UV Ceti star

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Artist's impression of DG Canum Venaticorum a binary star system , which consists of two red dwarf stars of the spectral type M4Ve, and therefore belongs to the group of UV-Ceti stars.

UV Ceti stars (based on their prototype UV Ceti ; also flare stars or flicker stars , GCVS system abbreviation :  UV ) belong to the class of eruptive variable stars . They are located at the lower end of the main sequence and are characterized by the occurrence of flares (aperiodic, temporary releases of large amounts of energy). The flare stars show a late spectral class mostly of type K or M with pronounced emission lines for hydrogen. Most of these stars have 0.08 to 0.5  solar masses and are correspondingly red dwarfs .

Flares

Flare on the sun's surface

The stellar flares correspond to the solar flares in terms of their formation and the energy released during the outbursts. However, since the UV Ceti stars have a lower absolute brightness than the sun , the outbreaks can also be observed in white light.

The cause of the outbreaks lies in magnetic short circuits of the stellar field lines in the corona . The energy released in the process accelerates particles into the chromosphere below the corona, where they collide with the denser matter. The plasma in the chromosphere is heated and accelerated back into the corona at high speed. The flares have been detected in the range of X-ray , ultraviolet and radio radiation as well as in visible light.

The course of a classic flare consists of a steep increase in the outbreak intensity and a slow exponential decay. Quasi-periodic pulsations that occur during and after the eruption can be superimposed on the flares. These are wave-like brightness variations in the light curve that have also been observed in the sun. The physical background of the quasi-periodic pulsations is unknown.

The frequency of the flares is up to 1.2 events per hour. Most eruptions only reach low amplitudes up to max. 5  magnitudine . The number of flares decreases logarithmically with the amplitude. The amplitude of a flare depends on the wavelength: it decreases steadily from the ultraviolet to the infrared.

Fast and slow flares

Flares are divided into:

  • fast flares; they have more energy and their course corresponds to the solar X-ray flares.
  • slow flares; they show an unusual course in which the ascent takes as long as the descent (more than 30 minutes). Their amplitudes are significantly lower than with fast flares.

Complex flare courses can be interpreted as a superposition of fast and slow eruptions.

Presumably, fast and slow flares only differ in their geometric arrangement:

  • The active region in which fast flares arise points towards earth; this makes the interaction of the flare with the star surface visible.
  • If, on the other hand, the active region is on the remote side, only the interaction of the accelerated electrons with the upper layers of the chromosphere and the corona can be detected on earth; the interaction is then observed as a slow flare.

Star spots

Image of a huge sunspot captured by ALMA .

On the surface of the UV Ceti stars there are star spots similar to sunspots . The star spots are an area of ​​low temperature because the magnetic field lines impede the transport of energy from the star's interior into the photosphere. If the star spots are detected photometrically , the stars are also assigned to the class of BY-Draconis stars . The star spots and the flares are two properties of magnetically active stars that 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 the generation of a global magnetic field. The constant X-ray luminosity is 10 18.5… 22.5 watts and is probably the result of a large number of nanoflares.

The rotation period , which is usually a few days, can be derived from photometric observations of the star spots . A comparison with the distribution of the flares shows that, contrary to simple models, there does not seem to be a large active region on the UV Ceti stars, but that the flares are evenly distributed. Therefore, several smaller active regions with corresponding star spots are likely to exist on the flare stars, in which the magnetic short circuits also occur, which are the cause of the flares.

properties

UV Ceti stars, along with the RS Canum Venaticorum stars , the BY Draconis stars and the FK Comae Berenices stars, are among the magnetically active stars. The UV Ceti stars are often found in regions with active star formation or in young open star clusters . The magnetic activity of the M dwarfs at the lower end of the main sequence decreases rapidly with age and for the M dwarfs with a spectral type earlier than M5.5 - as with the sun-like stars - there seems to be a cyclical activity . The activity cycles can be detected spectroscopically in the resting phases through the line strength of H-alpha , the H and K lines of calcium and the Na1 line of sodium. About 75% of all M dwarfs belong to the magnetically active stars and show the flares typical of UV Ceti stars.

