Star spot

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A star spot is a region with a lower temperature compared to the undisturbed surface of a star. Star spots are observed in magnetically active stars and are the result of reduced energy transport from the star's interior due to magnetic fields. The properties of the star spots correspond to the sun spots on the sun, although the size of the star spots can reach a few tenths of the star surface. Star spots can be detected directly with instruments such as the CHARA array .

Magnetically active stars with star spots

Star spots are just one form of magnetic activity. In addition to the star spots, the active stars as well as the sun show flares , torches , outbreaks in the radio, ultraviolet and X-ray range, strong chromospheres and corons . Magnetically active stars include:

  • The red dwarfs with masses between 0.08 and 0.5 solar masses and surface temperatures of 2500 to 4000 Kelvin . They make up around 75% of all stars in the Milky Way and are magnetically active stars in their youth. If star spots are detected on red dwarfs, the stars belong to the class of BY-Draconis stars . When flares, short eruptions lasting from minutes to hours, are detected, the red dwarfs are counted among the UV Ceti stars . The subdivision is historical, as intensive observations have shown that all BY Draconis stars also show flares and that star spots can also be detected on UV Ceti stars. The star spots typically cause a sinusoidal modulation of the red dwarf's light curve with an amplitude of around 0.1 mag .
  • In the case of the sun-like stars , only indirect signs of stellar activity were initially found, such as the changing equivalent width of the CaII H&K emission lines . In the meantime, slight fluctuations in brightness of a few percent due to the rotation of star spots over the visible hemisphere have been detected for all main sequence stars with a spectral type later than F7. The phase-shifted correlation between the CaII H&K and the fluctuations in brightness suggests that, as with the sun, there are activity centers on these stars that contain not only star spots but also bright regions with plagues and flares . The stellar activity decreases sharply with the age of the sun-like stars. This decrease is correlated with the rotational speed that occurs due to loss of torque due to stellar winds .
  • The T-Tauri stars are young main sequence stars with a spectral type G to M and irregular changes in brightness caused by a variable accretion from a circumstellar disk. Since these stars reach high speeds of rotation through accretion, strong global magnetic fields are formed on these stars, which are only a few million years old, probably due to the alpha-omega dynamo . The magnetic fields control accretion and lead to cyclical changes in brightness with amplitudes of up to 0.5 mag, which are caused, among other things, by large, cool star spots or groups of star spots.
  • The RS Canum Venaticorum stars are separate double star systems with a modulation of the light curve by star spots. The more massive component is a giant or sub-giant with a spectral type F to K and the orbital period of the binary star system is between a day and a few weeks. The amplitude of the brightness variations caused by star spots is up to 0.6 mag, which is interpreted as a degree of coverage by star spots of 50 percent.
  • The FK-Comae-Berenices stars are a small group of rapidly rotating giants with speeds between 50 and 150 km / s. Their spectral type is F to K and all FK-Com stars show signs of stellar activity such as flares, star spots and emission lines from active regions. They differ from the RS Canum Venaticorum stars in that they have a companion, which is either absent or contributes only insignificantly to the overall light. The amplitude of the changes in brightness is below 0.3 mag in the course of a rotation period.
  • The W-Ursae-Majoris stars are contact systems made up of two sun-like stars that exchange matter and energy via a common shell. Their orbital period is 3 to 22 hours and the surface temperature of the two components is always similar. Modeling the light curves of the W Ursae Majoris stars requires the presence of cool or hot regions. Reconstructions of the star surfaces of these binary stars by Doppler imaging techniques always show star spots on both components, with the more massive component always appearing to be the more active. This distribution of the star spots leads to the O'Connell effect and W phenomenon in the W-UMa stars. The O'Connell effect describes the different brightness of the primary and secondary maxima. The W phenomenon is also a consequence of a stronger accumulation of star spots on the primary component of the binary star system, whereby the occultation of the smaller secondary star leads to a lower minimum than the occultation of the larger primary star.
  • When brown dwarfs star spots have been established only indirectly. Since the temperature in star spots is lower than on the undisturbed surface, only less energy can be radiated there. The star or brown dwarf must therefore expand in order to maintain the hydrostatic equilibrium in order to radiate the energy generated in its interior over the larger surface. This hypothesis explains, for example, why the more massive component of 2M0535-05, which consists of two brown dwarfs, is cooler and has a larger radius than the companion with its lower mass.
  • Interacting double stars of the Algol type consist of a hot, massive main sequence star with spectral classes B to F and a cooler companion. The companion is usually a G to K Unterriese, which fills its Roche interface and whose rotation period is identical to the orbit period of a few days. The high speed of rotation together with the convective energy transport on the subgiant creates a number of phenomena of magnetic activity including star spots. These are usually only detectable in the primary minimum, when the more luminous primary star is covered by the subgiant.

