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  • 523  (29)
  • Univ. Köln  (21)
  • Univ. Freiburg (Breisgau)  (8)
  • English  (29)
  • Russian
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  • 2010-2014  (29)
  • 1
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    Properties of quasistellar objects and their progenitors in the radio and mm wavelength regimes in the context of ULIRG-to-QSO evolution (2010)
    Univ. Köln
    Details
    Publication Date: 2020-10-07
    Description: The thesis at hand presents the results of a study of galaxies with active galactic nuclei (AGN) carried out in the radio and mm wavelength regimes at low and high angular resolutions. The studied objects cover a broad range of mechanisms responsible for powering the engines of the respective galaxies: ULIRGs (UltraLuminous Infrared Galaxies), a product of mergers between galaxies that are supposedly powered by starbursts, AGN or a mixture of the two, and QSOs (quasi-stellar objects) which are powered by AGN. Since ULIRGs and QSOs show similar properties (especially the infrared luminosities), it was proposed that they form the early and late phase of an evolutionary sequence where supposedly ULIRGs are the dust enshrouded progenitors of QSOs. As a prototypical ULIRG, Arp 220 represents an early-to-intermediate phase object in the scheme of the ULIRG-to-QSO evolution. For this object observations at multiple mm wavelengths and angular resolutions with the IRAM Plateau de Bure Interferometer (PdBI) are presented and analyzed. Arp 220 as a whole is studied in CO emission of the more extended structure. Furthermore, Arp 220-East is analyzed in more detail in the CO gas. Indications for emission ~10" towards the south, as well as to the north and to the west of the two nuclei, were found in the low resolution CO(1-0) maps ...
    Description: Zsfassung in dt. und engl. Sprache
    Description: thesis
    Keywords: 523 ; TBC 000 ; TIE 900 ; Radioastronomie ; Sonstige Sternensysteme {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 152 S.
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  • 2
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    Multi-telescope studies of neutron stars (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: Neutron stars are unique stellar remnants with extreme properties, as their density and magnetic field. Their study can be the key to a number of unanswered problems in fundamental physics and astronomy, ranging from stellar evolution to strong field gravity. One of the best ways of studying these objects is with observations at radio wavelengths, the efficiency of which can be vastly improved with the combination of data from multiple radiotelescopes. In this thesis, we use the largest European radiotelescopes for performing high quality studies of the properties of objects belonging into two separate categories of neutron stars, millisecond pulsars and magnetars. In the first part of this thesis, a complete description of the observing systems and calibration procedures for the multiple telescopes used is presented. Specifically, all observations were made with the European Pulsar Timing Array (EPTA) telescopes, which are the Effelsberg 100m radiotelescope in Germany, the Lovell 76m radiotelescope in UK, the Westerbork 94m equivalent synthesis radiotelescope in the Netherlands and the Nanc cay 94m equivalent decimetric radiotelescope in France...
    Description: Zsfassung in dt. und engl. Sprache
    Description: thesis
    Keywords: 523 ; THU 400 ; THK 000 ; TBC 000 ; Pulsare, Neutronensterne {Astronomie} ; Spektren und Spektroskopie von Sternen {Astronomie} ; Radioastronomie
    Language: English
    Type: monograph , publishedVersion
    Format: 152 S.
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  • 3
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    Heating of the corona in a 3D MHD forward model approach (2010)
    Univ. Freiburg (Breisgau)
    Details
    Publication Date: 2021-03-29
    Description: Observations of the solar corona show loop-like structures formed by plasma at temperatures of one million degrees and higher. Since the solar surface is much cooler than the corona, a heating mechanism must be responsible for the high temperatures. The dissipation of magnetic fields in the corona could provide such a heating mechanism. However, the process of transforming magnetic energy into thermal energy is still not yet understood in detail. To investigate this process and its impact on the heating of the corona, we employ a three-dimensional magneto-hydrodynamical model. This numerical model synthesizes the temporal evolution of the magnetic field above an Active Region. It includes the solar atmosphere from the photosphere up to the corona. The magnetic field in the corona is braided by foot point motions in the photosphere. This is done imilarly to the braiding through granulation in reality. The stressed agnetic field induces currents which are then dissipated in the corona. This dea is known as the DC model (direct current) and was proposed by Parker in 1972. The model reaches a quasi-stationary state, i.e. the energy input by hotospheric motions is counterbalanced by radiative losses in the optically thin corona. As a result, the described heating process creates and sustains a hot corona with a temperature of one million degrees and higher ...
    Description: thesis
    Keywords: 523 ; TGC 765 ; TGC 800 ; Korona {Astronomie: Sonne} ; Magnetfelder {Astronomie: Sonne}
    Language: English
    Type: monograph , publishedVersion
    Format: 108 S.
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  • 4
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    The fate of singularities in quantum cosmology and the application of generalized effective equations to constrained quantum systems (2010)
    Univ. Köln
    Details
    Publication Date: 2020-10-07
    Description: The first part of the thesis is concerned with the fate of singularities in quantum cosmology. The second part addresses the derivation of predictions from quantum cosmology. In the first part, two classes of cosmological models were studied. In the first class of models, the universe evolves to or emerges from a big-rip singularity. Here, energy density, pressure and scale factor diverge after a finite amount of time. This type of singularity arises rather generically in cosmological models with phantom dark energy. For each of these phantom-field models, the corresponding scenario with ordinary scalar field was studied. The scalar field induced a big-bang singularity. The second class of models studied was dominated by a big-brake singularity. At the big brake, the universe evolution comes to a halt due to an infinite deceleration. The motivation behind this choice of models was the occurrence of a singularity at large scale factor. The major question pursued was whether these types of singularity were resolved on the quantum level. If such singularities were resolved in quantum cosmology, this would imply that quantum gravitational effects can occur in the macroscopic universe. After devising classical models that contain the respective singularity, I subjected these models to quantization which was carried out in the geometrodynamical approach. The governing equation is then the WheelerDeWitt equation ...
    Description: Zsfassung in dt. und engl. Sprache
    Description: thesis
    Keywords: 523 ; TEC 800 ; Quantenkosmologie {Astronomie: Kosmologie}
    Language: English
    Type: monograph , publishedVersion
    Format: 228 S.
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  • 5
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    Searching for solar axions in the eV-mass region with the CCD detector at CAST (2010)
    Univ. Freiburg (Breisgau)
    Details
    Publication Date: 2021-03-29
    Description: The CERN Axion Solar Telescope (CAST) is searching for solar axions, which could be produced in the core of the Sun via the so-called Primakoff effect. For this purpose, CAST uses a decommissioned LHC prototype magnet. In its magnetic field of 9 Tesla axions could be reconverted into X-ray photons. The magnet is mounted on a structure built to follow the Sun during sunrise and sunset for a total of about 3 hours per day. The analysis of the data acquired during the first phase of the experiment with vacuum in the magnetic field region yielded the best experimental upper limit on the axion-to-photon coupling constant for axion masses up to about 0.02 eV. In order to extend the sensitivity of the experiment to a wider mass range, the CAST experiment continued its search for axions with helium in the magnet bores. In this way it is possible to restore coherence for larger masses. Changing the pressure of the helium gas enables the experiment to scan different axion masses. In the first part of this second phase of CAST, helium-4 has been used and the axion mass region was extended up to 0.4 eV ...
    Description: thesis
    Keywords: 523 ; TII 100 ; TCE 380 ; Dunkle Materie {Astronomie: Interstellare Materie} ; CCD, Photodetektoren {Astronomische Instrumente}
    Language: English
    Type: monograph , publishedVersion
    Format: 266 S.
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  • 6
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    Star-disc encounters in young star clusters (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: According to current knowledge, star formation occurs preferentially in clustered environments. As a byproduct of the star formation process young stars are found to be surrounded by accretion discs that are potential birth places of planets and planetary systems. Located in the hosting cluster, these protoplanetary discs are potentially subject to stellar interactions. These interactions give rise to a fundamental question of clustered star formation: How far does the cluster environment affect the evolution of protoplanetary discs and the formation of planets? The question is addressed in the present investigation in terms of the effect of stellar encounters on stars and their disc in young clusters by combining numerical simulations of isolated star-disc encounters and stellar dynamics of young star clusters. The investigation is composed of three key aspects. First, simulations of a dynamical model of the Orion Nebula Cluster (ONC) show that disc destruction is dominated by encounters with high-mass stars that act as gravitational foci for the lower mass stars in the cluster centre. The massive stars are thus subject to repeated encounters which can lead to a total disc destruction ...
    Description: thesis
    Keywords: 523 ; THX 000 ; TIC 000 ; Extrasolare Planetensysteme {Astronomie: Sterne} ; Sternhaufen {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 137 S.
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  • 7
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    Magnetic coupling of the solar photosphere and chromosphere (2010)
    Univ. Freiburg (Breisgau)
    Details
    Publication Date: 2021-03-29
    Description: The solar surface outside sunspots and active regions, i.e., the quiet Sun, shows the ubiquitous pattern of granulation in the photosphere. The quiet solar photosphere harbors small-scale magnetic structures inside and between granules. This thesis presents thermodynamic properties of the small-scale magnetic flux concentrations in the quiet Sun using high spatial and temporal resolution observations along with numerical simulations. Spectral line profiles of the Fe I 630 nm pair and Ca II H were used to trace the photospheric and chromospheric layers of the magnetic elements. In the presence of magnetic field spectral lines split and are polarized via the Zeeman effect. The difference of a spectral line profile, measured in left and right circular polarized light, is a Stokes-V profile with two lobes. In the absence of any gradients of velocity or magnetic field along the line of sight, Stokes-V profiles are anti-symmetric. The different area of the two lobes, the Stokes-V area asymmetry, provides information about the gradients of the magnetic and velocity fields along the line of sight. Comparing high resolution spectropolarimetric data with synthetic maps of a 3D MHD simulation, we found several magnetic elements in the photosphere showing a central region of negative Stokes-V area asymmetry surrounded by a peripheral region with larger positive asymmetry ...
    Description: thesis
    Keywords: 523 ; TGC 745 ; TGC 755 ; TGC 800 ; Photosphäre {Astronomie: Sonne} ; Chromosphäre {Astronomie: Sonne} ; Magnetfelder {Astronomie: Sonne}
    Language: English
    Type: monograph , publishedVersion
    Format: 146 S.
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  • 8
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    Periodic variability in active galactic nuclei (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: In this doctoral thesis, multi-frequency very long baseline interferometry observations together with multi-frequency total flux-density variability data of compact relativistic jets in active galactic nuclei are presented and analyzed. The main goal of the thesis is to investigate the physical mechanisms in relativistic jets responsible for such phenomena as the co-existence of moving and stationary jet components, jet wiggling and precession. We also aim to study the connection between the structural changes in the relativistic jets and flares in the total flux-density light curves and to find observational evidences for the appearance of a primary perturbation in the base of the jet and its further propagation. In this thesis we also investigate which physical mechanisms are responsible for periodical total flux-density variability and to search for periodicities as a sign of jet precession. In order to study the jet physics we used the multi-frequency very long baseline interferometry technique which gives the highest possible in astronomy resolution. We also compared jet structural changes with single-dish multi-frequency observations spanning more than 30 years together with optical and gamma-ray data. In particular, analysis of the long-term kinematics of two active galactic nuclei S5 1803+784 and 0605-085 shows evidence for jet precession ...
    Description: thesis
    Keywords: 523 ; TBC 000 ; TIE 500 ; Radioastronomie ; Aufbau und Struktur von Sternsystemen {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 201 S.
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  • 9
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    Star formation conditions in nearby galaxies (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: The understanding of gravitational stability of galactic disks against star formation, the interplay of atomic and molecular components of the interstellar medium in a galaxy and its physical conditions, namely temperature and density, are important to gain a deeper insight into star formation. Nearby galaxies like the spiral galaxy M51 or the southern starburst galaxies NGC4945 and Circinus offer excellent opportunities to investigate these topics over a complete galactic disk or parts of the galaxy. A suitable tracer to study the molecular part of the gaseous component of galaxies is the CO molecule. It is the second most abundant molecule after H2 in the interstellar medium, its lower lying transition are excited at typical conditions of molecular clouds and the rotational transitions lie in the radio window. Thus, CO emission can penetrate throughout galactic disks and is observable from the ground. Apart from complementary data from the literature, we use observations of 12CO 21, 12CO 4--3, and 13CO 2--1 in this thesis. Additionally, the [CI] 3P1-3P0 fine structure line was observed to study the physical conditions in the central regions of galaxies. In the first part of this thesis, the nearby spiral galaxy M51 is used as a sample source to investigate the onset of star formation. CO, HI, radio-continuum, dust, and stellar maps of M51 are combined to study star formation rate, the H2/HI transition, the gas-to-dust ratios, the stability of the disk against gravitational collapse, and properties of the giant molecular clouds. Parts of the results have been published in Astronomy and Astrophysics (461,143) in 2007 and are submitted to Astronomy and Astrophysics, 2008 for publication. We use the first complete 12CO 2--1 map of M51 including the companion galaxy NGC5195 observed with HERA at the IRAM-30m telescope to trace the molecular gas. Complementary HI VLA data is used to study the atomic gas distribution and the combined total gas surface density ...
    Description: thesis
    Keywords: 523 ; TEG 000 ; Kosmogonie {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 180 S.
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  • 10
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    LINC-NIRVANA piston control and near-infrared polarization images of the Orion proplyds (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: This thesis is focussed on the development of optimized techniques to overcome limitations of astrophysical observations. The goal is an optimal signal estimation in noisy measurements by the consideration of underlying physical processes. This principle was applied to two different fields in astrophysics: intrumental design and analysis of polarimetric observations. In the observational part of this thesis near-infrared images of young stellar objects in the Orion constellation are studied. Limitations in resolution and sensitivity of current astronomical instruments prohibit the detailed analysis of interesting proto-stellar sources to improve theories of star formation. Radiation from the astronomical targets is not only characterized by its spectral energy, but also by polarization properties. The modeling of typical configurations of star-disk systems and the simulation of their polarization patterns helped to understand and interprete features, that were found in observations. For the case of a proto-stellar systems with both a disk and an envelope analysis techniques were developed, which are based on polarimetric effects of the scattering of light by dust. These techniques substantially improve the sensitivity and resolution and are reliable under different observing conditions. Although the obtained data did not allow investigations of substructures of the circumstellar material, the techniques are suitable to obtain constraints on star formation processes ...
    Description: thesis
    Keywords: 523 ; THK 000 ; Spektren und Spektroskopie von Sternen {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 168 S.
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  • 11
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    Studying martian dust devils by applying pattern recognition algorithms to multi-mission camera images (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: A pattern recognition and classification software was developed to detect dust devils automatically in surface images from Mars. The amount of images taken by spacecraft orbiting Mars is increasing continuously and the expenditure of time is too high to search every image for spatially and temporally highly variable features like dust devils. The pattern recognition method was therefore used to conduct a completely new kind of search for dust devils. Images from the three different Mars missions Viking, Mars Global Surveyor and Mars Express can be processed and for the first time automatically scanned for the desired objects. Viking images including dust devils were used as the database to filter unique dust devil features and the derived parameters built the feature vector. Various Classification methods have been tested resulting in a two-layer perceptron (neural network) as the best classifier. Necessary adjustments and increments complete the software so that it can be applied to Mars Global Surveyor Mars Orbiter Camera (MOC), Mars Express High Resolution Stereo Camera (HRSC) and probably coming images from future missions. It was shown that the standard dust devil is filtered and correctly classified. The two main features, the bright spot representing the dust column and the shadow, must be filterable from the background. Crater rims and hills are the most false-positive objects...
    Description: thesis
    Keywords: 523 ; TGG 545 ; TGG 515 ; TCM 000 ; TCE 380 ; Marsoberfläche {Astronomie} ; Marsbeobachtung, Raumflüge zum Mars {Astronomie} ; Mathematische und EDV-Verfahren {Astronomie} ; CCD, Photodetektoren {Astronomische Instrumente}
    Language: English
    Type: monograph , publishedVersion
    Format: 158 S.
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  • 12
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    Dissecting the host galaxies of active galactic nuclei at high angular resolution (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: The work presented here is concerned with issues related to the interplay of galaxy structure and the accretion phenomenon onto supermassive black holes (SMBH), i.e. active galactic nuclei (AGN). There is statistical evidence for a coevolution of SMBHs and the galaxy bulges they reside in. The detailed knowledge of the involved physical processes, however, is still far from being satisfactory. In order to test and improve theoretical models of how this coevolution is arising, high angular resolution observations of the host galaxies of AGN are necessary. Imaging and spectroscopy of the innermost region of galaxies have the power to reveal information on the structure, the chemical composition, and the dynamics of stars and gas in the presence of an SMBH, and allow us to separate energetic signatures related either to star formation or to the accretion onto the SMBH. Considering such observations over cosmological distances (redshifts) -- i.e. looking into the past because of the finite light-travel time -- we are also able to assess evolutionary effects on the beforementioned properties ...
    Description: thesis
    Keywords: 523 ; TIE 400 ; Spektren und Leuchtkraft von Sternsystemen {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 174 S.
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  • 13
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    Cophasing LINC-NIRVANA and molecular gas in low-luminosity QSO host and cluster galaxies (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: This thesis consists of two parts. The first part reports on the development of a fringe tracking system for LINC-NIRVANA, the near-infrared homothetic imaging camera for the Large Binocular Telescope (LBT). LINC-NIRVANA allows for a coherent combination of the light entering the two primary mirrors. The fringe pattern in the focal plane is required to be stable in time to preserve the high spatial frequency information in long exposure images. Atmospheric differential piston causes the position of the fringes to change with time. A fringe tracker is a servo system that measures and corrects for atmospheric differential piston in real-time. In the first part, numerical simulations of atmospheric turbulence are used to investigate the problem of angular anisoplanatism of differential piston. The fringe tracking concept and the top-level requirements are introduced, followed by a presentation of the mechanical design of the system. This design includes a solution for a device that is capable of positioning a detector with high precision within a cryogenic environment. A fitting algorithm determines the amount of differential piston in the point-spread function of a reference target. The performance of this algorithm under different signal-to-noise conditions is investigated. First polychromatic fringes obtained with a testbed interferometer conclude the first part ...
    Description: thesis
    Keywords: 523 ; TII 500 ; Emissionsnebel {Astronomie: Interstellare Materie}
    Language: English
    Type: monograph , publishedVersion
    Format: 186 S.
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  • 14
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    High-resolution observations of the solar photosphere and chromosphere (2010)
    Univ. Freiburg (Breisgau)
    Details
    Publication Date: 2021-03-29
    Description: Observations of the sun are almost always impaired by the turbulent motion of air in Earth's atmosphere. The turbulence would limit the theoretical resolution of modern large telescopes to that of amateur telescopes without additional tools.Today however, high-resolution data of the Sun are necessary to invesitgate its small-scale structure. This structure is likely to be connected to the radially outward increasing temparature distribution of the solar atmosphere. An introduction into further details of this topic that has also been the motivation for this work is presented in Chapt. 1. A theory of atmospheric turbulence that builds the basis for several results of this work is described in Chapt. 2. Here, two modern tools to enhance the resolution of groundbased observations are reviewed, on the one hand adaptive optics (AO) systems and on the other hand speckle interferometry. Until recently, these two techniques were only used separately. In Chapt. 3 the necessary modifications for analytical models of transfer functions are developed that include the changes made by an AO system to the incoming wave front, thus making a combination of AO systems and speckle interferometry possible ...
    Description: thesis
    Keywords: 523 ; TGC 100 ; TGC 740 ; Sonnenbeobachtung {Astronomie} ; Sonnenatmosphäre
    Language: English
    Type: monograph , publishedVersion
    Format: 137 S.