As studies on open star clusters have shown, in UV Ceti stars all signs of magnetic activity decrease with age and the rate of rotation. This applies both to late dwarfs with fully convective energy transport and to stars like the sun with radiative energy transport in the core. With the latter, the effect is stronger and the stellar magnetic field is created in the Tachocline region , the transition layer between the core and the outer layer with convective energy transport; in the case of fully convective stars, however, it is not known why a stellar magnetic field is formed.

The magnetic activity of later dwarfs is much more pronounced in binary star systems than in single stars . The influence of a companion on stellar activity can come about because the companion already limits the life of the accretion disk during star formation . This means that less rotational energy is dissipated via accretion- driven stellar winds .

In the case of close binary stars, the tidal effects lead to a synchronization of the period of rotation with the period of orbit ( bound rotation ); these stars show strong signs of magnetic activity such as flares or in the intensity of the hydrogen emission lines of the Balmer series .

Fast rotating ancient red dwarfs can also be the result of interacting with a planet in a tight orbit. These Hot Jupiters are deforming near their star, and the dissipated deformation energy further reduces the orbital radius. This leads to a corotation of the star and the planet, whereby the speed of rotation of the red dwarf increases again. At the end of this process, the planet and the red dwarf can merge , causing the star to gain considerable angular momentum .

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists around 1000 stars with the abbreviation  UV , which means that around 2% of all stars in this catalog belong to the class of UV Ceti stars.

Examples

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

Individual evidence

  1. ↑ Akiko Uzawa et al .: A Large X-ray Flare from a Single Weak-lined T Tauri Star TWA-7 Detected with MAXI GSC . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1108.5897v1 .
  2. ↑ B. Fuhrmeister, S. Lalitha, K. Poppenhaeger, N. Rudolf, C. Liefke, A. Reiners, JHMM Schmitt, J.-U. Ness: Multi-wavelength observations of Proxima Centauri . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1109.1130v1 .
  3. ↑ Qian S.-B., Zhang J., Zhu L.-Y., Liu L., Liao W.-P., Zhao E.-G., He J.-J., Li L.-J. , Li K. and Dai Z.-B .: Optical flares and flaring oscillations on the M-type eclipsing binary CU Cnc . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1204.6104v1 .
  4. ^ HA Dal, S. Evren: The Statistical Analyzes of Flares Detected In B Band Photometry of UV Ceti Type Stars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.3761 .
  5. ^ HA Dal and S. Evren: A New Method To Classify Flares Of UV Ceti Type Stars: Differences Between Slow And Fast Flares . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.5791 .
  6. ^ HA Dal and S. Evren: Rotation Modulations and Distributions of The Flare Occurrence Rates On The Surface Of Five UV Ceti Type Stars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.5792 .
  7. ^ John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
  8. ↑ I. Crespo-Chacon, G. Micela, F. Reale, M. Caramazza, J. Lopez-Santiago, and I. Pillitteri: X-ray flares on the UV Ceti-type star CC Eridani: a “peculiar” time- evolution of spectral parameters . In: Astrophysics. Solar and Stellar Astrophysics . 2007, arxiv : 0706.3552v1 .
  9. ^ Nicholas M. Hunt-Walker, Eric J. Hilton, Adam F. Kowalski: MOST observations of the Flare Star AD Leo . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.5019 .
  10. ↑ M. Moualla et al .: A new flare star member candidate in the Pleiades cluster . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1108.6278 .
  11. ^ J. Gomes da Silva, NC Santos, X. Bonfils, X. Delfosse, T. Forveille, and S. Udry: Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program I. Comparison of activity indices . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1109.0321v1 .
  12. ↑ DYLAN P. MORGAN, ANDREW A. WEST, ANE GARCE, SILVIA CATALA, SAURAV DHITAL, MIRIAM FUCHS, AND NICOLE M. SILVESTRI: THE EFFECTS OF CLOSE COMPANIONS (AND ROTATION) ON THE MAGNETIC ACTIVITY OF M DWARFS . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.6806 .
  13. ↑ Emeline Bolmont, Sean N. Raymond, Jeremy Leconte, and Sean P. Matt: Effect of the stellar spin history on the tidal evolution of close-in planets . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.2127v1 .
  14. ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved May 4, 2019 .

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