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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.

Black hole X-ray binaries : radiation and high-redshift feedback

Knevitt, Gillian Frances Grace January 2014 (has links)
The accretion of matter onto black holes results in their characteristic spectrum through which we can identify them and study their properties. Furthermore, this radiation can couple to their surroundings, resulting in complex interactions between black holes and their environments. In this thesis, I study the accreting properties of stellar mass black holes, and examine the effect that such interactions may have had on the early universe. I also consider the observational characteristics of the lowest luminosity stellar mass black hole binary systems in our own galaxy. Approximately one billion years after the Big Bang, the universe underwent a huge baryonic phase change, in which neutral hydrogen became ionized by the first sources of radiation. Massive stars are thought to drive this process, but their ionizing lifetimes could have been extended by a later phase in their evolution: black hole X-ray binary formation. However, the extent of this enhancement is not known, and has been highly debated in recent literature. In this thesis, I show that X-ray binaries were unlikely to be present in sufficient numbers to exert a significant effect on the intergalactic medium. Using a stellar population synthesis model of a single starburst event, I show that radiation from X-ray binaries dominates the ionizing power of a cluster after the most massive stars have ended their lives. However, their high energy spectra and short lifetimes mean their ionizing timescales are too long for them to affect the progress of reionization. Even so, the high escape fraction of X-rays from galaxies still provides scope for low level heating and ionization of the distant intergalactic medium under different circumstances, such as in the context of continuous star formation. I also assess the detectability of the dimmest black hole binary systems in the Milky Way. Using a catalogue of black hole binaries in our galaxy, I find that there is a statistically significant lack of short orbital period systems, when compared to the neutron star binary population. I show that these sources may be hidden from view, rather than being truly absent, due to radiatively inefficient accretion, in which energy is lost to the black hole. However, this conclusion requires that the switch to inefficient accretion occurs sharply at a threshold mass accretion rate. In the case of a smoother switch, alternative observational or evolutionary arguments must be put forward to explain this dearth.

Range analysis of binaries with design procedures

Barrett, Edward January 2014 (has links)
In the past few years, there has been increased interest in automating the reverse engineering and verification of binary executable code. The importance of this subject has b,een highlighted by the growing relevance of security, of reliability and of legacy code. Since dynamic analysis is of limited use for whole-program analyses, there has been a renewed enthusiasm for the development of automated static analyses, which can prove a property holds over all paths of the program. The abstract interpretation framework serves this purpose and has been widely adopted in both academic and industrial circles. Yet, since its introduction in 1977, standard abstract interpretation has been formulated as the least solution of a set of fixpoint equations. The work in this thesis deviates from the standard approach to static analysis, proposing that recent advances in decision procedures could be leveraged to tackle the problem. The thesis can be considered to be a survey of the application of Boolean satisfiability (SAT) and linear optimisation to the problem of static analysis, specifically range analysis of binary executable code. It is shown (with experimental results) that SAT and linear optimisation can be used to infer ranges of register values which, amongst others, are useful for control flow recovery and for detecting binary vulnerabilities, such as buffer and heap overflows.

Exotic binary stars as products of common-envelope evolution

Sener Satir, H. Tugça January 2015 (has links)
The aim of this project was to examine binary stars containing at least one unusual and hydrogen-deficient hot subdwarf, allowing us to explore possible outcomes of common-envelope ejection in a close binary. The first object studied was PG 1544+488, an exceptional short-period spectroscopic binary containing two helium-rich subdwarfs. I obtained and improved orbital and atmospheric parameters for each component. The orbital period P = 0.496 ± 0.002 d, dynamical mass ratio M(B)/M(A) = 0.911 ± 0.015, and spectroscopic radius ratio R(B)/R(A) = 0.939±0.004 indicate a binary containing nearly identical twins. The surfaces of both stars are slightly metal-poor (1/3 solar) and carbon-rich (0.3% by number). The best model for the origin of PG 1544+488 requires the ejection of a common envelope from a binary in which both stars are red giants with helium cores of nearly equal mass. The second object was 81Lyn and the first goal was to determine whether the relative motion of the two components could be resolved, and subsequently to obtain the dynamical mass ratio. Radial velocities of both components were measured but it was not possible to determine a period. An emission feature at Ha is also observed, which varies within a single night's observation. I discuss the origin of this feature. Additionally, 17 subdwarf stars, are classified according to their helium abundances. For two of these; I obtained and improved orbital and atmospheric parameters for [CW83] 1419-09 and recognised a variability of radial velocity for CD-314800. Exotic' binaries like PG 1544+488 and 81 Lyn place strong constraints on models for binary-star evolution. Further observations are required for BI Lyn, and to discover other exotic post-common-envelope binaries

