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Abteilung Astronomie

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Dissertation Thorsten Nagel

Synthetische Spektren und Vertikalschichtungen von Akkretionscheiben

Accretion disks are components of objects as diverse as protoplanetary systems, active galactic nuclei, cataclysmic variables or X-ray binaries. Often, a high fraction of the luminosity of these systems is generated by the accetion disk itself. To understand these objects and interpret the observational data of increasingly high quality a realistic physical model of the accretion disk is therefore necessary. The aim of this thesis is the development of a model for the calculation of synthetic spectra and vertical structures of accretion disks. The physical processes in the disk should be considered as accurate as possible.
A full three-dimensional detailed radiation hydrodynamic treatment is presently still impossible due the enormous numerical costs. In the case of a geometrically thin alpha-disk (Shakura & Sunyaev 1973), where the disk thickness is smaller than the disk diameter, the radial and vertical structures can be decoupled. Under the assumption of axial symmetry and by deviding the disk into concentric rings the determination of the vertical structure becomes a one-dimensional problem. The dissipated energy in each disk ring is radiated away at the disk surface, the energy flux can be expressed as effective temperature.
In the context of this work the program package AcDc (Accretion Disc code), consisting of the programs AcDc-LTE, AcDc-NLTE and AcDc-MakeDisk, has been developed for the detailed calculation of the vertical structures and the spectra of accretion disks. The equations of radiative and hydrostatic equilibrium as well as the rate equations for the population numbers of the atomic levels are solved consistently with the radiation transfer equation under the constraint of particle number and charge conservation. Irradiation of the accretion disk by the central object can also be considered. Subsequently, AcDc-MakeDisk integrates the spectra of the individual disk rings to a complete disk spectrum for different inclination angles, and the spectral lines are Doppler broadened according to the radial component of the Kepler rotation.
One of the objects examined with the developed program package is AMCVn, the prototype of a class of binary systems with two interacting White Dwarfs and an accretion disk, almost entirely composed of helium, around the primary star. It could be shown that the radial extension of the accretion disk strongly affects the spectrum. Due to the larger and at the same time cooler radiating surface, a larger outer radius leads to an increase of the spectral line strengths of neutral helium compared to those of ionized helium. The inclination also has a considerable influence on the spectrum. The larger the inclination angle, the stronger the spectral lines are broadened due to the increasing radial component of the Kepler velocity. A change of the Reynolds number, which parameterizes the viscosity, by 20 per cent does not have a significant effect on the spectrum.
Another object analyzed is 4U1626-67, an ultracompact X-ray binary, consisting of a white dwarf and a neutron star. Its accretion disk does not show signs of hydrogen or helium, but consists of metals. The models shown here represent the very first detailed calculations of hydrogen and helium deficient accretion disks performed so far. In particular, the influence of the irradiation of the central object on the vertical structure and the spectrum of the accretion disk is examined. Irradiation leads to a strong rise in temperature in the outermost layers. Additionally, numerous absorption lines turn into emission. From comparison of a HST UV spectrum with model spectra it follows that only a model with irradiation can reproduce the observed spectrum, in particular the emission lines of O V and C IV.
Key words: accretions disk , synthetic spectrum , vertical structure , AMCVn

Online-Publikation: http://nbn-resolving.de/urn:nbn:de:bsz:21-opus-8804

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