^ 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.
  9. ^ Reiners, Ansgar; Schmitt, Juergen H. M. M.; Liefke, Carolin (2007). "Rapid magnetic flux variability on the flare star CN Leonis". Astronomy and Astrophysics. 466 (2): L13–6. arXiv:astro-ph/0703172. Bibcode:2007A&A...466L..13R. doi:10.1051/0004-6361:20077095.
  10. ^ "Calling Dr. Frankenstein! : Interactive Binaries Show Signs of Induced Hyperactivity". National Optical Astronomy Observatory. 7 January 2007. Retrieved 2006-05-24.
  11. ^ Croswell, Ken (November 2005). "A Flare for Barnard's Star". Astronomy Magazine. Kalmbach Publishing Co. Retrieved 2006-08-10.
  12. ^ "V2500 Oph". The International Variable Star Index. Retrieved 18 November 2015.
  13. ^ 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.
  14. ^ Osten, Rachel; Drake, Steve; Tueller, Jack; Cameron, Brian; "Swift Observations of Stellar Flares", Swift Team Meeting, 1 May 2007

External links

AD Leonis

AD Leonis (Gliese 388) is a red dwarf star. It is located relatively near the Sun, at a distance of about 16 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.

AZ Cancri

AZ Cancri (AZ Cnc) is a M-type flare star in the constellation Cancer. It has an apparent visual magnitude of approximately 17.59.

BO Microscopii

BO Microscopii (BO Mic) is a star in the constellation Microscopium located about 170 light-years (52 parsecs; 11 million astronomical units) from the Sun. It has been dubbed "Speedy Mic" because of its very rapid rotation. The projected rotational velocity at the equator of this star is about 135 km/s (84 mi/s), which, with an estimated inclination of 70° to the line of sight from the Earth, means it completes a rotation every 0.380 ± 0.004 days (9.120 ± 0.096 hours). The photosphere of this star shows a high level of magnetic activity, with multiple star spots and prominences observed at the same time. As many as 25 prominences have been observed simultaneously, extending outward as far as 3.6 times the radius of the star. BO Mic is a flare star that undergoes sudden increases in X-ray and ultraviolet emissions. These events can emit a hundred times more energy than large solar flares. Speedy Mic is one of the most active stars in the vicinity of the Sun.

DT Virginis

DT Virginis, also known as Ross 458 AB, is a binary star in the constellation of Virgo. Both of the stars are low-mass red dwarfs with at least one of them being a flare star. This binary system has a circumbinary planet detected by direct imaging, currently the planetary-mass object with the widest known orbit around a binary star.

DX Cancri

DX Cancri is the variable star identifier for a small star in the northern zodiac constellation of Cancer the crab. With an apparent visual magnitude of 14.81, it is much too faint to be seen with the naked eye. Visually viewing this star requires a telescope with a minimum aperture of 16 in (41 cm). Based upon parallax measurements, DX Cancri is located at a distance of 11.8 light-years (3.6 parsecs) from Earth. This makes it the 18th closest star (or star system) to the Sun.

This star has a stellar classification of M6.5V, identifying it as a type of main sequence star known as a red dwarf. It has about 9% of the mass of the Sun and 11% of the Sun's radius. The outer envelope of the star has an effective temperature of 2,840 K, giving it the cool red-orange glow of an M-type star. This is a flare star that has random, intermittent changes in brightness by up to a fivefold increase.

This star has been examined for excess emission of infrared radiation caused by cold circumstellar dust, but none was found. It is a proposed member of the Castor Moving Group of stars that share a common trajectory through space. This group has an estimated age of 200 million years.

EQ Virginis

EQ Virginis is a flare star in the constellation of Virgo. It is an orange dwarf of spectral type K5Ve and is a member of a IC 2391 moving group. The moving group of stars is between 30 and 50 million years old.

EV Lacertae

EV Lacertae (EV Lac, Gliese 873, HIP 112460) 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.On 25 April 2008, NASA's Swift satellite picked up a record-setting flare from EV Lacertae. This flare was thousands of times more powerful than the largest observed solar flare. Because EV Lacertae is much farther from Earth than the Sun, the flare did not appear as bright as a solar flare. The flare would have been visible to the naked eye if the star had been in an observable part of the night sky at the time. It was the brightest flare ever seen from a star other than the Sun.

EV Lacertae is much younger than the Sun. Its age is estimated at 300 million years, and it is still spinning rapidly. The fast spin, together with its convective interior, produces a magnetic field much more powerful than the Sun's. This strong magnetic field is believed to play a role in the star's ability to produce such bright flares. After the flare, the star was blue.

