Flare star

UV Ceti star (also called flare or star Flackersterne, GCVS nomenclature abbreviation: UV) belong to the class of eruptive variable stars. You are at the lower end of the main sequence and are characterized by the occurrence of flares ( non-periodic, temporary releases large amounts of energy ). The flare stars show a late spectral type K or M usually with strong emission lines of hydrogen.

Flares

The stellar flares correspond to solar flares in terms of their formation and the energy released during the eruptions. Since the UV Ceti stars have a low absolute magnitude compared to the sun, the outbreaks in the white light can be observed. The cause of the outbreaks is magnetic short -circuiting of the stellar field lines in the corona. The liberated energy accelerated particles in the underlying chromosphere to the corona, the conflict there with the denser matter. The plasma of the chromosphere is heated in the process and accelerated at high speed in the corona. Flares have been detected in the field of X-rays, radio waves, ultraviolet radiation and visible light. The course of a classic flare consists of a steep rise and a slow exponential decay of the outburst intensity. Flares can be superimposed quasi-periodic pulsations that occur during and after the eruption. It is wave-like variations in brightness in the light curve, which has been observed in the sun. The physical background of quasi-periodic pulsations is unknown. The incidence of flares is up to 1.2 events per hour, most eruptions achieve only low amplitudes. The amplitude can be up to 5 magnitudine, wherein the number of flares with the amplitude decreases logarithmically. The amplitude of a flare is wavelength dependent, which steadily decreases from the ultraviolet to the infrared.

Flares are divided into fast and slow eruptions. Fast flares have more energy and its course corresponds to the solar Röntgenflares. The slow flares show an unusual course, with the increase also lasts as long as the descent. Their amplitudes are significantly lower and the duration of the increase is more than 30 minutes. Flareverläufe complex can be interpreted as a superposition of faster and slower eruptions. It is believed that fast and slow flares differed only by the geometrical arrangement. The active region, arise in the fast flares shows during rapid flares in the direction of the earth. So that the interaction of the flare can be seen at the surface of the star. Is the active region, however, on the opposite side, it can be detected on the ground only the interaction of accelerated electrons with the upper layers of the chromosphere and the corona. The interaction is then observed as a slow flare.

Star spots

On the surface of the UV Ceti star to star spots are similar to sunspots. The star spots are a range of low temperature, because the magnetic field lines of the energy transport is hindered by the stellar interior to the photosphere. If the star spots detected photometrically, the stars are also attributed to the class of the BY Draconis stars. The star spots and flares are two properties of magnetically active stars, which do not differ in their physical properties. The magnetic activity is a consequence of the convective energy transport in the outer layers of the atmosphere in combination with a differential rotation. This leads to a movement of the ionized plasma and to generate a global magnetic field. The constant X-ray luminosity is 1025.5 to 1029.5 erg / s and is probably the result of a large number of Nanoflares.

From photometric observations of star marks the rotation period can be derived, which is usually in the range of a few days. A comparison with the distribution of flares shows that it is not a large active region seems to be on the UV Ceti stars against simple models. The flares are evenly distributed and therefore are likely to Flaresternen several smaller active regions exist with corresponding star spots, where the magnetic short circuits, which are the cause of the flares.

Properties of the UV Ceti stars

UV Ceti stars belong to the group of magnetically active stars along with the RS Canum - Venaticorum - stars, the BY Draconis stars and the FK Comae Berenices - stars. The UV Ceti stars are often found in regions of active star formation or young open clusters. The magnetic activity of M dwarfs at the bottom of the main sequence increases with age quickly and it seems to be for the M dwarfs with spectral types earlier than M5.5 as the Sun-like stars a cyclic activity. The activity cycles are detected spectroscopically during rest periods by the line width of H -alpha, H- and K- line of calcium and the Na1 - line of sodium. About 75 % of all M dwarfs are among the magnetically active stars and show the typical UV Ceti star flares.

The magnetic activity is much more pronounced later dwarfs in binary systems than in single stars. It has long been known that all the signs of magnetic activity with age and the rotation rate to lose weight, have shown how studies of open star clusters. This applies both for late dwarfs with fully convective energy transport as well as stars with a core with radiative energy transport as the sun, the effect being more pronounced in the latter. For stars with a radiative core, the stellar magnetic field in the tachocline region, the transition layer between the core and the outer layer with convective transport of energy is produced. For fully convective stars, the cause of the generation of a stellar magnetic field is not known. The influence of a companion on the stellar activity may result from the fact that the companion already limits the lifetime of the accretion disk during star formation. This means that less rotational energy is dissipated via akkretionsgetriebene stellar winds. In close binary stars occurs through tidal effects to a synchronization of the rotational period with the duration of train circulation and these stars show strong signs of magnetic activity such as flares or in the intensity of the hydrogen emission lines of the Balmer series.

Rapidly rotating old red dwarfs can also be the result of an interaction with a planet in a tight orbit. These hot Jupiter deform near its star and the dissipated strain energy decreases further the orbit radius. This results in a co-rotation of the star and of the planet, whereby the rotation speed of the red gnome is accelerated again. At the end of this process may lead to a merger of the planet and the Red Dwarf, which the star is gaining considerable momentum.

Examples

Known UV Ceti stars are UV Cet, YZ Cet, AD Leo, EV Lac, Ross 248 and CN Leo ( Wolf 359).