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  • 523  (111)
  • 914  (48)
  • English  (159)
  • 2010-2014  (159)
  • 1980-1984
  • 1
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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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  • 2
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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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  • 3
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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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  • 4
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    Surface spots on cool giant stars probed by spectro-astrometry (2010)
    Univ. Hamburg
    Details
    Publication Date: 2021-03-29
    Description: Spectro-astrometry is a technique to obtain information about the structure of an astronomical source on sub-diffraction limited spatial scales down to milli- or micro-arcseconds. The method exploits the shift of the photocentre of a structured source over emission/absorption lines in its spectral energy distribution. Sub-diffraction limited resolution can be achieved because the method only uses wavelength-differential information of longslit spectra. Although spectro-astrometry has already been devised by Bailey (1998) and some studies have been performed to date on different astronomic targets, so far, no systematic analysis of the potential of this technique has been undertaken. The aim of this work was to analyse the potential of the method, develop the tools to exploit this potential and write the software which is needed to perform the analysis procedure. To verify the practical feasibility, observations were done with the VLT/CRIRES facility to demonstrate the capabilities of the method with respect to detecting structures on giant star surfaces. All steps of data reduction were analysed with respect to their influence on the spectro-astrometric quantities and a new method for bad pixel detection was devised and implemented ...
    Description: thesis
    Keywords: 523 ; THT 600 ; THK 000 ; THT 400 ; Riesensterne {Astronomie} ; Spektren und Spektroskopie von Sternen {Astronomie} ; Kalte Sterne {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 109 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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    Accretion, winds and jets (2010)
    Univ. Hamburg
    Details
    Publication Date: 2021-03-29
    Description: This thesis deals with the high-energy emission from young stellar objects, which are on the one hand still actively accreting from their disk, and on the other hand are no longer deeply obscured by their natal cloud. Stars of spectral type B and A are called Herbig Ae/Be (HAeBe) stars in this stage, all stars of later spectral type are termed classical T Tauri stars (CTTS); strictly speaking both types are defined by spectroscopic signatures, which are equivalent to the evolutionary stage described above. In this thesis CTTS and HAeBes are studied through high-resolution X-ray and UV spectroscopy and through detailed physical simulations. Spectroscopic X-ray data is reduced and presented for two targets: The CTTS V4046 Sgr was observed with Chandra for 100 ks, using a high-resolution grating spectrometer. The lightcurve contains one flare and the He-like triplets of Si XIII, Ne IX and O VII indicate high densities in the X-ray emitting regions. The second target is the HAeBe HD 163296, which was observed with XMM-Newton for 130 ks. The lightcurve shows only moderate variability, the elemental abundance follows a pattern, that is usual for active stars. The He-like triplet of O VII exhibits line ratios similar to coronal sources, indicating that neither a high density nor a strong UV-field is present in the region of the X-ray emission ...
    Description: thesis
    Keywords: 523 ; THP 000 ; Thermodynamische Vorgänge in Sternen {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 84 S.
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  • 7
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    Explorative approach to the dust evolution in binary star systems (2010)
    Univ. Berlin
    Details
    Publication Date: 2021-03-29
    Description: This thesis discusses dust formation in binary systems, in particular for binary systems consisting of a Mira like star and a brown dwarf. A Mira-like star is an intermediate mass star in a late stage of their stellar evolution on the Asymptotic Giant Branch (AGB), and a brown dwarf, is a sub-stellar object with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores. In their radial pulsating elevated convective atmospheres, Mira-stars often develop strong stellar winds, which are driven by radiation pressure on the dust and lead to a substantial mass-loss of the star. Stellar winds are of central importance for the development of medium-heavy-AGB stars. Also, they are a reliable source for the production of dust particles and heavy elements for the interstellar medium and the chemical evolution of galaxies. In fact, most stars are in binary or multiple star systems.For a complete description of the processes of dust formation in binary star systems it is necessary to study the perturbative influence of a second star in the vicinity of a AGB-star with a strong stellar wind. This endeavour is embedded in a long standing tradition of scientific investigation of dust formation at the Zentrum für Astronomie und Astrophysik (ZAA) at the Technical University of Berlin...
