Magnetic activity in the Hertzsprung-Russell diagram

Magnetic activity describes the observable manifestations of a dynamo-generated stellar magnetic field. The most well-known example of an active star is our Sun; the 11-year solar cycle is a clear indication for its dynamo. Solar-like dynamos require (differential) rotation and convection and are, therefore, only found in stars with an interior structure analogous to that of the Sun. Nevertheless, strong signatures of magnetic activity are seen also in fully convective, very low-mass stars suggesting that some other kind of dynamo mechnism takes over.

The temperature in the atmospheres of active stars increases outward as a result of magnetic heating. The outer atmospheric layers (chromosphere, transition region and corona) exist only thanks to the stellar dynamo. They become manifest through radiation at successively shorter wavelengths, from the optical and UV to the X-ray regime. Multi-wavelength studies of magnetic activity are, therefore, a crucial tool for understanding the structure of stellar atmospheres.

Identification of young stars in X-ray surveys

Fig.: Color-magnitude diagram with photometry from the Gaia mission for stars that are candidate members of the TW Hya association (TWA). The position in the HR diagram above the main sequence (thick black line) and high X-ray luminosity (expressed by the color code) indicate young age. The TWA has an age of about 10 million years. (M.Bogner - master thesis)

After the dispersal of the accretion disk (see topic "star formation") the optical and IR emission of young stars is similar to that of a main-sequence star which has already reached the phase of nuclear fusion. These so-called "weak-line" T Tauri stars are, therefore, difficult to identify. However, during the whole pre-main sequence phase -- independent of the presence or absence of circumstellar material -- the X-ray emission is up to a factor thousand stronger than on the main-sequence. This makes observations of star forming regions in the X-ray band an efficient means for finding "weak-line" T Tauri stars. X-ray surveys are, therefore, complementary to surveys at IR wavelengths which detect predominantly young stars with accretion disks (so-called "classical" T Tauri stars). Currently, we use X-ray data from the eROSITA instrument to establish complete samples of pre-main sequence stars in star forming regions and nearby co-moving groups (see Fig. on the right).

Activity cycles

Fig.: Activity cycl of the 400 million year old solar-like star eps Eridani. Observations in the chromospheric Calcium lines are shown in black, averaged over each observing season. X-ray observations are red (XMM-Newton) and blue (eROSITA) symbols. During the years 2014 - 2018 the optical and the X-ray lightcurves displayed a high correspondence but in 2020 the Calcium emission has dropped sharply and this has no equivalent decrease of the X-ray emission. (M.Coffaro)

The well-known 11-year cycle of our Sun is caused by the periodically repeating reconfiguration of the solar magnetic field. It is, therefore, reasonable to expect that other stars that harbor magnetic fields undergo activity cycles as well. The solar cycle is manifest throughout the electromagnetic spectrum through variability on the characteristics 11-year timescale. Already in the 1990s it was established that other solar-like stars (spectral type G) have cycles, with periodicities between about 2-20 years. These measurements were obtained through optical spectroscopy using the chromospheric Calcium H&K emission lines. The equivalents of these chromospheric cycles in the outer atmosphere, the corona, have remained difficult to identify. Through repeated snapshot observations with the X-ray observatory XMM-Newton we were able to detect coronal cycles on two young solar-like stars, iota Hor und eps Eri (see Figure). In the meantime we have started a detailed characterization of the coronae of these stars. We have developed a new method that allows us to establish the presence and size of different types of magnetic structures magnetischen (called "Active Regions", "Cores of Active Regions" and "Flares") in the stellar corona, although it is not possible to spatially resolve the surface of the stars. This method consists in modelling of the stellar XMM-Newton data with results from our Sun on which the abovementioned magnetic structures were directly observed.

Activity and rotation in low-mass stars

Fig. from Magaudda et al. (2021): X-ray activity versus Rossby number. The X-ray activity is here measured as fraction of the total stellar luminosity (L_bol) that is emitted in the X-ray band (L_x). The Rossby number is defined as the ratio between rotation period (P_rot) and convective turnover time. It is considered to represent the efficiency of the stellar dynamo. The most recent data obtained with the instruments eROSITA (L_x) and TESS (P_rot) are highlighted in color.