Observation techniques and reconstruction methods

Photometric measurements of the brightness of stars led to the first discovery of star spots. They are still the most important source of knowledge about star spots today, as photometry can already be performed with small telescopes. The technique of light curve modeling is used to infer the position, temperature and diameter of the star spots from the changes in brightness. However, the solutions are not clear even when using multi-color photometry for single stars. A better resolution and the uniqueness of the reconstruction of the spot distribution is achieved with light curve solutions only when analyzing the covering light curves in double stars.

The investigation of star spots by means of spectroscopic methods requires a high spectral resolution and a telescope with a diameter of several meters. The following reconstruction techniques can be used based on spectra:

  • The Doppler imaging technique reconstructs the distribution and size of star spots based on changes in the line profiles of absorption lines due to the rotation of the star spots across the surface
  • The Zeeman-Doppler imaging technique is based on the splitting of spectral lines by magnetic fields. This change in the absorption lines also changes due to the rotation of the magnetic fields and the star spots anchored in them across the surface of the star
  • Molecular bands of TiO and VO in the spectra of stars with a spectral type earlier than M are an indication of the presence of a zone of low temperature, i.e. a star spot. The change in the intensity and, due to the Doppler effect, the wavelength of the molecular bands allows the analysis of the distribution and size of the star spots

properties

From the amplitude of the brightness variations of up to 0.65 mag, sizes of star spots of up to 40% of the visible hemisphere in the T-Tauri star V410 Tau have been derived. The temperature difference between star spots and the undisturbed photosphere decreases with decreasing spectral class. The temperature difference is over 2000 Kelvin for stars of the spectral class G0 and only 200 K for the spectral type M4. Apparently there is no correlation with the luminosity class . Therefore, the nature of the star spots is the same for giant stars and dwarfs. In late stars, however, the proportion of the penumbra to the umbra is possibly greater and a lower temperature difference is only a consequence of insufficient resolution.

According to polarimetric measurements, the magnetic fields causing the star spots have magnetic flux densities of around 3000 Gauss in the umbra and much less reliable of 1500 Gauss in the penumbra. The so-called filling factor, the proportion of the part of the star's surface covered with star spots, appears to increase with decreasing temperature.

The lifespan of starspots, like that of sunspots, depends on their size, with smaller sunspots disintegrating faster. The survival time of larger star spots with filling factors of more than 20% is probably limited by the differential rotation of the stars. However, photometric observations seem to indicate that some pre-main sequence stars can last for more than 20 years. However, these could be active centers consisting of many smaller star spots instead of one large, permanent spot.

Centers of activity can be observed over several years, especially with RS-CVn stars. They are not assigned a fixed length , but migrate over time over the star's surface. In RS-CVn stars, young stars and the FK-Comae stars, there are often two active centers that are arranged at a distance of about 180 ° on the star sphere. Sometimes one region is dominant, and sometimes the other. The differential rotation can be calculated from the migration of the centers of activity around the star sphere. The period of rotation at the poles always seems to be longer than at the equator, like the sun.

The distribution of star spots is controversial. This is especially true in connection with the presence of star spots at the poles of the stars, which can also be simulated by misinterpretations and measurement errors. Polar star spots have never been observed on the sun. Long-term observations of stars with high rotation rates seem to show that all star spots arise close to the equator and then migrate towards the poles over the course of years.

Activity cycles

The magnetic activity of the sun can be determined using sunspots in the form of the sunspot relative number or as the degree of coverage (sum of the area of ​​the sunspot to the total area of ​​the photosphere). Other measurands are the 10.7 cm radio flux index , the area of ​​the flares (light spots) or the strength of the emission lines of calcium or magnesium. All mentioned indices show the Hale cycle with a cycle length of 11 or 22 years.