GJ 3379

GJ 3379 (Giclas 99-49) is the nearest star in the Orion constellation, being around 17.5 lightyears away from the Sun. The main sequence star is a red dwarf with the spectral class M3.5V. It has an apparent magnitude of 11.33 and an absolute magnitude of 12.68, therefore, the star is not visible with the naked eye. It is located in the left upper part of the Orion constellation, below Betelgeuse. Its radial velocity is +30.0 kilometers per second. According to the SIMBAD database, the star is classified as a flare star.

In the past, this star may have had a close encounter with the Solar System. Some 163,000 ± 3,000 years ago, it achieved a minimum distance of 4.30 ± 0.10 ly (1.32 ± 0.03 pc).

Gliese 3685

Gliese 3685 is a star in the constellation of Leo. It is extremely faint; its apparent magnitude is 13.3, and can only be seen with a ten-inch (25 cm) telescope (see Limiting magnitude). Based on a parallax of 50.30 milliarcseconds, the system is located 65 light years (20 parsecs) away from the Earth.

This is a part of a binary star system consisting of two components separated by 24″. The primary component, Gliese 3685 (also known as Gliese 3685 A), is a very old red dwarf that is also a flare star. A 20-minute flare was observed in 2004 by the GALEX satellite. Its companion, Gliese 3686, is another faint red dwarf with a spectral type of M5. It is also known as LP 613-50 and is also located roughly the same distance as its primary.

Gliese 49

Gliese 49 (GJ 49 / HIP 4872) is a star in the constellation of Cassiopeia. Visually, it is located 106 arc minutes north of the bright star γ Cassiopeiae. With an apparent magnitude of 9.56, it is not observable with the naked eye. It is located, based on the reduction of parallax data of Hipparcos (100.40 ± 1.52 Millisecond of arc), 32.5 light-years away from the Solar System.

Gliese 49 is a red dwarf of spectral type M1.5V. Much dimmer than Sun, it has a total luminosity that is 5.0% that of the Sun; it is, however, much brighter than other nearby red dwarfs such as Proxima Centauri or Wolf 359. It has an effective temperature of 3454 ± 50 K. Its mass is 57% that of the Sun and its radius is 2/3 that of the Sun.

It rotates on its axis with a projected rotation speed of 2.5 km/s, which means that its rotation period does not exceed 13.4 days. It has a metallic content similar to that of the Sun, with its index of metallicity [M / H] = +0.03. Although its age is not known exactly, it is less than 250 million years.

Gliese 49 has a similar proper motion to the red dwarf flare star V388 Cassiopeiae. The visual separation between the two is 295 arcseconds, which implies that the real distance between them is over 2900 AU. Both stars are associated with the Hyades, as suggested by its young age and chromospheric activity levels.

Gliese 693

Gliese 693 is a flare star of spectral type M2 located in the constellation Pavo, 18.95 light-years from Earth.

LHS 292

LHS 292 is a red dwarf in the constellation Sextans. It is far too faint to be seen with the unaided eye and requires a large amateur telescope to be seen visually. It lies relatively close to the Sun at a distance of about 14.8 light years. It is a flare star, which means it can suddenly increase in brightness for short periods of time.

It has the space velocity components [U, V, W] = [28, −16, −14] km/s.

L 1159-16

L 1159-16 is a red dwarf in the northern constellation of Aries. With an apparent visual magnitude of 12.298, it is too faint to be seen by the naked eye, although it lies relatively close to the Sun at a distance of about 14.5 light-years (4.4 parsecs). It is a flare star, which means it can suddenly increase in brightness for short periods of time.

Lacaille 8760

Lacaille 8760 (AX Microscopii) is a red dwarf star in the constellation Microscopium. Although it is generally too faint to be seen without a telescope, it is one of the nearest stars to the Sun at about 12.9 light-years' distance. At an apparent magnitude of +6.7, it may only be visible to the unaided eye under exceptionally good viewing conditions, under dark skies. It was originally listed in a 1763 catalog that was published posthumously by the French Abbé Nicolas Louis de Lacaille. He observed it in the southern sky while working from an observatory at the Cape of Good Hope.In the past Lacaille 8760 has been classified anywhere from spectral class K7 down to M2. In 1979 the Irish astronomer Patrick Byrne discovered that it is a flare star, and it was given the variable star designation AX Microscopii. As a flare star it is relatively quiet, only erupting on average less than once per day.

Lacaille 8760 orbits around the galaxy with a relatively high ellipticity of 0.23. Its closest approach to the Sun occurred about 20,000 years ago when it came within 12 light-years (3.7 parsecs). Due to its low mass (60% of the Sun), it has an expected lifespan of about 75 billion (7.5 × 1010) years, seven times longer than the Sun's.

Despite efforts by astronomers, as of 2011 no planets have been detected in orbit around this star.Lacaille 8760 is one of the largest and brightest red dwarfs known, with about 60% the mass and 51% the radius of the Sun.

