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Zerstörungsfreie Tomographie von Raumladungs- und Polarisationsverteilungen mittles Wärmepulsen
(2007)
Non-destructive, three-dimensional imaging of space-charge and polarization distributions in electret materials has been implemented by means of laser-induced thermal pulses. In pyroelectric films of poled poly(vinylidene fluoride), images of up to 45 x 45 pixels with a depth resolution of less than 0.5 mu m and a lateral resolution of 40 mu m were recorded, the latter being limited by fast thermal diffusion in the absorbing metallic front electrode. Initial applications include the analysis of polarization distributions in corona-poled piezoelectric sensor cables and the detection of patterned space-charge distributions in polytetrafluoroethylene films.
X-ray spectroscopy is a sensitive probe of stellar winds. X-rays originate from optically thin shock-heated plasma deep inside the wind and propagate outwards throughout absorbing cool material. Recent analyses of the line ratios from He-like ions in the X-ray spectra of O-stars highlighted problems with this general paradigm: the measured line ratios of highest ions are consistent with the location of the hottest X-ray emitting plasma very close to the base of the wind, perhaps indicating the presence of a corona, while measurements from lower ions conform with the wind-embedded shock model. Generally, to correctly model the emerging Xray spectra, a detailed knowledge of the cool wind opacities based on stellar atmosphere models is prerequisite. A nearly grey stellar wind opacity for the X-rays is deduced from the analyses of high-resolution X-ray spectra. This indicates that the stellar winds are strongly clumped. Furthermore, the nearly symmetric shape of X-ray emission line profiles can be explained if the wind clumps are radially compressed. In massive binaries the orbital variations of X-ray emission allow to probe the opacity of the stellar wind; results support the picture of strong wind clumping. In high-mass X-ray binaries, the stochastic X-ray variability and the extend of the stellar-wind part photoionized by X-rays provide further strong evidence that stellar winds consist of dense clumps.
Giacconi et al. (1962) discovered a diffuse cosmic X-ray background with rocket experiments when they searched for lunar X-ray emission. Later satellite missions found a spectral peak in the cosmic X-ray background at ~30 keV. Imaging X-ray satellites such as ROSAT (1990-1999) were able to resolve up to 80% of the background below 2 keV into single point sources, mainly active galaxies. The cosmic X-ray background is the integration of all accreting super-massive (several million solar masses) black holes in the centre of active galaxies over cosmic time. Synthesis models need further populations of X-ray absorbed active galaxy nuclei (AGN) in order to explain the cosmic X-ray background peak at ~30 keV. Current X-ray missions such as XMM-Newton and Chandra offer the possibility of studying these additional populations. This Ph.D. thesis studies the populations that dominate the X-ray sky. For this purpose the 120 ksec XMM-Newton Marano field survey, named for an earlier optical quasar survey in the southern hemisphere, is analysed. Based on the optical follow-up observations the X-ray sources are spectroscopically classified. Optical and X-ray properties of the different X-ray source populations are studied and differences are derived. The amount of absorption in the X-ray spectra of type II AGN, which are considered as a main contributor to the X-ray background at ~30 keV, is determined. In order to extend the sample size of the rare type II AGN, this study also includes objects from another survey, the XMM-Newton Serendipitous Medium Sample. In addition, the dependence of the absorption in type II AGN with redshift and X-ray luminosity is analysed. We detected 328 X-ray sources in the Marano field. 140 sources were spectroscopically classified. We found 89 type I AGN, 36 type II AGN, 6 galaxies, and 9 stars. AGN, galaxies, and stars are clearly distinguishable by their optical and X-ray properties. Type I and II AGN do not separate clearly. They have a significant overlap in all studied properties. In a few cases the X-ray properties are in contradiction to the observed optical properties for type I and type II AGN. For example we find type II AGN that show evidence for optical absorption but are not absorbed in X-rays. Based on the additional use of near infra-red imaging (K-band), we were able to identify several of the rare type II AGN. The X-ray spectra of type II AGN from the XMM-Newton Marano field survey and the XMM-Newton Serendipitous Medium Sample were analysed. Since most of the sources have only ~40 X-ray counts in the XMM-Newton PN-detector, I carefully studied the fit results of simulated X-ray spectra as a function of fit statistic and binning method. The objects revealed only moderate absorption. In particular, I do not find any Compton-thick sources (absorbed by column densities of NH > 1.5 x 10^24 cm^−2). This gives evidence that type II AGN are not the main contributor of the X-ray background around 30 keV. Although bias effects may occur, type II AGN show no noticeable trend of the amount of absorption with redshift or X-ray luminosity.
In der Nanotechnologie und der molekularen Biologie werden immer kleinere Strukturelemente, wie beispielsweise einzelne Atomlagen oder Molekülgruppen, manipuliert, um bestimmte Funktionen zu erzielen. Veränderungen in solchen Systemen laufen auf atomarer Längen- und Zeitskala ab. Für das physikalische Verständnis dieser ultraschnellen Prozesse ist ein anschauliches Bild wichtig. Dank ihrer hohen Struktur- und Zeitauflösung liefert die Femtosekunden-Röntgenbeugung Bildsequenzen atomarer Bewegung von Molekülen und Festkörpern und ermöglicht somit Rückschlüsse über die komplexe Wechselwirkung zwischen Elektronen- und Kernbewegungen. Die aktuellen und zukünftigen Möglichkeiten, Atomen bei ihren Bewegungen zuzusehen, diskutiert der Referent an aktuellen Beispielen.
Proteins are chain molecules built from amino acids. The precise sequence of the 20 different types of amino acids in a protein chain defines into which structure a protein folds, and the three-dimensional structure in turn specifies the biological function of the protein. The reliable folding of proteins is a prerequisite for their robust function. Misfolding can lead to protein aggregates that cause severe diseases, such as Alzheimer's, Parkinson's, or the variant Creutzfeldt-Jakob disease. Small single-domain proteins often fold without experimentally detectable metastable intermediate states. The folding dynamics of these proteins is thought to be governed by a single transition-state barrier between the unfolded and the folded state. The transition state is highly instable and cannot be observed directly. However, mutations in which a single amino acid of the protein is substituted by another one can provide indirect access. The mutations slightly change the transition-state barrier and, thus, the folding and unfolding times of the protein. The central question is how to reconstruct the transition state from the observed changes in folding times. In this habilitation thesis, a novel method to extract structural information on transition states from mutational data is presented. The method is based on (i) the cooperativity of structural elements such as alpha-helices and beta-hairpins, and (ii) on splitting up mutation-induced free-energy changes into components for these elements. By fitting few parameters, the method reveals the degree of structure formation of alpha-helices and beta-hairpins in the transition state. In addition, it is shown in this thesis that the folding routes of small single-domain proteins are dominated by loop-closure dependencies between the structural elements.
In der vorliegenden Arbeit werden Methoden der Erdsystemanalyse auf die Untersuchung der Habitabilität terrestrischer Exoplaneten angewandt. Mit Hilfe eines parametrisierten Konvektionsmodells für die Erde wird die thermische Evolution von terrestrischen Planeten berechnet. Bei zunehmender Leuchtkraft des Zentralsterns wird über den globalen Karbonat-Silikat-Kreislauf das planetare Klima stabilisiert. Für eine photosynthetisch-aktive Biosphäre, die in einem bestimmten Temperaturbereich bei hinreichender CO2-Konzentration existieren kann, wird eine Überlebenspanne abgeschätzt. Der Abstandsbereich um einen Stern, in dem eine solche Biosphäre produktiv ist, wird als photosynthetisch-aktive habitable Zone (pHZ) definiert und berechnet. Der Zeitpunkt, zu dem die pHZ in einem extrasolaren Planetensystem endgültig verschwindet, ist die maximale Lebenspanne der Biosphäre. Für Supererden, massereiche terrestrische Planeten, ist sie umso länger, je massereicher der Planet ist und umso kürzer, je mehr er mit Kontinenten bedeckt ist. Für Supererden, die keine ausgeprägten Wasser- oder Landwelten sind, skaliert die maximale Lebenspanne mit der Planetenmasse mit einem Exponenten von 0,14. Um K- und M-Sterne ist die Überlebensspanne einer Biosphäre auf einem Planeten immer durch die maximale Lebensspanne bestimmt und nicht durch das Ende der Hauptreihenentwicklung des Zentralsterns limitiert. Das pHZ-Konzept wird auf das extrasolare Planetensystem Gliese 581 angewandt. Danach könnte die 8-Erdmassen-Supererde Gliese 581d habitabel sein. Basierend auf dem vorgestellten pHZ-Konzept wird erstmals die von Ward und Brownlee 1999 aufgestellte Rare-Earth-Hypothese für die Milchstraße quantifiziert. Diese Hypothese besagt, dass komplexes Leben im Universum vermutlich sehr selten ist, wohingegen primitives Leben weit verbreitet sein könnte. Unterschiedliche Temperatur- und CO2-Toleranzen sowie ein unterschiedlicher Einfluss auf die Verwitterung für komplexe und primitive Lebensformen führt zu unterschiedlichen Grenzen der pHZ und zu einer unterschiedlichen Abschätzung für die Anzahl der Planeten, die mit den entsprechenden Lebensformen besiedelt sein könnten. Dabei ergibt sich, dass komplex besiedelte Planeten heute etwa 100-mal seltener sein müssten als primitiv besiedelte.