Diese Arbeit stellt eine neue Tomographiemethode vor, die Bedeckungstomographie in physikalischen Parametern (
Physical Parameter Eclipse Mapping
). Sie ist entwickelt um die Struktur von Akkretionsscheiben in kataklysmischen Veränderlichen direkt in Form der grundlegenden physikalischen Parameter zu kartieren.
Kataklysmische Veränderliche sind enge Doppelsternsysteme, in denen Materietransfer von einem der Sterne, typischerweise ein Hauptreihenstern, zum anderen einem weißen Zwerg, stattfindet und zu der Ausbildung einer Akkretionsscheibe führt. Akkretionsscheiben sind von generellem Interesse in der Astrophysik, da sie in einer Vielzahl von Objekten mit Masseneinfall auftreten, wie z.B. aktiven
galaktischen Kernen und jungen stellaren Objekten. Bedeckende kataklysmische Veränderliche sind ideale Laboratorien um solche Akkretionsprozesse zu untersuchen da mit Veränderung der Bahnphase unterschiedliche Teile der Scheibe sichtbar werden.
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Table of Contents
1 Preface
2 Accretion in Cataclysmic Variables
2.1 Cataclysmic Variables
2.2 The observations
2.2.1 The accretion disk
2.2.2 The white dwarf
2.2.3 The boundary layer
2.2.4 The bright spot and the gas stream
2.2.5 The red star
2.2.6 The outburst state
2.3 Theories of accretion phenomena
2.3.1 The viscosity
2.4 The outbursts
2.4.1 The Disk Instability model
2.4.2 The Mass Transfer Burst model
2.4.3 DI model vs. MTB model
3 IP Peg and HT Cas
3.1 The "puzzling" system IP Pegasi
3.1.1 A Dwarf Nova in the spotlight
3.1.2 The spectral appearance
3.1.3 The primary component
3.1.4 The secondary component: The red star
3.1.5 Setting the scene
3.1.6 The eclipses
3.1.7 Summary of the disk parameters
3.2 The "Rosetta Stone" HT Cassiopeiae
3.2.1 An unusual Dwarf Nova
3.2.2 The spectral appearance
3.2.3 The primary component
3.2.4 The secondary component: The red star
3.2.5 Setting the scene
3.2.6 The eclipses
3.2.7 Summary of the disk parameters
4 Eclipse Mapping
4.1 Tomography methods
4.2 Theory
4.2.1 The Entropy
4.2.2 The initial and the default image
4.2.3 Fit to the observations
4.2.4 Illustration of the MEM algorithm
4.3 Images of accretion disks
4.3.1 IP Peg & HT Cas
4.3.2 The general picture
5 Eclipse Mapping in Emission Lines
5.1 Emission Line Mapping of HT Cas
5.2 Discussion
6 Physical Parameter Eclipse Mapping
6.1 The new idea
6.2 Description of the method
6.2.1 Polar grid
6.2.2 Spherical white dwarf
6.2.3 White Dwarf spectra
6.2.4 The uneclipsed component
6.2.5 Use of a grid of model spectra
6.2.6 Use of passband response functions
6.3 Physical Models
6.3.1 Black body
6.3.2 A uniform LTE slab model
7 Application of the method to synthetic data
7.1 Test of the Temperature Mapping
7.2 Comparison with classical Eclipse Mapping
7.3 LTE-slab-version
7.3.1 Study of the model
7.3.2 Test of the LTE slab version
7.3.3 Discussion
8 Application of the method to real data
8.1 IP Peg on decline from outburst
8.1.1 Optically thick accretion disk
8.1.2 Discussion
8.1.3 Comparison to the results from Bobinger et al.
8.1.4 Comparison to the superoutburst light curve from HT Cas
8.2 HT Cas in quiescence
8.2.1 Optically thick disk in quiescence
8.2.2 Optically thin solution
8.2.3 Discussion
8.2.4 Fitting the white dwarf simultaneously
8.2.5 Fit with different distances
8.2.6 Comparison to Wood, Horne & Vennes 1992
8.3 Further improvements of the method
9 Discussion
Research Objectives and Core Topics
The primary objective of this thesis is to introduce and apply a new tomographic method, termed "Physical Parameter Eclipse Mapping," to reconstruct the structure of accretion disks in cataclysmic variables directly in terms of fundamental physical parameters such as temperature and surface density.
- Development of the Physical Parameter Eclipse Mapping algorithm.
- Comparative analysis of accretion disk models (e.g., Disk Instability vs. Mass Transfer Burst).
- Reconstruction of radial temperature and surface density profiles for dwarf novae.