List of nearest stars and brown dwarfs

There are 52 stellar systems beyond our own Solar System that currently lie within 5.0 parsecs (16.3 light-years) of the Sun. These systems contain a total of 63 stars, of which 50 are red dwarfs, by far the most common type of star in the Milky Way. Much more massive stars, such as our own, make up the remaining 13. In addition to these "true" stars, there are 11 brown dwarfs (objects not quite massive enough to fuse hydrogen), and four white dwarfs (extremely dense objects that remain after stars such as our Sun exhaust all fusable hydrogen in their core and slowly shed their outer layers while only the collapsed core remains). Despite the relative proximity of these objects to Earth, only nine (not including the Sun) are brighter than 6.5 apparent magnitude, the dimmest magnitude visible to the naked eye from Earth. All of these objects are located in the Local Bubble, a region within the Orion–Cygnus Arm of the Milky Way.

Based on results from the Gaia telescope's second data release from April 2018, an estimated 694 stars will possibly approach the Solar System to less than 5.0 parsecs (16 light-years) over the next 15 million years. Of these, 26 have a good probability to come within 1.0 parsec (3.3 light-years) and another 7 within 0.5 parsecs (1.6 light-years). This number is likely much higher, due to the sheer number of stars needed to be surveyed; a star approaching the Solar System 10 million years ago, moving at a typical Sun-relative 20–200 kilometers per second, would be 600–6,000 light years from the Sun at present day, with millions of stars closer to the Sun. The closest encounter to the Sun so far predicted is the low-mass orange dwarf star Gliese 710 / HIP 89825 with roughly 60% the mass of the Sun. It is currently predicted to pass 19,300 ± 3,200 astronomical units (0.305 ± 0.051 light-years) from the Sun in 1.280+0.041−0.039 million years from the present, close enough to significantly disturb our Solar System's Oort cloud.The easiest way to determine stellar distance to the Sun for objects at these distances is parallax, which measures how much stars appear to move against background objects over the course of Earth's orbit around the Sun. As a parsec (parallax-second) is defined by the distance of an object that would appear to move exactly one second of arc against background objects, stars less than 5 parsecs away will have measured parallaxes of over 0.2 arcseconds, or 200 milliarcseconds. Determining past and future positions relies on accurate astrometric measurements of their parallax and total proper motions (how far they move across the sky due to their actual velocity relative to the Sun), along with spectroscopically determined radial velocities (their speed directly towards or away from us, which combined with proper motion defines their true movement through the sky relative to the Sun). Both of these measurements are subject to increasing and significant errors over very long time spans, especially over the several thousand-year time spans it takes for stars to noticeably move relative to each other.

Ross 47

Ross 47 is a flare star of spectral type M4 located in the constellation Orion, 19 light-years from Earth.

TVLM 513-46546

TVLM 513-46546 is an M9 ultracool dwarf at the red dwarf/ brown dwarf mass boundary in the constellation Boötes. It exhibits flare star activity, which is most pronounced at radio wavelengths. The star has a mass approximately 80 times the mass of Jupiter (or 8 percent of the Sun's mass). The radio emission is broadband and highly circularly polarized, similar to planetary auroral radio emissions. The radio emission is periodic, with bursts emitted every 7054 s, with nearly one hundredth of a second precision. Subtle variations in the radio pulses could suggest that the ultracool dwarf rotates faster at the equator than the poles (differential rotation) in a manner similar to the Sun.

Wolf 359

Wolf 359 is a red dwarf star located in the constellation Leo, near the ecliptic. At a distance of approximately 7.9 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 (including Proxima Centauri), 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.

Wolf 359 is one of the faintest and lowest-mass stars known. At the light-emitting layer called the photosphere, it has a temperature of about 2,800 K, which is low enough for chemical compounds to form and survive. The absorption lines of compounds such as water and titanium(II) oxide have been observed in the spectrum. The surface has a magnetic field that is stronger than the average magnetic field on the Sun. As a result of magnetic activity caused by convection, Wolf 359 is a flare star that can undergo sudden increases in luminosity for several minutes. These flares emit strong bursts of X-ray and gamma ray radiation that have been observed by space telescopes. Wolf 359 is a relatively young star with an age of less than a billion years. No companions or disks of debris have been detected in orbit around it.

YZ Ceti

YZ Ceti is a red dwarf star in the constellation Cetus. Although it is relatively close to the Sun at just 12 light years, this star cannot be seen with the naked eye. It is classified as a flare star that undergoes intermittent fluctuations in luminosity. YZ Ceti is about 13 percent the mass of the Sun and 17% of its radius.

This star is unusually close to Tau Ceti, a star of spectral class G8. The two are only about 1.6 light years apart, a little more than a third of the distance from the Sun to the Solar System's nearest neighbor, Proxima Centauri.

Pulsating
Eruptive
Cataclysmic
Rotating
Eclipsing

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