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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

The stability of short-period extrasolar giant planets

Koskinen, T. T. January 2008 (has links)
A three-dimensional coupled thermosphere-ionosphere model for extrasolar giant planets (EXOTIM) has been developed. This is the first such model reported in the literature. This thesis contains an extensive description of the model and the methods adopted in modelling the different physical processes expected in the upper atmospheres and ionospheres of extrasolar giant planets. Modelling the upper atmosphere is important because the stability of the atmosphere against thermal evaporation is controlled by the conditions in the thermosphere. The thermosphere is heated by the absorption of EUV and X ray (XUV) radiation emitted by the host star. The radiation also ionises the neutral species in the upper atmosphere, which is expected to be composed mainly of molecular and atomic hydrogen, and atomic helium. Ionisation and subsequent photochemistry leads to the formation of the H+, Hf, H3", and He+ ions (and small quantities of HeH+). H3" emits strongly in the infrared and may act as a significant coolant in gas giant thermospheres. Assuming photochemical equilibrium, the absorption of XUV radiation and ion photochemistry were modelled in a self-consistent fashion. The 3D model can also simulate strong winds affecting the upper atmosphere, and account for both advection and diffusion of the neutral species around the planet. The results indicate that within 1.0 AU from a solar-type host star, the upper atmospheres of Jupiter-type EGPs can be substantially cooler and more stable than implied by studies that ignore the possibility of radiative (Hf) cooling. In this context, a limiting distance, or a stability limit, was identified for such EGPs that depends on the composition of the upper atmosphere and ionosphere, and within which the atmospheres of the planets undergo hydrodynamic escape. Under restricted conditions, this limit is located around 0.15 AU from a Sun-like host star. The model was also used to simulate a newly found transiting planet HD17156b, which orbits its host star on a highly eccentric orbit.
2

A new semi-analytical treatment of the effect of supernovae on ULIRG spectral energy distributions

Jenner, Clare Elizabeth January 2008 (has links)
This work presents a method for generating synthetic spectra of Ultra-Luminous Infrared Galaxies (ULIRGS) using AGN, HII region and supernovae source functions. The AGN element represents the far-infrared contribution to the ULIRG spectrum from an energetic central engine. It is modelled using a quasar source embedded in an axi-symmetric dusty torus. The radiative transfer of flux (RT) is then simulated and the AGN emergent spectral energy distributions (SEDs) generated. The HII region solution is then developed. A stellar evolutionary synthesis code is used to generate instantaneous burst (ISB) source functions which decay in time. The evolution of the gas and dust density in a spherically-symmetric, dense GMC, under the influence of a time-dependent ionizing source flux, is derived. Having irradiated the dust distribution with the source cluster SED, the RT is calculated and the HII region SEDs obtained. The impact of supernovae energy on a GMC already ionized by stellar flux is then considered. Using the standard pressure-driven expansion model of e.g., Weaver et al. (1977) the radial evolution of a superbubble expanding under the influence of a continuous super novae energy function is derived. The superbubble is modelled in both an adiabatic rapid expansion phase and in an isothermal momentum-conserving phase. As the superbubble expands, upstream gas is swept into a thin shell trapped on its surface and the gas density enhancement is modelled using simple shock physics. Having generated expressions for the evolution of the shell gas temperature, it is linked to the dust density via a temperature dependent condensation factor. Finally expressions are developed to quantify the evolution of the optical depth along a line of sight. It is found that the star formation efficiency (SFE) has a profound effect on the radial evolution of the optical depth distributions in GMCs generating marked differences in behaviour between and high and low SFEs. Low SFE models have shells below the dust condensation temperature at the GMC boundary R2w and the extinction, having initially been in decline, recovers to more substantial values in a dust reformation scenario. These systems tend to be optically thick for most of their evolution. Those models classified high SFE have supershell temperatures in excess of the dust condensation temperature at R2w and the extinction distribution drops precipitously and reaches very low values (< 1) at R2w- It then remains low for some time before a small recovery in extinction occurs as the shell dust condenses out. These systems tend to be optically thin for most of their life-times. It is the more powerful supernovae source functions included in the modelled space which generate the high SFE extinction behaviour and vice versa. Having derived the dust density distributions they are irradiated by the appropriate central source cluster SED and the RT simulated to generate the emergent SEDs. These are similarly categorised as low and high SFE. The low SFE model SEDs appear to be representative of systems where the dust acts as an enshrouding bolometer and most it not all UV and optical radiation is reprocessed and re-emitted into the infrared. Conversely, the high SFE model SEDs are optically revealed and exhibit substantial, only mildly attenuated source flux at short wavelengths for the majority of their evolution. The emergent AGN and starburst (ISB HII region and supernovae) SEDs are then combined in pairs to form a ULIRG SED Library. These SEDs are matched to the published data for a sample of six nearby (redshift z < 1) ULIRGS. No model ULIRG SED is found to have a better than 40% probability of belonging to the same population distribution as the published data. This is found to be most likely the result of using an ISB source function. The starburst SED library is therefore extended to approximate constant star formation (CSFRA) using a time decay parameterization and the ULIRGs refitted. In each case a model ULIRG SED was found to match the observations with a better than 5% probability of non-random fit, which suggests that a constant rather than ISB star formation mode is perhaps more appropriate in ULIRGs. Using the CSFRA component of the best-fitting model ULIRG SED, estimates are made for the star formation rate, starburst age and the implied merger and interaction state for each ULIRG. In all case these quantities agree favourably with the literature. Each ULIRG was fitted with a CSFRA SED element originating in the high SFE group of models. This was found to be a direct result of fitting the upper limits to the short wavelength (A < 3m) flux data points, as it is the high SFE SEDs that are optically revealed.
3

