Magnetic Activity from Stars to Planets
We work on the physics of stars and stellar systems, the understanding of their magnetic dynamos, and the search for extrasolar planets.We observe the stars and their planets using astronomical telescopes around the world, and we are developing instrumentation for high-precision spectroscopy.
Our projects are funded by the European Research Council (ERC), Deutsche Forschungsgemeinschaft (DFG), Bundesministerium für Bildung und Forschung (BMBF), and Bundesministerium für Wirtschaft und Technologie (BMWi).
Stars, Brown Dwarfs and Planets
Our Galaxy is made of stars, one of them is the Sun that comfortably heats our planet Earth. Billions of other stars are known, most of them less massive than our Sun, and a few hundred planets and planetary systems around other stars are known today. The majority of stars in our Galaxy are less than half as massive and half as large as the Sun. Planets, on the other hand, are at least 100 times lighter than the Sun. Between stars and planets, another group exists - objects not hot enough to burn hydrogen into helium, but formed like stars. These objects, called brown dwarfs, do not reach a stable state. They become cooler during their entire lifetime, which by far exceeds the age of the Universe.Magnetic Activity
The Sun exhibits an 11-year cycle of activity during which dark spots, flares, faculae, and other transient events occur. These events are of magnetic nature, the magnetic field being induced by the vast amount of moving plasma that act as a gigantic dynamo. Magnetic activity also occurs on other stars, often much stronger, manifested as huge spots, massive energy release, and very strong magnetic fields. The physical mechanisms behind magnetic activity in stars and brown dwarfs, and the influence of planets are not well understood, not even in the Sun.Solar, stellar and substellar magnetic activity are manifestations of the same processes occurring in physically related objects that possess very different atmospheres and surface structure. We investigate the physical principles of magnetic activity carrying out high precision spectroscopic experiments in stars and brown dwarfs; we investigate their atmospheres, magnetic fields, surface properties, and search for immediate drivers of magnetic activity. The main tools for our observations are large ground-based telescopes like the Very Large Telescope (VLT), Keck observatory, the Hobby Eberly Telescope, and others. We are also analyzing high precisio photometric data from satellite missions, for example data for some 100,000 sun-like stars from the Kepler mission.
The Search for Extrasolar Planets
The detection of Earth-like planets in stellar habitable zones is one of the principal goals of exoplanet research. After more than than 20 years of exoplanet hunting, it is well known that planets outside our solar system can be detected, the sample has grown to a size that allows statistical statements about the frequency of planets, and future instrumentation will allow their characterization.
The last years have seen tremendous progress not only in the rapidly growing amount of discovered planets, but also in the speed at which the roadmap towards the detection of Earth-like planets is evolving. Ground and space-based photometric missions are surveying different parts of the sky with the goal to discover transiting exoplanets, and instrumentation allowing to measure radial velocities suitable to find Earth-like planets already exists. For the future, missions are under investigation that should search for transiting planet candidates around nearby stars in a wider region across the sky and the next generation of ground- and space-based telescopes (e.g., E-ELT, JWST) will provide the opportunity to study the atmospheres of a few of the most interesting planets.In our group, we search for extrasolar planets and work on methods to find them using the radial velocity technique. One of our main areas is the CARMENES project that will survey a sample of 300 low-mass stars to find habitable Super-Earths and provide a detailed picture of the host stars.
Instrumentation Projects
Our optical lab we carry out spectroscopic experiments and develop new instrumentation for the search for extrasolar planets. We are operating a Fourier Transform Spectrograph Bruker IFS 125HR. The spectrograph has a maximum optical path difference of 208cm and operates between 400nm and 15μm. It can be fed with external light through a focussed beam and through a parallel beam that we equipped with a fibre feed.
Our Fourier Transform Spectrograph, a Bruker HR125.
Our institute is also operating a 50cm siderostat on the roof of the building. Light from the siderostat can be fed into the optical laboratory. The combination of a siderostat and a FTS in our optical laboratory allows us to obtain high-quality spectra from the Sun, Earth's atmosphere, and bright stars at our institute. This allows a variety of possible science programs. One important program is the development of frequency standards for the search for extrasolar planets. For this program, we use the FTS to characterize the light from wavelength standards like hollow cathode lamps, electrodeless microwave discharge lamps, etalons, gas absorption cells, etc. Fed by the siderostat, the FTS can provide high-quality spectra of the spatially resolved Sun, which allows investigation of the solar limb, its centre, or sunspots.
The Laboratory
This image shows a schematic setup of our laboratories.
Line Profiles, spots, and rotation
Here are some links to javascript visualizations of line profiles (and their Fourier transform) in stars: