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  • 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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    Methods for the reconstruction of large scale anisotropies of the cosmic ray flux (2010)
    Univ. Siegen
    Details
    Publication Date: 2020-10-07
    Description: In cosmic ray experiments the arrival directions, among other properties, of cosmic ray particles from detected air shower events are reconstructed. The question of uniformity in the distribution of arrival directions is of large importance for models that try to explain cosmic radiation. In this thesis, methods for the reconstruction of parameters of a dipole-like flux distribution of cosmic rays from a set of recorded air shower events are studied. Different methods are presented and examined by means of detailed Monte Carlo simulations. Particular focus is put on the implications of spurious experimental effects. Modifications of existing methods and new methods are proposed. The main goal of this thesis is the development of the horizontal Rayleigh analysis method. Unlike other methods, this method is based on the analysis of local viewing directions instead of global sidereal directions. As a result, the symmetries of the experimental setup can be better utilised. The calculation of the sky coverage (exposure function) is not necessary in this analysis. The performance of the method is tested by means of further Monte Carlo simulations. The new method performs similarly good or only marginally worse than established methods in case of ideal measurement conditions. However, the simulation of certain experimental effects can cause substantial misestimations of the dipole parameters by the established methods, whereas the new method produces no systematic deviations. The invulnerability to certain effects offers additional advantages, as certain data selection cuts become dispensable.
    Description: thesis
    Keywords: 523 ; TFE 000 ; Kosmische Elektrodynamik {Astrophysik}
    Language: English
    Type: monograph , publishedVersion
    Format: 177 S.
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  • 3
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    Modeling light curves of type Ia supernovae (2010)
    Univ. Hamburg
    Details
    Publication Date: 2020-10-07
    Description: The main topic of this work is the introduction of time dependence into PHOENIX. This has been achieved for both the radiation field and the matter in the SN Ia model atmosphere. First, time dependence in the radiative transfer equation has been implemented. Two discretization schemes have been used for the implementation of the time derivative. Test calculations have been performed to confirm the correctness of the implementations. The radiation time scale computed with the time dependent radiative transfer is comparable to a simple analytic approach. Perturbations of the inner boundary condition of the atmosphere move through the whole atmosphere. For instance, an atmosphere with a sinusoidally varying inner light bulb leads to an atmosphere where the luminosity varies sinusoidally everywhere. For the next extension of time dependence for the matter, a simple hydrodynamical solver has been implemented. It computes the changes in the energy of an SN Ia atmosphere by considering the homologous expansion, energy transport and the deposition of energy by g -ray emission due to the radioactive decay of 56Ni and 56Co. Test calculations verified that each part of the solver works correctly. The energy deposition heats the atmosphere and increases the observed luminosity, whereas the adiabatic e expansion cools the atmosphere. The energy transport always pushes the temperature structure of the atmosphere towards the radiative equilibrium state ...
    Description: thesis
    Keywords: 523 ; THI 000 ; THU 162 ; Helligkeit von Sternen {Astronomie} ; Supernovae {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 107 S.
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  • 4
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    Hydrogen-deficient central stars of planetary nebulae (2010)
    Univ. Potsdam
    Details
    Publication Date: 2020-10-07
    Description: Central stars of planetary nebulae are low-mass stars on the brink of their final evolution towards white dwarfs. Because of their surface temperature of above 25,000 K their UV radiation ionizes the surrounding material, which was ejected in an earlier phase of their evolution. Such fluorescent circumstellar gas is called a "Planetary Nebula". About one-tenth of the Galactic central stars are hydrogen-deficient. Generally, the surface of these central stars is a mixture of helium, carbon, and oxygen resulting from partial helium burning. Moreover, most of them have a strong stellar wind, similar to massive Pop-I Wolf-Rayet stars, and are in analogy classified as [WC]. The brackets distinguish the special type from the massive WC stars.
    Description: thesis
    Keywords: 523 ; TEG 000 ; Kosmogonie {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 102 S.
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  • 5
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    Stochastic cosmological inflation (2010)
    Univ. Bielefeld
    Details
    Publication Date: 2020-10-07
    Description: In this thesis we apply methods from statistical physics to stochastic inflation. Those methods, the replica field theory and the Gaussian variational methods, have to our knowledge never been applied before in this context, and allow us to compute the power spectrum of a scalar test field in the most general set-up. It provides a framework to perform calculations in regions of arbitrarily large quantum fluctuations and may also serve as a starting point to address the issue of back reaction. We first give an introduction to cosmological inflation, cosmological perturbation theory and cosmic microwave background anisotropies. Then we explain the idea of stochastic inflation, including some detailed derivations, and give an overview over major progress in this field. This is followed by an introduction to replica field theory, presented in a way directly applicable to stochastic inflation. Our work continues with a detailed calculation of the power spectrum of a scalar test field in a Friedmann Universe. We show the effect of the quantum fluctuations on the spectrum and derive explicit expressions showing its dependence on time and other important parameters. The effect of self-interactions and possible effects on the cosmic microwave background are discussed. We conclude with a summary of our results and give an outlook. One part of our major results has been published in Phys. Rev. D 78, 103501 (2008), where for the first time we present a replica field-theoretical approach to stochastic inflation in which we find a manifestation of the phenomena of so-called dimensional reduction. It implies under certain conditions inevitable infra-red divergencies of correlation functions on large-scales. These conditions are examined in detail in Phys. Rev. D 79, 44009 (2009), where we find that generically for a wide class of circumstances the divergencies are pushed exponentially fast well beyond observable scales ...
    Description: thesis
    Keywords: 523 ; TA 100 ; TEC 000 ; Theoretische Grundlagen {Astronomie} ; Kosmologie {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 132 S.
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  • 6
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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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  • 7
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    Massive stars in the Galactic Center Quintuplet cluster (2010)
    Univ. Potsdam
    Details
    Publication Date: 2021-03-29
    Description: The presented thesis describes the observations of the Galactic center Quintuplet cluster, the spectral analysis of the cluster Wolf-Rayet stars of the nitrogen sequence to determine their fundamental stellar parameters, and discusses the obtained results in a general context.The Quintuplet cluster was discovered in one of the first infrared surveys of the Galactic center region (Okuda et al. 1987, 1989) and was observed for this project with the ESO-VLT near-infrared integral field instrument SINFONI-SPIFFI. The subsequent data reduction was performed in parts with a self-written pipeline to obtain flux-calibrated spectra of all objects detected in the imaged field of view ...
    Description: thesis
    Keywords: 523 ; TIC 000 ; THT 800 ; TIE 662 ; Sternhaufen {Astronomie} ; Massenreiche Sterne {Astronomie} ; Struktur der Milchstrasse {Astronomie}
    Language: German
    Type: monograph , publishedVersion
    Format: 115 S.
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  • 8
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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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  • 9
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    Zeeman-Doppler Imaging of active late-type stars (2010)
    Univ. Potsdam
    Details
    Publication Date: 2021-03-29
    Description: Stellar magnetic fields, as a crucial component of star formation and evolution, evade direct observation at least with current and near future instruments. However investigating whether magnetic fields are generated by a dynamo process or represent relics from the formation process, or whether they show a behavior similar to the sun or something very different, it is essential to investigate their structure and temporal evolution. Fortunately nature provides us with the possibility to indirectly observe surface topologies on distant stars by means of Doppler shift and polarization of light, though not without its challenges. Based on the mentioned effects, the so called Zeeman-Doppler Imaging technique is a powerful method to retrieve magnetic fields from rapid rotating stars based on measurements of spectropolarimetric observations in terms of Stokes profiles. In recent years, a large number of stellar magnetic field distributions could be reconstructed by Zeeman-Doppler Imaging (ZDI). However, the implementation of this method often relies on many approximations because, as an inversion method, it entails enormous computational requirements.
    Description: thesis
    Keywords: 523 ; THR 000 ; TEG 000 ; Elektrische und magnetische Felder {Sterne} ; Kosmogonie {Astronomie}
    Language: German
    Type: monograph , publishedVersion
    Format: 107 S.
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  • 10
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    Radiative transfer in the exosphere of Saturn's moon Titan (2010)
    Univ. Berlin
    Details
    Publication Date: 2021-03-29
    Description: This work focuses on the distribution of atomic hydrogen in the exosphere of Saturns̉ moon Titan. This work is based on measurements performed by the Hydrogen Deuterium Absorption Cell (HDAC) aboard the Cassini spacecraft, that should directly determine the D/H ratio from the UV emission of atomic hydrogen and deuterium at 121.567 and 121.533 nm, respectively. HDAC measurements were performed on December 26, 2005 during the ninth Titan flyby of the Cassini orbiter. Unfortunately the amount of deuterium in the absorption cell could not be determined. Thus, the purpose, HDAC was designed for could not be achieved. For this reason, HDAC has performed measurements only once. However, the amount of atomic hydrogen in the absorption cell is well known, hence the data using only the hydrogen cell can be used in order to investigate Titans̉ exosphere. This work aims at simulating the HDAC measurements performed in order to investigate Titan’s exosphere, e.g. to determine the distribution of atomic hydrogen. A model of Titan’s exosphere including atomic hydrogen and methane has been developed and the transfer of solar radiation is calculated in order to simulate the HDAC measurements. The radiative transfer is solved using the Monte Carlo method. For the exospheric model, two different atomic hydrogen density distributions were applied, which determine the distribution in the exosphere by different approaches. Density profiles calculated by both models mainly differ in the lower exosphere ...
    Description: thesis
    Keywords: 523 ; TGG 655 ; Saturnmonde {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 171 S.
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