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  • 15
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    Astrophysical phenomena related to supermassive black holes (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: All projects aim at pushing the limits of our knowledge about the interaction between a galaxy and a supermassive black hole (SMBH) at its center. The development of a new instrument can be as valuable as combining different datasets. I follow both approaches and developed projects which deal with new instrumentation and telescope technology, combine datasets from different wavelengths and resolutions, and incorporate recent theoretical models and predictions, which can be verified empirically. The first two of the six chapters compile astrophysical and technical background of the individual projects, which are presented in the following four chapters. While the first project (Chapter 3) deals with observations of the innermost parsec of our Galaxy, Chapter 4 presents data of the inner kpc of an active galaxy. The subjects of Chapters 5 and 6 are very luminous AGN/host systems, so-called QSOs. Whereas Chapter 5 presents global, spatially unresolved properties of SMBH/host systems, the radio jet, analyzed in the final Chapter 6, combines all size scales. It is investigated from close to its origin out to several kpc. The accretion onto the black hole of the Milky Way (Chapter 3) is extremely inefficient and the SMBH possibly interacts dominantly via tidal forces only. The next discussed system (Chapter 4) is the prototype of moderately luminous Seyfert 2 AGN, NGC 1068. Here a strong local influence of the nuclear X-ray radiation is observed. Chapter 5 deals with the possible global importance of radiative interaction between highly luminous QSO AGN and the host. The radio jet in Chapte 6 definitely shows signs of interaction with the matter of its host several kpc away from the nucleus. Chapters 3-6 include a dedicated introductory and a conclusive section, which put the results obtained in the larger astrophysical context of the observation ...
    Description: thesis
    Keywords: 523 ; TEG 440 ; TIE 662 ; Schwarze Löcher {Astronomie: Kosmogonie} ; Struktur der Milchstrasse {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 147 S.
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  • 16
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    High resolution spectroscopy of photospheric bright points (2010)
    Univ. Freiburg (Breisgau)
    Details
    Publication Date: 2021-03-29
    Description: Photospheric bright points are proxies for small-scale magnetic flux concentrations.These magnetic elements are thought to be the building blocks of the solar magnetic field and are important to understand many physical processes on the Sun, e.g. the variability of the solar irradiance.Because of their small size (100-200 km in diameter) the observations of photospheric bright points put special requirements on the seeing conditions, instrumental setup and post-processing techniques ...
    Description: thesis
    Keywords: 523 ; TGC 800 ; TGC 745 ; Magnetfelder {Astronomie: Sonne} ; Photosphäre {Astronomie: Sonne}
    Language: English
    Type: monograph , publishedVersion
    Format: 105 S.
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  • 17
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    Superconducting hot electron bolometers on silicon nitride membranes for terahertz waveguide mixers (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: The topic of this work is the development of Hot Electron Bolometer (HEB) mixers on freestanding membrane substrates for terahertz high resolution heterodyne receiver systems. The radio astronomy in the THz frequency spectrum is still in the beginning. At present much effort is put into the development of novel telescopes as well as in detector technologies. This work focuses on the development of THz waveguide detectors with superconducting Hot Electron Bolometer (HEB) mixers. These are primarily aimed for the use in the airborne Stratospheric Observatory For Infrared Astronomy (SOFIA) (1.9 THz) and the Atacama Pathfinder EXperiment (APEX) telescope (1.4 THz) in northern Chile ...
    Description: thesis
    Keywords: 523 ; TBC 000 ; Radioastronomie
    Language: English
    Type: monograph , publishedVersion
    Format: 169 S.
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  • 18
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    The H_1n+3 + H2 isotopic system (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: Dense cold molecular clouds reckoned to be stellar nurseries are the scene of an extreme molecular deuteration. Despite the cosmic D/H ratio of 10^{-5}, molecular species in prestellar cores are observed to contain nearly as much deuterium as hydrogen. This astonishing deuterium enrichment promoted by low temperatures is the work of H3+. It is the key species which unlocks the deuterium from its HD reservoir via reactions like H3+ + HD -〉 H2D+ + H_2 and drags it further to other species in successive reactions. For this reason, the H3+ + H2 isotopic system is outstandingly critical for the astrochemistry of cold environments. However, its understanding is yet incomplete and insufficient. This thesis thus focuses on the H3+ + H2 isotopic system from a theoretical, experimental and astronomical point of view giving a particular look into the role of nuclear spins...
    Description: thesis
    Keywords: 523 ; TJM 000 ; Kosmochemie {Weltraumforschung}
    Language: English
    Type: monograph , publishedVersion
    Format: 202 S.
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  • 19
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    Mass distribution at the galactic centre (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: In the framework of this PhD thesis, the study of the distribution of an extended mass close to the super-massive black hole (SMBH) at the centre of our Galaxy is addressed using observational data and theoretical modelling. The main emphasize is on establishing a distinction between the fraction of dark mass present in the form of a black hole and that in an extended form. Despite the significant observational and theoretical progress in the understanding of SMBHs in the last ten years, the formation of SMBHs and the interplay with their host galaxy are still poorly understood. The work presented here extends our understanding of the dynamics in the vicinity of the SMBH. Already in 1974, it was proposed that the radio source SgrA* could be a SMBH. With observations during the following years, it became more clear that the centre of our Galaxy hides an amount of dark mass close to 3 million solar masses. However, the strongest evidence for the existence of a SMBH, was only after it became possible to trace for the first time stellar orbits of fast moving stars, the so-called S-stars, around SgrA*. This was achievable using the SHARP near-infrared speckle camera at la Silla in Chile as well as the near-infrared camera NAOS/CONICA at the Very Large Telescope (VLT) at Paranal in Chile.
