Starspot

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 decreased energy transport from the stellar interior due to magnetic fields. The star spots correspond in their properties to the sunspots on the sun, but with the expansion of the star spots can reach several tenths of the stellar surface. Star spots can not be observed directly, but can be detected only indirectly through brightness fluctuations, changes of the spectrum or polarimetric measurements.

Magnetically active stars with starspots

Star spots are just a form of magnetic activity. In addition to the star spots show the active stars like the sun flares, flares, eruptions on the radio, ultraviolet and X-ray range, strong chromospheres and coronas. The magnetically active stars include:

• The red dwarfs with masses from 0.08 to 0.5 solar masses and surface temperatures 2500-4000 Kelvin. You make a good 75 % of all stars in the Milky Way and are in their youth magnetically active stars. Will the Red Dwarf Star spots proved so count the stars to the class of BY Draconis stars. In the detection of flares, short flares with a duration of minutes to hours, the red dwarfs are counted as UV Ceti stars. The subdivision is a history where intensive observations have shown that all BY Draconis stars show flares and star spots can be detected in UV Ceti stars. The star spots typically cause a sinusoidal modulation of the light curve of the red dwarf with an amplitude of about 0.1 mag.
• In the sun-like stars only indirect signs of stellar activity were first found as the variable equivalent width of the CaII H & K emission lines. Meanwhile, low brightness fluctuations of a few percent by the rotation of star spots on the visible hemisphere could be detected for all main sequence stars with a spectral type later than F7. The phase-shifted correlation between the Ca II H & K and the brightness variations suggests that there are, as in the sun on these stars activity centers, which in addition to stellar spots and bright regions Plages and torches. The stellar activity decreases strongly with the age of the Sun-like star. This decrease is correlated with the rotational speed which occurs due to a torque loss due to wind rating.
• The T Tauri stars are young main sequence stars with a spectral type G to M and irregular changes in brightness, which is caused by a variable accretion from a circumstellar disk. As these stars reach through the accretion high rotational speeds, to strong global magnetic fields form on this only a few million years old stars probably by the Alpha - Omega Dynamo. The magnetic fields control the accretion and lead to cyclical changes in brightness with amplitudes of up to 0.5 like, caused among other things by large cool star spots or star spot groups.
• The RS Canum - Venaticorum stars are separated binary systems with a modulation of the light curve by star spots. The more massive component is a giant or supergiant with a spectral type F to K and the orbital period of the binary system is between one day and several weeks. The amplitude of the brightness variations due to starspots is up to 0.6 like what is interpreted as a degree of coverage by starspots of 50 percent.
• The FK Comae Berenices - stars are a small group of rapidly rotating giant with speeds between 50 and 150 km / s Your spectral type of F to K and all FK Com stars show signs of stellar activity such as flares, starspots and emission lines of active regions. They differ from the RS Canum - Venaticorum - stars by the companion, which is absent or only marginally contributes to the total light. The amplitude of the luminance change is below 0.3 may during a period of rotation.
• The W Ursae Majoris stars are contact systems of two Sun-like stars that take advantage of a common envelope matter and energy. Their orbital period is 3 to 22 hours and the surface temperature of the two components is always similar. The modeling of the light curves of the W Ursae Majoris stars requires the presence of cool or hot regions. Reconstructions of the stellar surfaces of these double stars by Doppler imaging techniques show always two components star spots, where the more massive component appears to be the more active always. This distribution of 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 collection of star spots on the primary component of the binary system, making the cover of the smaller secondary star leads to a deeper minimum than the coverage of the larger primary star.
• For brown dwarfs star spots have so far been detected only indirectly. In star spots since a lower temperature than is present on the undisturbed surface, only less energy can be radiated there. The star or brown dwarf must therefore expand to the maintenance of hydrostatic equilibrium to radiate the energy generated in the interior of the larger surface area. 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 smaller mass.
• Interacting Binary Stars from Algol type consist of a hot massive main sequence star of spectral type B to F and a cooler companion. The attendant is usually a G to K subgiant, which fills its Roche- interface and its period of rotation is identical to the railway orbital period of a few days. The high rotational speed along with the convective energy transport on the sub- giant produces a series of phenomena of magnetic activity including star spots. These are usually only detectable in the primary minimum when the luminous stronger primary star is covered by the sub- giant.

Observation techniques and reconstruction methods

Photometric measurements of the brightness of stars have led to the first discovery of stellar spots. You are still the most important source of knowledge about stellar spots as photometry can already be done with small telescopes. To close from the brightness changes of the position, temperature and diameter of the star spots, the technique of light curve model is used. However, the solutions are not unique, even when using Mehrfarbenphotometrie for single stars. A better resolution and the uniqueness of the reconstruction of the spot distribution is achieved in the light curve solutions only in the analysis of eclipsing light curves in binary stars.

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

• Doppler Imaging technique to reconstruct 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 over the surface
• Zeeman Doppler imaging technique is based on the splitting of spectral lines by magnetic fields. This change in the absorption lines also changed due to the rotation of the magnetic fields and enshrined in them star spots on the surface of the star
• Molecular bands of TiO and VO in the spectra of stars with spectral type earlier than M a are an indication of the existence of a low temperature zone, so one star spot. The change in the intensity and because of the Doppler effect, the wavelength bands of the molecule allows the analysis of the distribution and size of the star spots

Properties

Like from the amplitude of the brightness variations of up to 0.65 have been derived sizes of star spots by up to 40 % of the visible hemisphere in the T Tauri star V410 Tau. The temperature difference between starspots and the undisturbed photosphere is decreasing spectral always lower. So is the temperature difference over 2000 Kelvin for stars of spectral type G0 and only 200 K for the spectral type M4. There seems to be no correlation with the luminosity class. Therefore, the nature of the star spots the same for giant stars and dwarfs. You may be late in stars but also the proportion of penumbra to umbra larger and a smaller temperature difference is only a consequence of insufficient resolution.