R-matrix calculations of electron-molecule collisions with C2 and C-2

Halmova, Gabriela January 2008 (has links)
The C2 molecule is very important astrophysically as well as terrestrially. Its spectrum is known from the comets, the interstellar mediums and the atmospheres of cool carbon stars. C2 is present in flames and electric discharges through carbon-containing materials, and could be important at the edge of fusion plasmas. Nevertheless, there seem to be no preceding published studies of electron collisions with neutral C2. On the other hand, there are a number of studies available for C2 ion. The challenge of studying electron collisions with C2 is the unusually large number of low-lying electronic states, which are themselves difficult to represent using standard ab initio methods. Furthermore, the system supports several bound states even though the exact number is not firmly established. This thesis describe the electron collision processes with the dicarbon molecule and its anion at the energies up to 10 eV. Specific attention is given to determine the formation of the bound states of C2 and a low-lying resonance of C as a function of internuclear separation. While the calculations on the system used the standard implementation of the UK i-matrix method, those on C required both the molecular .ft-matrix method with pseudo-states and the partitioned /2-matrix method to be employed. The effectiveness of these procedures for this problem is discussed.
4

A statistical study of the solar and seasonal control of E-layer critical frequency

Kouris, Stamatios S. January 1972 (has links)
No description available.
5

Colours and motions in the atmosphere of Jupiter

Browne, G. C. January 1976 (has links)
Pole-to-pole microdensitometer scans of Jupiter images, taken at red, green, blue and UV wavelengths, for the 1970, 1972, 1973 and 1974 apparitions have been used to study the colours on Jupiter. Variations with latitude of the colour ratios B/R and UV/R have been attributed to variations in the intensity of the 'reflected' radiation at blue and UV wavelengths. The colour ratio G/R appears to be approximately constant with latitude. The variations in B/R and UV/R correspond to the visual appearance of Jupiter; high values of B/R and UV/R coincide with zonal regions and low values occur in belted regions. The use of a simple scattering model and a simplified solution to radiative transfer theory have shown that Jupiter's atmosphere in the zonal regions can be approximated by a scattering atmosphere above a reflecting layer. The scattering atmosphere must consist predominantly of molecules, but with some admixture of aerosols. To explain the colour of the belted regions it is necessary to invoke the existence of chromophores (i.e. particles that absorb preferentially at blue and UV wavelengths). From 1972 to 1974 there seems to be a systematic colour difference between the northern and southern zones. This appears to be due to changes in the albedos of these regions with time. These changes are attributed to changes in cloud properties. The scattering model indicates that Jupiter's aerosol layer may vary in density, thickness and type of aerosol with time, on a global scale. There is an almost linear decrease in the colour ratios G/R, B/R and UV/R towards the centre of the Great Red Spot. This suggests that the top of the Great Red Spot consists of chromophores in the form of an anvil-shaped cloud, similar to some storm systems on Earth. There is a strong correlation between high 5? emission and low values of B/R, and vice versa. There appears to be no such correlation at 7.5?, 8-14? and 20?. The lifetimes of dark spots and red spots appear to be greater than those of light spots by a factor of five or more. Observations suggest that dark spots and red spots can undergo longitudinal oscillations, but light spots cannot. All spots remain at a constant latitude throughout their lifetime.
6