    Description: thesis
    Keywords: 523 ; TEG 000 ; TIC 000 ; TEG 440 ; Kosmogonie {Astronomie} ; Sternhaufen {Astronomie} ; Schwarze Löcher {Astronomie: Kosmogonie}
    Language: English
    Type: monograph , publishedVersion
    Format: 122 S.
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  • 20
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    Design and analysis of a broadband SIS-mixer for the Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: In this thesis the design and analysis of SIS-mixers (SIS: superconductor-isolator-superconductor) for the use as low-noise and broad-band detectors in the submm-region for astronomical observations is described. The SIS-mixers have been developed for the frequency band 2 (636-802 GHz) of the Heterodyne Instrument for the Far Infrared (HIFI) on ESA’s space-observatory Herschel. The required performance baseline has been defined by the HIFI-consortium in terms of the estimated noise temperature contribution of the mixer to be 110 K at 636 GHz and 150 K at 802 GHz. In the frequency region between 80 GHz and 900 GHz Heterodyne SIS mixers are established as the best devices for low noise mixing with a high spectral resolution (mixing of the signal radiation with local oscillator, LO). For the HIFI band 2 mixers, the technology of Nb/Al-Al2O3/Nb-junctions has been used with junction areas of 0.5-1.0 um2, a target value for the current density jc of 15 kA/cm2 and a gap-voltage of 2.75-2.77 mV. For the compensation of the SIS-junction’s intrinsic capacitance and the optimized power coupling to the tunnel junction, two types of matching circuits have been theoretically modelled and experimentally studied: (1) three-step transformer single junction devices and (2) double-junction devices. This has been done for two material combinations: for an all-superconductive micro strip NbTiN/SiO2/Nb, and for the superconductor/normal-conductor combination NbTiN/SiO2/Al...
    Description: thesis
    Keywords: 523 ; TBG 000 ; TCE 230 ; Infrarot-Astronomie ; Weltraumbasierte UV-, Optische und Infrarotteleskope {Astronomische Instrumente}
    Language: English
    Type: monograph , publishedVersion
    Format: 183 S.
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  • 21
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    The three-dimensional topology of magnetic fields in and around sunspots (2010)
    Univ. Freiburg (Breisgau)
    Details
    Publication Date: 2021-03-29
    Description: The most prominent features on the surface of the solar disc are the sunspots, which have been studied since their detection in the 17th century. Despite this years and centuries of scientific investigations, surprisingly many facets of sunspots are not well understood. In this thesis, the properties of a sunspots' penumbra are derived from the observed spectra by means of an inversion, a forward modeling technique. A method for the construction of a 3-dimensional model of the field topology from the inversion results is presented. It offers the possibility to build a toy-model of the spot. This allows to investigate the relationships between the geometry and other properties of the sunspot like flow velocities, intensity, temperature, or field strength. The application of the procedure to time series of observations will allow to study the temporal evolution of the geometry and the other properties, to determine the physical processes happening and their respective drivers. A second part of the thesis is devoted to the analysis of isolated field concentrations in the surroundings of the sunspot. These isolated magnetic elements are assumed to be the elementary building blocks of the solar surface magnetism. In the turbulent surroundings of the solar atmosphere, only relatively strong concentrations with a field strength above 1 kG are assumed to be stable at all ...
    Description: thesis
    Keywords: 523 ; TGC 800 ; Magnetfelder {Astronomie: Sonne}
    Language: English
    Type: monograph , publishedVersion
    Format: 166 S.
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  • 22
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    Development of a wide bandwidth array acousto optical spectrometer for the Herschel Satellite Mission (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: The Herschel satellite observatory will explore the universe at far-infrared, submillimetre, and millimetre wavelengths. This regime of the electromagnetic spectrum is difficult to observe, because water vapour in the Earth’s atmosphere absorbs the signals of almost all the astronomical sources. With the 3.5m telescope (the largest ever placed aboard a spacecraft), it will be possible to observe various atomic and molecular lines with exceptionally high spatial resolution. The receiver system employs the heterodyne technique, and the spectral information is obtained by means of real-time spectrometers. Within this thesis, the requirements, specifications, design concept, and development of a wide bandwidth array acousto-optical spectrometer (WBS) is discussed...
    Description: thesis
    Keywords: 523 ; TC 200 ; TBG 000 ; Astronomische Beobachtungen vom Weltraum aus ; Infrarot-Astronomie
    Language: English
    Type: monograph , publishedVersion
    Format: 177 S.
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  • 23
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    Development of ultrathin niobium nitride and niobium titanium nitride films for THz hot-electron bolometers (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: The main focus of this work is the development of ultrathin NbN and NbTiN films. A reproducible and reliable deposition process for ultrathin NbN and NbTiN films for the use in phonon-cooled HEB devices was established. The ultrathin films were deposited on silicon (Si) substrates and on 2æm Si3N4 membranes by DC reactive magnetron sputtering. A method for the precise control of the nitrogen partial pressure by monitoring the target voltage has been introduced to deposit high quality, ultrathin NbN (3-4nm, Tc=8.5 K) and NbTiN (4-5nm, Tc=8K) films. Substrate heating of at least 600ʿC during the deposition is essential for the fabrication of ultrathin NbN and NbTiN films on Si substrates and Si3N4 membranes. The fabrication process required for HEB devices to be used in a quasi-optical mixer was developed. The ultrathin film was patterned by electron beam lithography (EBL), resulting in bolometer devices that measure areas of about 0.4 æm x 4 æm. The nature of the contact determines the interface transparency between the bolometer and the contact structure. Different cleaning processes have been performed and the influence on the contact resistance has been instigated. A better interface transparency gives less RF losses and could improve the HEB sensitivity and local oscillator (LO) requirement ...