    Description: thesis
    Keywords: 523 ; THT 900 ; TII 900 ; Doppelsterne {Astronomie} ; Interstellarer Staub {Astronomie: Interstellare Materie}
    Language: English
    Type: monograph , publishedVersion
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  • 8
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    The structure and spectra of irradiated secondaries in close binaries (pre-CVs) (2010)
    Univ. Hamburg
    Details
    Publication Date: 2021-03-29
    Description: Many stars evolve not alone but along one or more companions. While the evolution of single isolated stars is well predicted by theoretical calculations, close binary systems with their interactions are on the one hand far more difficult to handle, but on the other hand well constrained in their masses and sizes. One mechanism thought to produce such close binaries is the Common Envelope Evolution (CEE). Especially young post CEE binaries show great temperature differences between both components of about a factor 10 at a separation of just a few solar radii. This work focuses on close binary systems with a hot primary component (e.g. a subdwarf O-type star (sdO)) and a much cooler secondary component (e.g. a main-sequence star), which is heavily influenced by external radiation originating from the first. The technique to include external radiation into the stellar atmosphere code PHOENIX, i.e., the deviation of the outer boundary condition and the change in the temperature correction method, is described in detail. To concentrate on the irradiation effect, other difficulties such as asymmetric effects due to tear-drop shaped stars, ongoing mass accretion or shadows of a circumbinary disc, are avoided by selecting pre-cataclysmic variables (pre CVs) out of the class of close binaries as laboratory. The total eclipsing binary system UU~Sagittae is used as an example for massive irradiation and observed spectral features in the optical are fitted ...
    Description: thesis
    Keywords: 523 ; THK 000 ; THT 900 ; Spektren und Spektroskopie von Sternen {Astronomie} ; Doppelsterne {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 100 S.
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  • 9
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    Characterization of the stellar variability in CoRoT fields with BEST telescopes (2010)
    Univ. Berlin
    Details
    Publication Date: 2021-03-29
    Description: The first extrasolar planet 51 Peg b around the G-type star has been reported in 1995. The planet with few Jupiter masses orbiting its star very closely was detected by measurement of the oscillation in the radial velocity of the host star. In 1999 the first transit, when the planet is eclipsing the host star, of the extrasolar planet HD209458 b was observed with a small ground based photometric telescope. Ever since, new planets in distant systems are continuously being detected with new high precision instruments from the ground and from space. The department of Extrasolar Planets and Atmospheres at Deutsches Zentrum für Luft- und Raumfahrt, Berlin (DLR) is involved in the detection and characterization of extrasolar planets, through participation in the CoRoT space mission. Furthermore, two ground based photometric telescope systems are operated as a ground based support for the space mission CoRoT, dedicated to asteroseismology and to extrasolar planet search with the help of the transit method. The BEST project consists of two small aperture wide field-of-view photometric telescopes devoted to the search for transiting Jupiter-sized extrasolar planets and to the characterization of variable stars in CoRoT target fields...
    Description: thesis
    Keywords: 523 ; THX 100 ; TJ 900 ; TCE 220 ; Detektionsmethoden {Astronomie: Extrasolare Planetensysteme} ; Einzelne Weltraumprojekte ; Bodenständige UV-, Optische und Infrarotteleskope {Astronomische Instrumente}
    Language: English
    Type: monograph , publishedVersion
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  • 10
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    Chiral symmetry restoration and deconfinement in neutron stars (2010)
    Univ. Frankfurt (Main)
    Details
    Publication Date: 2021-03-29
    Description: Neutron stars are very dense objects. One teaspoon of their material would have a mass of five billion tons. Their gravitational force is so strong that if an object were to fall from just one meter high it would hit the surface of the respective neutron star at two thousand kilometers per second. In such dense bodies, different particles from the ones present in atomic nuclei, the nucleons, can exist. These particles can be hyperons, that contain non-zero strangeness, or broader resonances. There can also be different states of matter inside neutron stars, such as meson condensates and if the density is height enough to deconfine the nucleons, quark matter. As new degrees of freedom appear in the system, different aspects of matter have to be taken into account.The most important of them being the restoration of the chiral symmetry. This symmetry is spontaneously broken, which is a fact related to the presence of a condensate of scalar quark-antiquark pairs, that for this reason is called chiral condensate. This condensate is present at low densities and even in vacuum. It is important to remember at this point that the modern concept of vacuum is far away from emptiness. It is full of virtual particles that are constantly created and annihilated, being their existence allowed by the uncertainty principle. At very high temperature/density, when the composite particles are dissolved into constituents, the chiral consensate vanishes and the chiral symmetry is restored. To explain how and when chiral symmetry is restored in neutron stars we use a model called non-linear sigma model. This is an effective quantum relativistic model that was developed in order to describe systems of hadrons interacting via meson exchange ...
    Description: thesis
    Keywords: 523 ; THU 400 ; Pulsare, Neutronensterne {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 104 S.
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