Magnetic activity of planet-hosting stars

Magnetic activity in cool stars is a widely observed phenomenon, however it is still far from being understood. How fundamental stellar parameters like mass and rotational period quantitatively cause a stellar magnetic field which manifests itself in features such as spots, flares and high-energy coronal emission is a lively area of research in solar and stellar astrophysics. Especially for planet-hosting stars, stellar activity profiles are very interesting as exoplanets are affected by high-energy radiation, both at the time of planet formation as well as during the further lifetime of a star-planet system. In extreme cases, the atmosphere of a planet very close to its host star can be strongly heated by the stellar X-ray and EUV emission and finally escape the planet's gravitational attraction, so that the atmosphere of the planet evaporates over time.

Theoretically, planets can also affect their host star's magnetic activity. In analogy to processes in binary stars which lead to enhanced - both overall and periodically varying - activity levels, also giant planets might influence the stellar activity by tidal or magnetic interaction processes, however on a weaker level than in binaries. Some indications for such interactions exist from chromospheric measurements in stars with Hot Jupiters. In this thesis I investigate the magnetic activity of planet-hosting stars and especially possible effects from star-planet interactions with an emphasis on stellar coronae in X-rays.

I tested a complete sample of all known planet-hosting stars within 30 pc distance from the Sun for correlations of stellar X-ray properties with planetary parameters. A significant correlation exists between the stellar X-ray luminosity and the product of planetary mass and inverse semimajor axis. However, this could be traced back to a selection effect introduced by planetary detection methods. For stars in the solar neighborhood, planets are mainly detected by radial velocity shifts in the stellar spectra. This detection method introduces several trends in samples of planet-hosting stars which are investigated in detail in this thesis. On top of these selection effects, no significant other correlations which could be interpreted as manifestations of star-planet interactions were present in the sample.

I also monitored the chromospheric and coronal activity of a promising individual star-planet system over several months. This system consists of upsilon Andromedae, a cool main-sequence star, a Hot Jupiter and three more planets in wider orbits. Contrary to earlier findings by other authors, the star did not show planet-induced activity variations, but displayed variability with the stellar rotation period instead.

The star 51 Pegasi also hosts a Hot Jupiter; actually, it is the first exoplanet which was ever detected. In a detailed analysis of this star's coronal emission, I show that the star is in a Maunder minimum state, characterized by a very low coronal temperature of less than one million degrees and a persistent low activity level in coronal and chromospheric emission over sixteen years. The Hot Jupiter apparently does not enhance stellar activity in this system. I also present an analysis of the planet-hosting star tau Bootis, for which indications for a very short activity cycle of only one year duration have been published recently. The star rotates quickly compared to other stars of the same age, which might be due to a "spin-up" caused by its giant planet. My X-ray data that is available up to now suggests that a possible activity cycle is longer than thought so far; however, more data will be collected in 2011 and 2012 to allow a more detailed insight into this star's ac!

tivity.