Wind properties of blue supergiants

Petrov, Blagovest Vaskov January 2014 (has links)
The evolutionary state of blue supergiants is still unknown. Stellar wind mass loss is one of the dominant processes determining the evolution of massive stars, and it may provide clues to the evolutionary properties of blue supergiants. However, their mass-loss properties are not well understood. Therefore, in this thesis, we investigate the wind properties of blue supergiants by means of the non-L TE radiative transfer code cmfgen (Hillier & Miller 199B). The thesis describes two self-contained pieces of research which are linked through their connection with the wind properties of blue supergiants. The first involves a detailed analysis of the Ha line formation over a range in effective temperature between 30000 and 10000K. We find a maximum in the Hα equivalent width around 22500K which is always present in sets of models with various stellar and wind parameters. The non-monotonic Ha behaviour is related to the optical depth of the Lyα line, finding that at the "cool" branch the population of the 2nd level of hydrogen is enhanced in comparison to the 3rd level. This is expected to increase line absorption, leading to weaker Hα flux when Teff drops from 22500K downwards. We also show that at the bi-stability jump, Hα changes its character completely, from an optically thin to an optically thick line, implying that macro-clumping should play an important role at temperatures below the bi-stability jump. In the second part of the thesis, the physical ingredients that play a role in the line acceleration are explored. Our calculations confirm the bi-stability jump in mass-loss rates predicted by Vink et al. (1999). We also show that at temperatures around 10000 K, a second jump in mass-loss rates is produced if the observed velocity ratios are applied. The jump is caused by Felll/Fell recombination/ionisation as was suggested by Vink et al. (1999). Understanding the behaviour of the second jump my provide valuable science prospects for late B/A supergiants and LBVs, and therefore, a detailed investigation of this jump might be worth having.

Accretion variability in young stellar objects

Costigan, Gráinne January 2014 (has links)
Accretion is one of the dominant sources of radiation from a low mass young stellar object for the first few million years. This process regulates the flow of material and angular momentum from the surroundings to the central object, and is thought to play an important role in the definition of the long term stellar properties. Variability is a well documented attribute of accretion, and has been observed on time-scales ranging from days to years. However it is not clear where these variations come. The current model for accretion is magnetospheric accretion, where the stellar magnetic field truncates the disc, allowing the matter to flow from the disc onto the surface of the star. This model allows for variations in the accretion rate to come from many different sources, such as the magnetic field, the circumstellar disc or the interaction of the different parts of the system. This thesis sets out to use the intrinsic accretion variability to probe the inner regions of these systems. Two spectroscopic surveys were utilised that concentrated on the Hα emission line, which is known to be closely connected to the accretion process. Together, these surveys covered 24 object including low mass T Tauri, intermediate mass T Tauri stars and Herbig Ae stars, on time-scales of minutes, days, weeks, months to years. These two studies found the accretion variations to be less than 1 Mo/yr and dominated by time-scales close to the rotation period. A further photometric monitoring campaign was undertaken to confirm the short term variations found in the low mass sample. The results from all three of these works are in agreement with each other, and they suggest that the majority of the variations in typical accreting objects are the result of an asymmetric accretion flow.

