Signatures of stellar mass ejections

Today, we know more than 400 extrasolar planets, from which about 50% orbit around G-stars at various orbital distances. Planets orbiting close to their host stars experience non-negligible thermal and non-thermal mass loss. Model simulations have shown that the exoplanetary atmosphere can be eroded and evaporated, respectively, if the planet`s orbital distance is small and the star exhibits a strong XUV radiation environment and strong CME (Coronal Mass Ejection) activity, respectively. Stellar flaring is known since the 40ies of the last century and is well investigated. In contrast reports of stellar mass ejections are rare, mainly due to the difficulty of detection. Stellar flares are directly detectable in lightcurves, whereas stellar mass ejections are only indirectly detectable. On the Sun, CMEs are observable in white light and the evolution of the ejection can be followed in time series of images. Mass ejections, which are bubbles of plasma propagating outwards from the star with velocities of some tens of km/s to about 2500km/s (solar case) through the interplanetary medium, show also signatures in optical spectra. On the active dM star AD Leonis, a high velocity mass ejection was detected through a strong enhancement in the blue wing of the H-gamma spectral line of the Balmer series with a deduced velocity of 5800km/s. Similar but weaker signatures were reported also for other stars. For the search for signatures of stellar mass ejections observations are needed which provide sufficient spectral resolution together with a high time cadence, which ensures the feasability to follow the evolution of such signatures with time. Telescopes at ESO (European Southern Observatory), especially the VLT (Very Large Telescope) together with UVES (Ultra Violet Echelle Spectrograph) provide the required specifications mentioned above, therefore we will propose observations at ESO. These ESO observations will be performed coordinated with optical photometry from Kitt Peak and Fairborn Observatories in Arizona. Another approach for the search for signatures of stellar mass ejections uses data from longer wavelengths. Solar decameter type II radio bursts (slowly drifting radio bursts), which are the signature of shock waves propagating through the stellar atmosphere and the interplanetary medium, are known to be fully correlated with solar CMEs. Even for metric type II bursts a similar correlation exists but not that explicitly as for the decameter wavelength range. In 2007, an analysis of observations of AD Leonis by the World`s largest decameter array, the UTR-2 (Ukrainian T-shaped Radiotelescope-2nd modification), was dedicated to the search for type II bursts. Type III like structures (fast drifting radio bursts) were detected in the dynamic spectra having a high probability of being of stellar origin, but no type II bursts could be found. A possible reason for this non-detection is that solar decameter type II bursts might ocur on M-stars, such as AD Leonis, at shorter wavelengths, possibly in the meter or decimeter wavelength ranges. Therefore we proposed observations of AD Leonis and Kappa Ceti at the World`s largest meter array, the GMRT (Giant Meterwave Radio Telescope) in Pune /India. The observations of Kappa Ceti will be performed coordinated with the UTR-2, and the Lustbühel observatory (optical photometry) in Graz/Austria. Additionally we will also propose observations at the EVLA (Expanded Very Large Array) which provides high-sensitivity observations from 1-50GHz to access the adjacent deci- and centi-meter range, respectively. Optical spectroscopy of the Balmer lines provides not only access to signatures of possible mass ejections but also to signatures of prominences, which are elongated arching structures of plasma remaining stable for days or weeks reaching heights of several hundreds of Mm. Since the first detection of stellar prominence signatures in the 1980ies, about ten stars are known showing H alpha profiles with subtle absorption features moving in time along the spectral line profile. Oscillations of these structures are also detectable in optical spectra. If these oscillations will be detected in stellar spectra, it is possible to deduce prominence parameters, such as spatial scale and Alfven speed and magnetic field strength, respectively. Results from these investigations will be used as input for models which simulate the influence of mass ejections on atmospheres of exoplanets orbiting their host stars at a close distances. Further, the results will contribute to the general view of stellar activity of late-type main-sequence stars.

Coronal mass ejections (CMEs) belong to the most energetic activity phenomena on the Sun. They consist of charged particles embedded in magnetic field. CMEs can have velocities up to several thousand km/s. Their occurrence is closely related to flares and prominences. Since young stars have more frequent and more energetic flares, they could also have more frequent CMEs, possibly with higher masses and velocities. CMEs can interact with planetary atmospheres. This can be observed at Earth in form of geomagnetic storms which endanger our technological infrastructure. If a planet orbits a young star, frequent CME impacts could lead to the erosion of its atmosphere. Moreover, if the planet orbits in a closer distance than the Earth and/or has a weak magnetic field, it may lose its total atmosphere due to this process. In this case evolution of life would be impossible.Moreover, CMEs could contribute significantly to mass-loss in young stars, which would affect the early stellar evolution. Observations of CMEs on other stars is intrinsically difficult. Sporadic detections could be robustly identified as mass ejections, but up to now no statistics of relevant CME parameters are known for stars other than the Sun. Because of its vicinity, continuous monitoring, as well as preferable detection methods the Sun is the only star with well-investigated CME activity.This project combined several observing strategies to infer the CME activity on other stars, like radio observations and optical spectroscopy. Several flares and prominences were detected and studied. Moreover, we observed prominence oscillations enabling the estimation of their magnetic field strengths. However, no CMEs could be detected with these methods even on the youngest Sun-like stars and we found that high-quality data is necessary to detect possible stellar CMEs. We developed an empirical model to predict the CME rate of active stars and to compare with observations. Thereby we found that very massive and energetic CMEs are rare on young stars. However, no definitive conclusion can be made on their occurrence yet. Further observations of high quality and sufficient duration, as well as new observing strategies are needed. If young stars have indeed less CMEs than previously assumed, this could promote the emergence and evolution of habitable worlds.