    Description: thesis
    Keywords: 523 ; TBC 000 ; Radioastronomie
    Language: English
    Type: monograph , publishedVersion
    Format: 165 S.
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  • 24
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    Merger dynamics and stellar populations in the host galaxies of the quasi-stellar objects I Zw 1 and 3C 48 (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: The thesis focuses on two case studies of the host galaxies of the quasi-stellar objects (QSOs) I Zw 1 and 3C 48. The studies are motivated by the hypothesis that ultra-luminous infrared galaxies (ULIRGs) might represent the early stage of QSO evolution (Sanders et al. 1988). According to this hypothesis, galaxy mergers and interactions trigger gas inflow which is followed by starburst activity and by the formation of an active nucleus. As likely transitionary objects between the ultra-luminous infrared stage and the QSO stage, I Zw 1 and 3C 48 are promising candidates to investigate the active nucleus, the starburst, and the merger properties as the essential links between all stages of the proposed evolutionary sequence. The case study of I Zw 1 has an observational focus and is based on near-infrared (NIR) imaging and spectroscopy, carried out with ISAAC (Infrared Spectrometer and Array Camera) at the Very Large Telescope of the European Southern Observatory (ESO) on Cerro Paranal in Chile. The two-armed spiral host galaxy of I Zw 1 shows two bright knots in the north and in the west of the optical disk region, respectively. Here, the new ISAAC spectra indicate that the northern object is a projected foreground star, as previously assumed but recently challenged. In the surroundings of the western source, the ISAAC J-band image displays tidal features. This gives further support for the scenario that I Zw 1 is presently in a minor merger process with this source. The new spectra and the NIR colors of the western source indicate an old stellar population. The host of I Zw 1 is analyzed in a one-dimensional structural decomposition into bulge, disk, and halo components, which results in mean J-band mass-to-light ratios ...
    Description: thesis
    Keywords: 523 ; THS 000 ; TIE 900 ; TIE 400 ; Sternpopulationen {Astronomie} ; Sonstige Sternensysteme {Astronomie} ; Spektren und Leuchtkraft von Sternsystemen {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 123 S.
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  • 25
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    High resolution multi-wavelength study of active galaxies (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: This PhD thesis presents a study of active galaxies carried out from cm- to mm-wavelengths with high angular resolution. The mechanism of the activity in these objects is supposed to be strongly correlated with the accretion of matter onto a super-massive black hole in their centres. One approach to increase the understanding of these highly interesting sources is to observe and analyse the molecular gas. The characteristics of its distribution and dynamics are indispensible diagnostic tools to investigate the accretion processes at different angular scales. A second approach is the study of emission originating in the direct vicinity of the black hole, such as non-thermal radio emission. To account for different activity levels, five objects were chosen ranging from nearby, Low Luminosity Active Galaxies (LLAGs; NGC3718 at z=0.003 and NGC1068 at z=0.004) to higher redshifted, High LAGs (HLAGs; HE1029-1831 at z=0.039, 3C48 at z=0.367, and Q0957+561 at z=1.414). The first two sources are part of the NU(clei of)-GA(laxy) project, that aims at analysing the distribution and kinematics of the molecular gas at high angular resolution/sensitivity in a sample of 30 nearby LLAGs. HE1029-1831 is part of a complementary sample of nearby HLAGs with similar aims. All sources were observed in carbon-monoxide (CO), known to be a good molecular gas tracer, with the IRAM Plateau de Bure Interferometer (PdBI) providing high sensitivity and high angular resolution. CO emission was detected in all five galaxies. In NGC3718, the molecular gas disk, having roughly two hundred million solar masses, is highly warped. The distribution of the gas reveals large scale asymmetries witnessing a possible tidal tail interaction with a close companion, and as well small scale asymmetries most likely tracing accretion onto the nucleus ...
    Description: thesis
    Keywords: 523 ; TIE 400 ; Spektren und Leuchtkraft von Sternsystemen {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 207 S.
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  • 26
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    Catastrophic cooling in solar coronal loops (2010)
    Univ. Freiburg (Breisgau)
    Details
    Publication Date: 2021-03-29
    Description: Satellite observations with high spatial and temporal resolution have revealed the highly dynamic nature of the solar corona and enabled us to study physical processes in the outer atmosphere of our mother star in great detail. This thesis deals with dynamic processes in coronal loops, i.e. magnetic structures which may be thought of as the elementary building blocks of the solar corona. Using computational fluid dynamics as a tool, I calculate time-dependent models of solar coronal loops in order to address the following questions: Are dynamic processes in coronal loops, such as flows and transient brightenings, necessarily the result of a time-dependent driving mechanism? Can various observations of fast downflows be explained by a common mechanism? Which parameters determine the dynamics of coronal loops? It is found that coronal loops which are predominantly heated around their footpoints can develop a thermal instability in the upper part of the loop. This instability results in a self-amplifying catastrophic cooling process and leads to the formation of dense, cool plasma condensations. The first part of the work focuses on plasma condensations in short cool loops, which presumably constitute the solar transition region, and describes how the catastrophic cooling process leads to transient brightenings in spectral lines formed in the transition region ...
