Flaring- and CME-activity of late-type main-sequence stars

Stellar activity can have major consequences for the habitability of terrestrial planets in close orbits. In view of forthcoming habitable planet finding missions like Darwin (ESA) and TPF (NASA), a detailed knowledge of the activity characteristics of possible target stars is essential for the design of these missions. Recent studies have shown that CME (Coronal Mass Ejection) impacts can erode the atmospheres of close planets. This is of special importance for late-type (K, M) stars, since the habitable zones are much closer to the host star as for earlier types (F, G). It is a known fact that a large number of late-type stars show far stronger flaring activity than the present Sun. Since flares and CMEs on the Sun are associated spatially and temporally to a certain degree it is assumed that stars with stronger flaring activity are also stars with stronger CME activity (solar-stellar analogy). While flaring activity on other stars has been studied for years, there is still a major lack of information on their CME-activity, because CMEs are only directly observable on the Sun. Indirect methods are required when trying to investigate stellar CME activity. On a few K and M stars absorption features in UV spectra were detected which can be interpreted as indications for mass ejections. On the Sun, decametric radio type II bursts are reliable indicators for CMEs. Relying on the solar-stellar analogy, we are planning to search for stellar analogues of these characteristic bursts on nearby G, K, and M stars, which are the most favorable targets for habitable planet finding missions. We propose an observational campaign including radio observations and simultaneous UBVRI photometry and optical spectroscopy. The radio observations with the aim of detecting stellar analogues of radio type II bursts will be carried out by the World`s largest decameter array, the UTR-2 of the Ukrainian Academy of Sciences, located in Kharkov (Ukraine). The simultaneous photometry in the optical range will be performed at facilities of the Astronomical Institute of the Slovak Academy of Sciences to provide us with data on coinciding flares, the optical spectroscopy will be carried out at the Lustbühel Observatory in Graz. As a second approach, we are planning to use existing FUV-spectra (FUSE) to investigate possible line shifts and flux ratios as indicators for stellar mass expulsions. In addition, we are planning to compile a catalogue of M stars focusing on data concerning stellar activity. This catalogue then serves as a basis for the general study of the activity of M stars, especially in view of their role as possible habitable planet hosts. With this proposed project we are striving for a better understanding of stellar mass ejections and their connection with stellar flares using a combination of several observational and methodological approaches.

The number of detected exoplanets is growing fast, as well as methods for their characterization. Planets form a tight symbiosis with their host stars. The properties of the stars have therefore important implications for the origin and evolution of planets, their atmospheres, and habitability. From the Sun, highly-energetic radiation and particle emissions are known to cause major disturbances in the Earth`s atmosphere. It is obvious that stars more active than the Sun have much stronger influence on planets, especially if they are located closer to the star. Therefore, detailed knowledge of stellar activity properties and their influence on planets is crucial. Within this project, we use indirect indications of Coronal Mass Ejections (CMEs) known from the Sun to search for analogous phenomena on stars. Therefore, data from different wavelength regimes (radio and far ultraviolet/FUV) of different stars have been analyzed. As these signals are expected to be weak, sensitive instruments are needed for their detection and their sporadic nature require long-term observations. In the radio domain, several possible stellar bursts have been detected, which are however unlikely to be related to plasma ejections. Probably instruments with higher sensitivity than currently available are needed to detect CME-related bursts. In the FUV domain, weak indications of stellar CMEs have been found, in particular on one star. Because of short-term observations, limited sensitivity, and the transient nature of the events lead to these limited results. However, future instruments with higher sensitivity, as well as the exploitation of other wavelength regimes, will definitely improve studies of stellar CMEs. A second objective of the project was to compile a catalog of nearby stars of spectral type M, which have about half the mass of the Sun or lower and are much fainter. These stars comprise more than 70% of all stars in the galaxy, and are therefore promising targets for exoplanet searches. However, many of them are very active, so their capability of hosting life is still debated. The catalog includes various important data on the nearest 700 stars of this kind. It forms a rich basis for planetary, as well as stellar scientists, to select interesting and promising targets for future studies. The influence of stellar activity phenomena on the atmospheres of planets has also been addressed. In particular, the atmospheric evaporation caused by stellar high-energy radiation over the lifetimes of stellar systems has been studied. As a large number of exoplanets is known which orbit their host stars at distances even smaller than 1/10 the distance of Mercury, the influence of stellar emissions is expected to be much larger than for our Earth. However, we found that this process is not efficient enough to support the hypotheses that close-in rocky exoplanets, like the recently discovered CoRoT-7b, could be the cores of once more massive gas giants.