Fig.1: The Very Large Telescope at the European Southern Observatory, Paranal, Chile
Progress in astronomy and astrophysics is tied intimately to the capability of the instruments. While larger and larger telescopes permit observations of fainter objects and space based observatories open new observing capabilities, the instrumentation for these telescopes has to keep up with the new possibilities and adapt to scientific requirements defined by the astronomical community. An active participation in this process has a long tradition in our group, namely with instruments for the Very Large Telescope of the European Southern Observatory at Paranal in Chile (Fig. 1). The current instrumentation project (MUSE) is being developed and built as part of an international collaboration partly funded by the BMBF: a spectrograph that will provide optical spectra not only for single point sources but rather for a complete field of view of one square arcminute with a spatial resolution of 0.3 arcseconds (Fig. 2). While application for this instrument range from cosmologic to planetary science cases, the stellar astrophysics group in Göttingen is especially interested in using this instrument for stellar objects in the Milky Way and in nearby galaxies. the project passed the design phase successfully in 2009 and should start operation in 2012.
Fig.2: Computer Model of the Multi Unit Spectroscopic Explorer (MUSE) on the Nasmyth platform of the Unit 4 Telescope of the European Southern Observatory
Fig. 3: MONET Souh, a 1.2m robotic telescope operated by the stellar astrophysics group in Göttingen along with the South African Astronomical Observatory with the Milky Way in the background. (Picture: S.Potter/SAAO)
While observing time at large international observatories is very limited, long term projects are ideally conducted at dedicated telescopes. With funds from the Alfried Krupp von Bohlen und Halbach foundation, two robotic telescopes (MONET) have been built, one at McDonald Observatory in Texas, one at the South African Astronomical Observatory (Fig. 3). These telescopes can be operated remotely from Göttingen and allow long term monitoring projects, e.g. the search for extrasolar planets (see below). The remote observations make the MONET telescopes also ideal for educational purpose, half of the observing time is devoted for school use, providing access to professional astronomical equipment for school classes all over the world.
The stellar astrophysics group concentrates on extrasolar planets, low mass and solar-type stars as well as on late stages of stellar evolution. With the discovery of the first planet orbiting a star other than the sun in 1995 and the subsequent detection of more than 300 extrasolar planets, aspects of planet formation, evolution, and interaction with the host star have become key scientific topics. Our group participates in the direct search of extrasolar planets with various techniques. These techniques include gravitational lensing, a technique providing an unbiased search over a large range in our Milky Way, detection of radial velocity variations where we concentrate on very low mass planet host stars, and detection of transits, i.e. a partial eclipse of the planet host star by its planet. The latter method is especially interesting since a detailed photometric (Fig. 4) and spectroscopic analysis of the transit allows one to derive important information about the planet, e.g. its radius, mass, mean density, alignment of the stellar rotation axis with the planet's orbital axis, or the properties of the planetary atmosphere. These activities are closely linked to the emmy noether research group of Ansgar Reiners and are partially embedded in a DFG-funded Graduate Research Centre, “Extrasolar Planets and their Host Stars”, a collaborative project together with Hamburg Observatory and the Max Planck Institute for Solar System Research. While stellar evolution is quite well understood in general, specific questions are still open. An important aspect is the evolution of close binaries. in their late stage of evolution, the primary star turns into a compact object: a white dwarf, neutron star or stellar black hole. In very close binary systems, mass transfer from the secondary to the primary leads to accretion of the material on the compact object, resulting in intensive high energy radiation. In our group, the X-ray emission from accretion processes are used to probe these accretion processes. A special class of stars, the subluminous O and B stars are likely the product of close binary evolution. Their interior is explored through the analysis of their oscillations (Fig. 5). Recently, variations of these stellar oscillations led to the discovery of a planet orbiting such a star, casting light on the fate of planetary systems at the end of a star’s life.
Fig.4: Observed transit light curve for GJ 436b (points) with best fit from our analysis (line) phased to the central transit time. The bar on the right indicates the average error size. Bottom: residuals from the fit (points). The different point styles in both panels indicate data from the different visits. (Bean et al. 2008, A&A 486, 1039)
Fig.5: Discrete Fourier transform of the light curve of HS0702+6043 showing short period p-mode pulsations slightly below 3,000 µHertz and long period g-mode pulsations at about 300 µHertz. (Schuh et al. A&A 445, L31)
Most of the information of the physical condition of astronomical objects is obtained from the analysis of the emitted light. The derivation of temperature and density stratifications, chemical composition, velocity or magnetic fields require a comparison of simulated and observed spectra. These numerical simulations have to keep up with the advances in observing techniques. The steady development of sophisticated radiative transfer simulations is therefore an important tool in stellar astrophysics. A recent development of our group is the possibility to simulate spectra of the protoplanetary disks (Fig 6). Planets may be forming in such environments, so the derivation of the conditions in such disks is an important way of probing planet formation processes.
Fig.6: Infrared spectrum of the protoplanetary disk of GQ Lupi compared to a synthetic spectrum. (Hügelmeyer et al, A&A 498, 793)
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 Schuh, S., Huber, J., Dreizler, S. et al., 2006: HS 0702+6043: a star showing both short-period p-mode and long-period g-mode oscillations, Astronomy and Astrophysics, 445, L31
 Dreizler, S., Rauch, T., Hauschildt, P. H. et al., 2002: Spectral types of planetary host star candidates: Two new transiting planets, Astronomy and Astrophysics, 391, L17
 Werner, K., Dreizler, S. 1999: The classical stellar atmosphere problem, Journal of Computational and Applied Mathematics, 109, 65
 Dreizler, S., Heber, U. 1998: Spectral analyses of PG 1159 star: constraints on the GW Virginis pulsations from HST observations, Astronomy and Astrophysics, 334, 618
 Dreizler, S., Werner, K. 1996: Spectral analysis of hot helium-rich white dwarfs, Astronomy and Astrophysics, 314, 217