Superluminous supernovae and hostless transients from the Pan-STARRS1 medium deep survey

McCrum, M. G. I. January 2016 (has links)
The focus of this thesis is the study of superluminous supernovae (SLSNe) within the Pan- STARRSI Medium Deep Survey (PSI MDS). Previous work on SLSNe has displayed a trend in their host environments, in that they seem to preferentially occur in low luminosity or dwarf galaxies. This presented the motivation to search for supernovae in faint host galaxies in the hope of finding new SLSNe. The PS 1 MDS offered an ideal experiment for such a search, in the redshift range z = 0.3 - 1.5. During the 3 year period of study, approximately 400 PSI MDS hostless transients were catalogued and around half classified. Of the 22 spectroscopically confirmed supernovae which were not of Type la, over 50% have been classified as likely SLSNe. This led to the discovery and detailed study of PS1-11ap (in a faint host galaxy at z = 0.524). The object was discovered at the start of an observing season resulting in a superbly sampled light curve. Rest frame UV and optical spectra were obtained throughout its observing season, including a host spectrum. From comparisons with known SLSNe and modelling of a composite bolometric light curve a confident classification of PS 1-11ap as a rare, slowly evolving SLSNe-Ic is made. PSI-lOpm and PSI-I0ahf, at z = 1.206 and z = 1.158, were also discovered. Spectroscopic and photometric comparisons of PSI-lOpm with SLSNe-Ic place it comfortably within this class . PSI-lOahf sits better as a slowly evolving SLSN-Ic, however alternative progenitor possibilities are presented. A single spectrum and appropriate photometric magnitude for PS1-11acn (z = 0.61) offer limited evidence for SLSN typing. Rates for the two SLSNe-Ic classes are also estimated using Monte-Carlo simulations, at around 10"-4 and 10"-5 that of the normal CCSNe rate.

A study of spectral line surveys for four regions related to star formation

Bedikoglou, Isidore January 2009 (has links)
Star formation is a process that has been studied extensively during the last decades. With the advent of advanced instruments our knowledge has expanded beyond the point of general descriptions. From simple models of gravitational contraction of spherical clouds we have managed to give more detailed descriptions about the inner processes and the chemistry within the environment of proto stars and young stellar objects. Still there are several issues that have not been fully understood, such as the formation of stars with very large masses, the exact mechanism that generates strong bipolar outflows and the chemical routes through which complex molecules are formed within the young stellar envelopes. A wide field of research is associated with observations of the rotational transitions of molecules in the centimetre and (sub)millimetre ranges. Our study concentrates on the analysis of data from two spectral surveys one in the centimetre and one in the millimetre range. We have performed calculations of column densities and abundances of several species from four different sources of different masses and evolutionary stages. The sample includes two high-mass young stellar objects (AFGL 2591 and lRAS 20126), one low-mass binary system (NGC1333-IRAS 4), and a well known photon dominated region (Orion Bar). For AFGL 2591 temperature and density profiles are developed based on a combination of calculations and non-LTE radiative transfer modelling of spectral lines. The same analysis revealed great abundance enhancements for CH30H, and S02 of factors ≥103 suggesting the existence of a hot core. Related to this is the detection of CH30CH3, the first complex organic molecule detected towards this source. Furthermore, CH30H maps have revealed an unusual ring-like structure which has been explained by a scenario that requires the enhancement of CH30H in outflow regions of shocked gas. The study of the radio recombination lines in combination with radio flux measurements leads to the conclusion that the large radio source in the vicinity of the AFGL 2591 is an ultra compact HII-region associated with a different object in the background. For lRAS 20126 and IRAS 4 the conditions of the outer layers of their envelopes are estimated by non-LTE radiative transfer modelling of mainly centimetre lines. The results are about T≈30K and nH2≈105cm-3 typical in the outer regions of young stellar envelopes. Inverse P-Cygni profiles of H2CO at 14.488GHz, have been detected which imply kinematics of infall within the envelopes of both sources. This suggests that the sources are at an early evolutionary stage. The profile is more prominent towards lRAS 4, which is a complicated system of more than one component. We suggest that motions of infall might exist in outer layers of the surrounding cloud apart from the regions near the stellar components. Finally the maps of the Orion Bar have confirmed the clumpy medium by the detection of two separate clumps in the region. Their masses are estimated to be about IIM®. Based on radio recombination lines the temperature of the Orion nebula has been found to be 9017 K, reflecting a mature HIIregion. The findings of this study will be combined with the JCMT Spectral Legacy Survey, which is currently collecting data from observations in the submillimetre range towards the same sources.