X-ray emission from clusters of galaxies

Edge, Alastair Clouston January 1989 (has links)
Clusters of galaxies are the largest gravitationally bound systems known. The study of clusters can therefore provide information on the distribution of matter in the Universe on the largest scales. The X-ray emission from clusters is of particular interest, as the gas held within the gravitational potential well of a cluster responds to the total mass ('Dark' and 'Visible') of that cluster. This thesis is based upon a sample of 44 observations of clusters made by EXOSAT which provided both imaging and spectral data. The data give well determined luminosities, temperatures, iron abundances and mass flow rates for these clusters. The results provide constraints on models for cluster dynamics and formation. The theoretical models and previous observations of clusters are reviewed in Chapter 1, concentrating on the X-ray domain. The results from the imaging telescopes and proportional counters are given in Chapters 2 and 3. Detailed observations of Virgo, Coma and Perseus are described in Chapter 4. A correlation analysis of the X-ray, optical and radio results is presented in Chapter 5. The implications of these correlations are discussed in Chapter 6. The thesis concludes with a brief look forward to future missions.
7

Seyfert galaxies as X-ray sources

Elvis, Martin January 1978 (has links)
Using data from the Leicester University Ariel V Sky Survey Instrument (SSI), the work reported in this thesis establishes that Seyfert galaxies form a class of extragalactic x-ray source. The properties of Seyfert galaxies are reviewed and it is shown how standard arguments lead to a model of the structure of Seyfert galaxy nuclei containing three regions characterised by three types of emission: forbidden line, permitted line and 'non-thermal' continuum. Radio, infrared and optical parameters believed to derive from these three regions are then compared with x-ray flux densities from the SSI in an attempt to locate the source of x-ray emission. It is found that x-ray flux density correlates only with parameters describing the permitted line and continuum regions. Standard arguments presented earlier show that these regions are very small (=0.01 - 1pc diameter). Observable x-ray variability is thus possible and is indeed seen. The variability of x-ray emission from x-ray active galaxies is described using, in the main, SSI observations. Some suggestive similarities in their light curves are noted. An x-ray luminosity function for Seyfert galaxies is constructed by two methods whose results agree well. Using this luminosity function the Seyfert galaxies are found to contribute = 6% to the diffuse x-ray background, without evolution. They can account for the entire background if only moderate evolution is allowed. Finally, recent developments in the field are noted: the discovery of more Seyfert galaxies as x-ray sources, variability, and some first x-ray spectra are reviewed. An extension to the class of 'x-ray active galaxies' in the form of high excitation emission line galaxies is reported. The great possibilities for future research in the area of x-ray active galaxies provided by new x-ray observatories in flight or under construction are explored briefly. It is clear that this is a subject just begun.
8

Infrared photometry of eclipsing binary stars

Akinci, R. January 1978 (has links)
The infrared observations of several eclipsing binary stars are presented and discussed. Initially a brief review of the relevant literature is given together with a description of binary systems, in particular eclipsing binary stars. This is followed by an outline of the instrumentation and techniques used in making infrared observations of these systems. 1.2 and 2.2 micrometre observations of AB And and SW Lac, together with 2.2 micrometre observations of 44 i Boo are presented. These stars are all of the W UMa-type. The infrared light curves show a number of features not found optically and these features are discussed. All three systems appear to be very red for their spectral types. Observations are given of the cataclysmic variable AM Her at 1.2 and 2.2 micrometre. The 1.2 micrometre light curve is very similar to the optical light curves. However, the 2.2 micrometre light curve shows only a secondary eclipse, in this respect it is similar to the X-ray light curve. The system has a J - K colour of +0.60 at maximum light. These facts indicate that at 2.2 micrometre the system is emitting strong cyclotron radiation from near the white dwarf component. Finally, 1.2, 2.2 and 3.6 micrometre light curves of the Algol-type eclipsing binary RZ Set are presented. The infrared excess found is attributed to a gas stream and disk around the primary component of the system. The electron density in both the stream and the disk is ~1012 cm-3.
9