- Validation of the method using synthetic data and application to real observations (IP Pegasi and HT Cassiopeiae).
- Investigation of emission line behavior and optical depth in accretion disks.
Excerpt from the Book
The Disk Instability model
Paczynski & Schwarzenberg-Czerny (1980) suggested the disk instability model on the basis of observations of U Gem. Very simplified, it assumes that accretion on the white dwarf is low during quiescence while in outburst the accumulated matter in the disk is falling down on the white dwarf.
As seen in Section 2.3, for the effective temperature Teff and surface density Σ a hysteresis curve is realized in the Teff-Σ plane with a transition in the region of the ionization temperature of hydrogen. Outside of this region the relation T(Σ) is strictly monotonous. Qualitatively, the consequences can be described as follows (Osaki 1974): during quiescence, the mass accumulates in the outer regions of the disk, until an unknown instability process allows it to accrete onto the white dwarf. This scenario is supported by the fact that the observed ratio between the luminosity of the disk to the bright spot (≈ 1/5) is much larger than the ratio for steady accretion (≈ 1/100, Smak 1984). With increasing mass transfer rate the surface density increases as well. This leads to enhanced viscous friction and heating in the disk.
In thermal equilibrium the disk temperature is held constant, because the heat produced in the inner disk (z = 0) is equal to the energy radiated away. A decrease in the viscosity of the material leads to a decrease in the temperature and the mass transport through the disk. However, since matter is streaming in from outside, the surface density increases which leads in turn to an increase in temperature (A → B in Fig. 2.2).
Summary of Chapters
Preface: Provides an introduction to the motivation behind studying accretion disks in interacting binaries and outlines the thesis structure.
Accretion in Cataclysmic Variables: Summarizes properties of cataclysmic variables and reviews established theoretical frameworks for accretion phenomena, including viscosity and disk instability models.
IP Peg and HT Cas: Reviews available literature and specific characteristics of the two primary dwarf novae systems under investigation.
Eclipse Mapping: Explains the principles of the classical Eclipse Mapping method and the Maximum Entropy Method (MEM) used as the foundation for the new approach.
Eclipse Mapping in Emission Lines: Describes the application of classical mapping to hydrogen emission lines in HT Cas to investigate disk structure.
Physical Parameter Eclipse Mapping: Introduces the core concept of the thesis: a new method to map physical parameters rather than mere intensities.
Application of the method to synthetic data: Validates the new method by testing its ability to recover known parameter distributions.
Application of the method to real data: Presents the primary results of applying the new method to actual observations of IP Peg and HT Cas.
Discussion: Summarizes the thesis, evaluates the successes and limitations of the new method, and proposes future improvements.
Keywords
Accretion disks, Cataclysmic variables, Dwarf novae, Eclipse mapping, Physical parameter mapping, Maximum entropy method, IP Pegasi, HT Cassiopeiae, Surface density, Effective temperature, Viscosity, Stellar atmospheres, Binary systems, Tomography, Orbital mechanics.
Frequently Asked Questions
What is the primary focus of this research?
The work focuses on the tomographic reconstruction of accretion disks in cataclysmic variables, specifically developing a method to map fundamental physical parameters directly.
Which specific astronomical systems are analyzed?
The study primarily investigates the dwarf novae IP Pegasi and HT Cassiopeiae using observational data.
What is the core improvement over existing methods?
Unlike classical Eclipse Mapping which reconstructs intensity distributions, this new method reconstructs distributions of physical parameters (temperature and surface density) by fitting model spectra across multiple wavelengths.
What scientific algorithm serves as the basis for this mapping?
The method is based on the Maximum Entropy Method (MEM), ensuring the selection of the smoothest physical solution compatible with the observational data.
Does the method accurately reconstruct disk parameters?
Validation with synthetic data proves that the method is capable of reproducing temperature and surface density distributions, provided the physical model is well-understood.
What do the results indicate for HT Cassiopeiae?
The results for HT Cas suggest that the disk becomes optically thin in its central regions, which contradicts previous interpretations based on emission line mapping.
How does the method handle the white dwarf?
The method accounts for the white dwarf by treating it as a spherical object or as a separate entity with its own theoretical spectrum, depending on the specific model configuration.
What causes the artifacts noted in the mapping of HT Cas?
Artifacts, particularly in front of the white dwarf, suggest that the current treatment of the white dwarf and the boundary layer as a perfectly spherical object is a simplification that requires future improvement.
- Citar trabajo
- Sonja Isaacs (Autor), 1997, Unveiling Accretion Disks - Physical Parameter Eclipse Mapping of Accretion Disks in Dwarf Novae, Múnich, GRIN Verlag, https://www.grin.com/document/2964