Host galaxies and late-time evolution of super-luminous supernovae

Chen, Ting-Wan January 2015 (has links)
This thesis is aimed to understand the physical properties of a new type of violent explosions , which have been labelled super-luminous supernovae (SLSNe). These SLSNe are 10 to 100 times brighter than normal SNe. However, the standard paradigm of iron-core collapse cannot account for the origin of these events and the mechanism which powers such luminosities is still not well established. We have used the all -sky survey telescope (e.g. Pan-STARRS1) to identify super-luminous transients, and triggered a global observational network for follow-up observations. The facilities include 8-m Gemini telescopes, 4-m William Herschel Tele- scope, New Technology Telescope (PESSTO) and other 1 to 2-m telescopes. Deep images were collected in the SN late-time phases, which is essential to examine alternative lightcurve models and to distinguish potential energy sources. One crucial strategy is to study the host galaxies of SLSNe Ic, which provide a strong constrain to understand the stellar progenitors of SLSNe Ic, e.g. metallicity. The most reliable method to quantitatively determine oxygen abundances needed for the metallicity measurement, without calibration uncertainties is the "direct method" which requires the electron temperature to be estimated from the auroral [0III] λ4363 line. We presented SLSN hosts that to be low metallicity dwarfs: the host of SN 2010gx is the lowest metallicity host of any type of SNe ever discovered, with an oxygen abundance of 1/20 of the solar value. The hosts of PTF12dam, SN 2011 ke, SN 2012il and LSQ14an, they also follow this low-metallicity trend. We therefore propose that there appears to be a metallicity threshold, and the formation of SLSNe only occurs below 12 + log(O/H) = 8.10 (direct method). The low-metallicity environment is favourable for both the magnetar scenario (as massive stars rotating more rapidly) as well as pair-instability models.

Magnetic fields and X-ray emission in pre-main sequence stars

Johnstone, Colin Philip January 2012 (has links)
In this thesis, I use numerical models of stellar coronae to investigate coronal magnetic fields, X-ray emission, and accretion geometries of classical T Tauri stars. This is based on recently published Zeeman-Doppler Imaging (ZDI) magnetograms. I also investigate the effects of time-variable eclipsing of stellar flares on their observed lightcurves. I investigate how our ability to model stellar magnetic processes is affected by missing magnetic flux in observed ZDI magnetograms and find that the loss of unresolved small-scale field regions has a significant effect on our ability to model magnetically confined X-ray emitting coronae. However, it has little effect on predicted large scale field structures. I survey the sample of classical T Tauri stars with existing ZDI magnetograms and find that the field complexity is correlated with field strength and stellar rotation rate, such that rapidly rotating stars have weak complex fields, and slowly rotating stars have strong simple fields. It is not clear whether this is a result of the finite resolution of the ZDI technique, magnetic star-disc interactions, or the evolution of pre-main sequence stars. Using observed X-ray emission measures and temperatures for each of these stars, I model the closed X-ray emitting coronae and find that they typically extend several stellar radii from the stellar surface. The coronal extent is primarily determined by the complexity of the magnetic field, with simple fields extending a large distance from the stellar surface, and more complex fields being truncated closer to the stellar surface. Using observed mass accretion rates, I predict circumstellar disc truncation radii for these stars and find that they are typically several stellar radii from the stellar surface, with the locations of accretion footpoints being a strong function of the field strengths and complexities. In several cases, the disc is truncated significantly outside the maximum radius at which the corona can extend. This result is significant as studies into magnetospheric accretion generally assume that the magnetic field has a closed geometry at the inner edge of the disc. The lightcurve of a typical stellar flare consists of a single impulsive rise phase followed by a slower exponential decay. However, a large number of the observed flares do not possess typical morphologies, and instead show multiple rise phases, or large dips in their lightcurves. Using the largest flares observed by the Chandra Orion Ultradeep Project, I show that these atypical lightcurves could have been caused by the time variable eclipsing of typical flares due to the rotation of the host stars. However, this interpretation is unable to account for the large number of atypical lightcurves in the COUP flare sample, and so other physical mechanisms must be involved. Significantly, I find that most flares that are eclipsed by their host stars still retain typical lightcurves showing no obvious signs of eclipsing.

Mass loss from hot, luminous stars

Burnley, Adam Warwick January 2003 (has links)
No description available.

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