The course of the magnetic activity has also been measured for other magnetically active stars. Various reconstruction methods have been used for this, but mostly the strength of the calcium emission lines is used. In contrast to an equivalent of the speckle relative number, only one spectrum is required every couple of periods of rotation to determine an indicator of stellar activity.

Very young, rapidly rotating stars show a high level of magnetic activity, which at best can be described as chaotic and does not follow a pronounced cycle like the sun. From an age of more than a billion years, magnetically active stars show average activity levels and, in some cases, a cyclical variability of the indicators over a period of years to decades. Slowly rotating stars like the sun show little activity and well-defined cycles. Some stars show no signs of magnetic activity at all. It is still controversial whether this can be interpreted as an indication of Maund minima . Observations also show that the magnetic activity of stars in the late stages of development is extremely low. Along with this, the statement that the magnetic activity of a star is strongly correlated with its age is sufficiently well confirmed. While young stars become somewhat fainter at times of maximum activity, old stars like the sun are somewhat brighter when they are at maximum activity. This implies a development from a more star-spot-dominated photosphere to a greater influence of the flares as the active stars evolve, which is characterized by a decrease in magnetic activity over time due to torque losses.

Flip flop effect

The flip-flop effect describes observations of the sun, sun-like stars, the RS-CVn stars and some FK-Comae stars, according to which the development of two active regions on the star's surfaces is coupled. When the star spots regress in one active region, a second region in the other hemisphere becomes more active, and in this region the star spots take on a larger extension. The cycles of the flip-flop effect depend on the star type. While the length of the flip-flop effect corresponds to that of an activity cycle in RS-CVn double stars, the flip-flop cycle in sun-like stars and in the sun is 3 to 4 times shorter than the activity cycle. In addition to flip-flops, there are flip-flop-like phase changes of the star spots. During these phase jumps, the length of the stellar activity also changes by leaps and bounds, but the new active region is not offset by 180 ° from the length of the old active region on the star's surface.

Effects of star spots

The radii of stars can be measured with great accuracy in eclipsing stars . The radii of BY-Draconis stars are apparently between 3 and 12% larger than those of red dwarfs with no evidence of magnetic activity. In addition, the temperatures in the undisturbed photosphere seem to be 3% below the expected values. Both are effects of the star spots. The cooler star spots on the surface lead to a reduced radiation and the star reacts to this with an expansion in order to restore the hydrostatic equilibrium .

In cataclysmic variable is close binary systems consisting of a white dwarf and a companion who is a late sub giant or red dwarf. Most of the energy released comes from the potential energy from a flow of matter from the companion to the white dwarf. The accretion rate and thus the luminosity are subject to large fluctuations in some stars of this class and this is attributed to star spots at the Lagrange point L1 , which modulate the mass flow.

With the help of variable eclipse stars, it is possible to determine the period of revolution of a binary star system with high accuracy by photometric measurement of the time of minimum light. The light curve is changed by star spots and this can lead to a shift of the minimum. Star spots can therefore simulate that the orbital period is not constant and that a redistribution of the torque in the binary star system has taken place.

With the help of star spots, the orbital plane of extrasolar planets can also be determined. The transit of an exoplanet in front of the disk of its central star can be demonstrated by means of covering light curves ( transit method ). If the planet runs over a star spot, the light curve changes. When the exoplanet moves over the star spot again at the next passing, the plane of the planet's orbit and the plane of rotation of the star are approximately coplanar . With the help of the reconstruction of star spots from the light curve, the inclination of the axis of rotation of the star can also be derived with an accuracy of up to 5 degrees.

Flares and their relationship to star spots

Like star spots, flares are viewed as a sign of stellar activity and are also caused by magnetic fields in the upper layers of the atmosphere. 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 of the chromosphere is heated and accelerated into the corona at high speed. The flares have been detected in the range of X-rays , radio radiation , ultraviolet radiation and visible light. The course of a classic flare consists of a steep rise and a slow exponential decay of the outbreak intensity.

In contrast to star spots, flares have also been observed in stars with an early F and A spectral class. Since these stars should not have convective energy transport in their photosphere, only a weak magnetic field can be present in these stars as a remnant from the phase of star formation. Nevertheless, the energy released in the flares of these early stars is comparable or even greater than that of classical active stars. It is believed that the flares arise from a magnetic short circuit between the magnetic field of the early star and that of a magnetically active companion. Therefore, flares can also occur with stars without star spots, but all stars with star spots also show flares.

Individual evidence

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