Historically, the first indication of dynamo-generated magnetic fields in stars other than our Sun came from an empirical relation between stellar rotation rate and magnetic activity (measured through the X-ray luminosity). Nevertheless, up to now the rotation-activity relation has remained poorly constrained especially for very low-mass stars of spectral type M. M stars are fully convective, and a qualitative change of the magnetic activity with respect to solar-like stars is expected. M dwarfs are slow rotators, with rotation periods of weeks up to months that require long monitoring campaigns for detection. These long rotation periods together with their low (X-ray) luminosities make the observational study of the rotation-activity relation of these objects difficult. Presently we deal with this problem through combined measurements of X-ray luminosities (as a means to assess activity) and high-precision optical light curves from the Kepler and K2 missions (from which we determine rotation periods).

Flares

Flare on M dwarf observed with the K2 mission — Fig. from Raetz et al. (2020): Portion of an optical lightcurve of an M dwarf star obtained with the K2 mission. The modulation of the lightcurve resulting from star spots on the surface that rotate with the star has been removed such that the flux outside flare events is constant. The main flare parameters are illustrated in the figure. In addition the energy emitted during the flare can be calculated as the area under the flare curve.

Flarefluss in der Habitablen Zone — Fig.: Maximum energy flux that reaches the habitable zone of an M dwarf star during stellar flares. Each data point represents one observed flare event. Downward pointing triangles are calculated for planets at the inner edge of the habitable zone, and upwards pointing triangles are the outer edge of the habitable zone. (M.Bogner - Bachelor thesis)

Flares are stochastic brightness outbursts on magnetically active stars that are caused by a reconfiguration of the stellar magnetic field ("magnetic reconnection") which sets free energy and converts it into electromagnetic radiation. The radiation at different wavelengths is produced in different layers of the stellar atmosphere: In the photosphere the flare phenomenon is manifest in optical emission, while the corona produces X-ray flares.

Huge numbers of optical flares are recorded by the photometric space missions Kepler and TESS. These satellites have been devised for the search of exoplanets. However, they yield light curves with ample information on stellar magnetic activity (see top Fig. on the right). Next to our observational studies of optical flares we develop algorithms for the identification of flares with the future ESA mission PLATO.

Our research on stellar flares has the main goal to better understand the magnetic processes on the star but it is also relevant for exoplanet studies. The illumination of planets by their host stars, and in particular the variability of the stellar variation, are crucial parameters in calculations of planetary habitability. How much energy a planet receives from its host star depends on many factors, amongst others the brightness of the star and the separation between star and planet (see bottom Fig. on the right).

Activity on ultracool dwarfs

Fig. from Stelzer et al. (2021): X-ray luminosity versus spectral type in the ultracool dwarf domain. Red and blue are new X-ray detections of UCDs obtained from the eROSITA All-Sky survey. These data have tripled the number of UCDs known to be X-ray emitters. The X-ray luminosities of these objects are relatively high with respect to the sparse data obtained with previous X-ray instruments. This is due to the relatively low sensitivity of eROSITA that is, however, compensated by its all-sky capability.

Objects of spectral type M7 and later are called "ultracool dwarfs". This group comprises both stars of very low mass and brown dwarfs. The interiors of ultracool dwarfs are fully convective. Therefore, it is expected that their magnetic activity -- if any -- is of different type as that observed on solar-like stars. Moreover, the photospheres of ultracool dwarfs are characterised by a low degree of ionisation which makes the coupling between matter and magnetic field, a necessary condition for magnetic activity, questionable. We examine the activity of ultracool dwarfs with radio, optical and X-ray observations and compare the results to analogous observations of stars of spectral types GKM that have solar-like interiors. Currently an instrument of prime interest for our studies is eROSITA. Thanks to its all-ksy survey eROSITA is capable of identifying many X-ray sources that were unknown so far. Already during the first of 8 planned sky surveys eROSITA has tripled the number of known X-ray emitting UCDs (see Fig. on the right). A detailed investigation of the variability and temperature of the corona of these faint objects is underway.