Theoretical studies of ionospheric radiowave propagation

Foley, Gerald January 1975 (has links)
This Thesis is concerned with radiowave propagation within and below the Ionosphere at frequencies in the HF and VLF portions of the radio spectrum. The first part of the thesis is devoted primarily to a study of HF propagation based on Ray Theory concepts. An account of Ray Theory is given and the approximations used in the classical treatment are discussed. In particular, the thesis considers the so-called Complex Ray Theory in which the effect of electron collisions on the ray path is introduced. Original results are produced which detail the behaviour of the reflection and coupling points of the propagation modes for both classical and Complex Ray Theory applications. The second part of the thesis is concerned with propagation of VLF waves. The theory is formulated in terms of Waveguide Modes. An account is given of this Theory as it has been applied to the propagation of VLF waves in the Earth- Ionosphere waveguide. Reflection from the Ionosphere is discussed in terms of the classical and generalised Full Wave Theories so that the propagation of these waves may be considered below realistic model ionospheres. A computer programme based on these theories, and which enables the waveguide mode parameters to be calculated, is outlined. Original results are presented on the parametric behaviour of the waveguide modes as a function of frequency and azimuth, using tabulated night time ionospheric profiles. For one of these profiles an interesting feature of the results is the apparent interchange of identity between the two lowest order modes. Further calculations have enabled a comparison of the mode parameters using classical (non-generalised) and generalised Full Wave Theories, and a comparison of the amplitude and phase of measured and theoretically predicted field strengths at distances of less than 1000 Km from a transmitter. Some work on the application of Greens function techniques to discontinuous waveguides is presented.
10

High resolution temporal studies of Cygnus X-1

Giles, Alan B. January 1978 (has links)
Rapid variability in X-ray sources implies a compact object such as a white dwarf, neutron star or black hole. Following a review of the best known examples and their associated models, the observational history of Cygnus X-1 is outlined. This source is generally believed to be a black hole since it appears to exhibit rapid variability (millisecond bursts) and has a mass above the theoretical neutron star limit. This thesis is concerned with an attempt to study the rapid variability of the source using a multiwire array deploying detector system with a sensitive area of 4,000 cm2 (1.5 - 15 keV). The chance expectation value of millisecond bursts as seen by previous smaller experiments would be decreased from 10-2 to 10-9. The anticipated high count rate required the use of a data interactive electronics system together with a large buffer store to optimise use of the available telemetry data rate. This allowed timing of the X-ray events to 2 mS together with a large transient handling capacity so that the system was no longer severely limited at the very moment that the information was of most interest. The experiment was flown from Woomera in November 1976 and functioned correctly but the attitude control system failed providing only 6 seconds of Cygnus X-1 data at reduced sensitivity. No other interesting observations were made while the experiment's field of view drifted across the celestial sphere. Investigations of the shot noise character of Cygnus X-1 are not inconsistent with previous observations, but the short data set is unsuitable for such studies. A search for rapid bursts using the same analysis methods as previous workers was also inconclusive, the presence of the calibration source flux possibly being crucial in limiting the probability of a number of borderline features. The statistical interpretation of burst phenomena is discussed leading to the concept that the size of the features seen by SL-1306 are similar numerically to those reported by Rothschild et al. (1976) and therefore represent activity at approximately one-third the intensity. Such features occurring during the non-enhanced low states would not be detected with the customary analysis by previous smaller detectors. A frequency of ~ 1 per sec is suggested for these mini bursts, the duration being 0.5 mS, the bursts appearing to be stronger in the latter 0.25 mS. The author notes with considerable regret that two re-flight proposals were not approved.

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