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 that 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. The flares were initially explained by postulating giant planets in very close orbits, such that the magnetic fields of the star and planet were linked. The orbit of the planet would warp the field lines until the instability released magnetic field energy as a flare. No such planet has shown up as a Kepler transit and this theory has been abandoned. All superflare stars show quasi-periodic brightness variations interpreted as very large starspots carried round by rotation. Spectroscopic studies found spectral lines that were clear indicators of chromospheric activity associated with strong and extensive magnetic fields. This suggests that superflares only differ in scale from solar flares. Attempts have been made to detect past solar superflares from nitrate concentrations in polar ice, from historical observations of auroras, and from those radioactive isotopes that can be produced by solar energetic particles. Although three events and a few candidates have been found in the carbon-14 records in tree rings, it is not possible to associate them definitely with superflare events. Solar superflares would have drastic effects, especially if they occurred as multiple events. Because they can occur on stars of the same age, mass and composition as the Sun, this cannot be ruled out, but no indication of solar superflares have been found for the past ten millennia. Solar-type superflare stars are very rare and are magnetically much more active than the Sun; if solar superflares do occur, it may be in well-defined episodes that occupy a small fraction of its time.
Superflare stars A superflare star is not the same as a flare star, which usually refers to a very late spectral type red dwarf. The term is restricted to large transient events on stars that satisfy the following conditions:
The star is in spectral class F8 to G8 It is on or near the main sequence It is single or part of a very wide binary It is not a rapid rotator It is not exceedingly young Essentially such stars may be regarded as solar analogues. Originally nine superflare stars were found, some of them similar to the Sun.
Original superflare candidates The original paper identified nine candidate objects from a literature search:
Type gives the spectral classification including spectral type and luminosity class. V (mag) means the normal apparent visual magnitude of the star. EW(He) is the equivalent width of the 5875.6Å He I D3 line seen in emission. The observations vary for each object. Some are X-ray measurements, others are visual, photographic, spectroscopic or photometric. The energies for the events vary from 2 × 1033 to 2 × 1038 ergs.
Kepler discoveries The Kepler spacecraft is a space observatory designed to find planets by the method of transits. A photometer continually monitors the brightness of 150,000 stars in a fixed area of the sky (in the constellations of Cygnus, Lyra and Draco) to detect changes in brightness caused by planets passing in front of the stellar disc. More than 90,000 are G-type stars (similar to the Sun) on or near the main sequence. The observed area corresponds to about 0.25% of the entire sky. The photometer is sensitive to wavelengths of 400–865 nm: the entire visible spectrum and part of the infrared. The photometric accuracy achieved by Kepler is typically 0.01% (0.1 mmag) for 30 minute integration times of 12th magnitude stars.
… excerpt ends here. Continue reading the full article.