    Description: thesis
    Keywords: 523 ; TGC 765 ; TBI 000 ; Korona {Astronomie: Sonne} ; Ultraviolett-Astronomie
    Language: English
    Type: monograph , publishedVersion
    Format: 161 S.
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  • 27
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    Solving the system of radiation magnetohydrodynamics for solar physical simulations in 3d (2010)
    Univ. Freiburg (Breisgau)
    Details
    Publication Date: 2021-03-29
    Description: In this study we present a finite-volumen scheme for solving the equations of radiation magnetohydrodynamics in two and three space dimensions. Among other applications this system is used to model the plasma in the solar convection zone and in the solar photosphere. It is a non--linear system of balance laws derived from the Euler equations of gas dynamics and the Maxwell equations; the energy transport through radiation is also included in the model. The starting point of our presentation is a standard explicit first and second order finite-volume scheme on both structured and unstructured grids. We first study the convergence of a finite-volume scheme applied to a scalar model problem for the full system of radiation magnetohydrodynamics. We then present modifications of the base scheme. These make it possible to approximate the system of magnetohydrodynamics with an arbitrary equation of state; they reduce errors due to a violation of the divergence constraint on the magnetic field, and they lead to an improved accuracy in the approximation of solution near an equilibrium state. These modifications significantly increase the robustness of the scheme and are essential for an accurate simulation of processes in the solar atmosphere ...
    Description: thesis
    Keywords: 523 ; TGC 400 ; Strahlung der Sonne {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 305 S.
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  • 28
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    Global modeling of comets: nucleus, neutral and ionized coma of comets 67P/Churyumov-Gerasimenko and 46P/Wirtanen (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: Models of the thermal behavior of a cometary nucleus, the evolution of the neutral gas coma, the ionized cometary coma and of the interaction of the cometary plasma with the solar wind are studied in this work. The general aim is to develop a global model of the comet and its environment in order to characterize the physical conditions around comets 67P/Churyumov-Gerasimenko and 46P/Wirtanen with respect to the heliocentric distance. The results also provide estimates of the effects of the cometary environment on the radio science investigations experiment (RSI) aboard the spacecraft Rosetta. After the launch that is scheduled for February 2004, the Rosetta mission is planned to encounter comet 67P/Churyumov-Gerasimenko and accompany it on its orbit. Comet 46P/Wirtanen has been the original target comet, but serves now as back-up target due to the postponement of the Rosetta launch in January 2003. The model of the heat diffusion within the cometary nucleus is one-dimensional. A grid of one-dimensional models is distributed over the nucleus in order to determine the temperature distribution and the sublimation characteristics of the comet on the whole surface of the comet. A heat balance equation is applied as boundary condition on the surface. Many parameters that have to be accounted for in a heat diffusion model are not precisely known to date. The variation of these parameters within reasonable limits yields a wide range of possible results. The heat diffusion within the cometary nucleus is derived from an energy conservation equation that includes heat conduction through the porous cometary material and heat convection due to the transport of latent heat by the gas phase within the nucleus. Model results are evaluated by a comparison of modeled and observed global gas production rates ...
    Description: thesis
    Keywords: 523 ; TGL 280 ; TC 200 ; Einzelne Kometen {Astronomie} ; Astronomische Beobachtungen vom Weltraum aus
    Language: English
    Type: monograph , publishedVersion
    Format: 153 S.
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  • 29
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    Differentiation of asteroids in the early solar system from Hf-W systematics (2010)
    Univ. Köln
    Details
    Publication Date: 2021-03-29
    Description: The first major differentiation in primitive planetesimals is the formation of metal- or metal-sulfide cores. To constrain the time of asteroidal differentiation, meteoritic metals and silicates coexisting with metals were analyzed for Wisotopes and Hf and W concentrations. The study focused on silicate inclusions in non-magmatic IAB iron meteorites and the corresponding metal phases as well as on winonaites and acapulcoites were analyzed. IAB iron meteorites formed by crystallization of metal ponds under low pressure conditions at or near the surface of their asteroidal parent body. The presence of abundant silicate inclusions reflects either incomplete metal-silicate separation or mixing of metal and silicates by impacts. Hafnium-W measurements were performed on magnetic and non-magnetic separates from seven IAB iron meteorites to constrain the exact timing of metal separation, silicate differentiation and metamorphism. IAB metals have a deficit in 182W of - 3.1 ± 0.2 -units (relative to terrestrial standard materials)...
    Description: thesis
    Keywords: 523 ; TGG 900 ; TG 200 ; Planetoide {Astronomie} ; Kosmogonie des Sonnensystems {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 118 S.
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