The star spots causing the magnetic fields have by polarimetric measurements over magnetic flux densities of about 3000 Gauss in the umbra and clearly uncertain of 1500 Gauss in the penumbra. The so-called filling factor, the proportion of the area covered with starspots part of the stellar surface seems to increase with decreasing temperature.

The life of star spots depends on the analog of sunspots on their size, with smaller sunspots decay faster. Survival time greater star spots with filling factors of more than 20% is likely to be limited by the differential rotation of the star. However, in some Vorhauptreihensternen seem to indicate photometric observations that they can pass over 20 years. This could however be active sites consisting of many smaller star spots, act, rather than by a large stain resistant.

Activity centers can be observed especially in RS CVn - stars over several years. They are not assigned to a fixed length, but wander over time on the stellar surface. For RS - CVn - stars, young stars and the FK Comae - stars often are two active centers, which are located at a distance of about 180 ° on the star ball. It is sometimes one and sometimes the other dominant region. From the migration of the activity centers around the star sphere, the differential rotation can be calculated. The period of rotation of the poles always seems to be longer than at the equator as in 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 star, which can be simulated by misinterpretation and measurement error. Polar star spots have never been observed in the sun. But long-term observations of stars with high rotation rates seem to show that all the star spots arise äquatornah and then in the course of years migrate poleward.

Activity cycles

The magnetic activity can be determined in the sun based on sunspots in terms of the relative sunspot number or as coverage ( sum of the areas of sunspots for the entire surface of the photosphere ). Other measurements are 10.7 cm radio flux index, the area of the torches (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.

In other magnetically active stars the course of the magnetic type has been measured. For various reconstruction methods have been applied, but usually the thickness of the emission lines of calcium is used. In contrast to an equivalent number of spots relative only a spectrum every few rotation periods required to determine an indicator of the stellar type.

Very young rapidly rotating stars show a high magnetic activity that can be described as chaotic at best, and not a distinct cycle follows as in the sun. From the age of more than a billion years show magnetically active stars mean activity level and partly 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. If this can be interpreted as an indication of Maunderminima is still controversial. From observations it appears in addition that the magnetic activity of stars in late stages of development is extremely low. Along with this is the statement that the magnetic activity of a star is strongly correlated with its age, a sufficiently good confirmation. While young stars to times of maximum activity are somewhat fainter, old stars like the sun are a little brighter in the activity maximum. This implies the development of a multi- star spots dominated photosphere to a larger influence of the torch in the course of development of the active star, which is characterized by a decrease of the magnetic type in the course of time due to torque loss.

Flip -flop effect

The flip- flop effect describes observations of the sun, with sun-like stars, the RS CVn - stars and some FK Comae stars, according to which the development of two active regions on the star runs coupled surfaces. If the star spots regress in an active region, a second region becomes more active in the other hemisphere, which assume in this region the star spots a greater extent. The cycles of the flip- flop effect are dependent on the type of star. While in RS CVn - binary stars, the length of the flip- flop effect corresponds to an activity cycle, the flip -flop cycle of solar-like stars and the sun by a factor of 3 to 4 is shorter than the activity cycle. In addition to flip- flops occur flip-flop -like phase change of star spots. In these phase jumps, the length of the stellar activity also changes by leaps and bounds, but the new active region is not displaced by 180 ° relative to the length of the old active region on the stellar surface.

Effects of star spots

The radii of stars can be measured with great accuracy in eclipsing stars. Here, apparently, the radii of BY Draconis stars 3-12 % larger than red dwarfs without signs of magnetic activity. In addition, the temperatures in the undisturbed photosphere seem to be around 3% below the expected values. Both are effects of star spots. The cooler star spots on the surface lead to a reduced emission and the star responds with an expansion to restore the hydrostatic equilibrium.

In cataclysmic variables are close binary systems consisting of a white dwarf and a companion who is a later subgiant or red dwarf. Most of the energy released comes from the potential energy of a Materiefluß of the companion to the white dwarf. The accretion rate and hence the luminosity is limited with some stars of this class to large fluctuations and this is attributed to star spots at the L1 Lagrange point, modulating the mass flow.

With the help of eclipsing stars, it is possible orbital period of a binary system with high accuracy to be determined by the photometric measurement of the time of minimum light. By star spots the light curve is changed and this can lead to a shift of the minimum. Star spots can therefore pretend that the web orbital period is not constant and there has been a redistribution of torque in a double star system.

With the help of stellar spots and the orbital plane of extrasolar planets can be determined. The pre-transition of an exoplanet in front of the disk of its parent star can be detected by means of cover light curves ( transit method ). Runs the planet while on a star spot, you will alter the light curve. If the exoplanet moves at the next pre-transition again about the star spot, the orbital plane of the planet and the plane of rotation of the star approximately coplanar. With the reconstruction of star spots from the light curve can also be derived with an accuracy of up to 5 degrees, the inclination of the rotation axis of the star.

Flares and their relation to star spots

Flares are like star spots viewed as an indication of stellar activity and also caused by magnetic fields in the upper atmosphere. 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-ray radiation, the 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.

In contrast to star spots flares have also been observed in stars with a spectral type early F and A. Since these stars should have no convective energy transport in their photospheres, in these stars only a weak magnetic field as a remnant may be present from the phase of star formation. Nevertheless, these early stars the released in the flare energy is comparable to or even greater than with traditional active stars. It is believed that the flares arise in a magnetic short circuit between the magnetic field of early star and that of a magnetically active companion. Therefore flares can occur without star spots even with stars, but all the stars with star spots also show flares.