We present a spectral analysis of the spectrum from the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) of the southern VY Scl nova-like variable BB Doradus, obtained as part of a Cycle 20 HST/COS survey of accreting white dwarfs (WDs) in cataclysmic variables. BB Dor was observed with COS during an intermediate state with a low mass accretion rate, thereby allowing an estimate of the WD temperature. The results of our spectral analysis show that the WD is a significant far-ultraviolet (FUV) component of the spectrum with a temperature of about 35,000-50,000 K, assuming a WD mass of 0.80 M-circle dot (log(g) = 8.4). The disk, with a mass accretion rate of approximate to 10(-10) M-circle dot yr(-1), contributes about 1/5 to 1/2 of the FUV flux.

We present infrared spectroscopy of the classical nova V339 Del, obtained over an similar to 2-yr period. The infrared emission lines were initially symmetrical, with half width half-maximum velocities of 525 km s(-1). In later (t greater than or similar to 77 d, where t is the time from outburst) spectra, however, the lines displayed a distinct asymmetry, with a much stronger blue wing, possibly due to obscuration of the receding component by dust. Dust formation commenced at approximately day 34.75 at a condensation temperature of 1480 +/- 20 K, consistent with graphitic carbon. Thereafter, the dust temperature declined with time as T-d alpha t(-0.346), also consistent with graphitic carbon. The mass of dust initially rose, as a result of an increase in grain size and/or number, peaked at approximately day 100, and then declined precipitously. This decline was most likely caused by grain shattering due to electrostatic stress after the dust was exposed to X-radiation. The appendix summarizes Planck means for carbon and the determination of grain mass and radius for a carbon dust shell.

Neste trabalho foi desenvolvido um novo m´etodo de s´ntese espectral para modelar o disco de acres¸cao de vari´aveis catacl´smicas (VCs) nao magn´eticas. O principal objetivo deste trabalho ´e analisar a emissao do cont´nuo e das linhas em uma ampla faixa espectral no ultravioleta (UV). O disco ´e separado em an´eis concentricos e, para cada anel uma atmosfera de disco com vento ´e calculada. Na base, as atmosferas sao calculadas consistemente com o vento, tendo a distribui¸cao de densidade dos modelos de atmosferas de disco de Wade e Hubeny. A estrutura ´e calculada no sistema co-m´ovel com um perfil de velocidade vertical obtido da solu¸cao da equa¸cao de Euler para um disco de acres¸cao. O comportamento das linhas e do cont´nuo como fun¸cao da inclina¸cao orbital ´e consistente com as observa¸coes. Tamb´em foi verificado que a taxa de acres¸cao influi sobre a temperatura do vento levando `as mudan¸cas correspondentes nas intensidades relativas das linhas. Foi encontrado que a massa da prim´aria tem uma forte influencia na profundidade dos perfis de absor¸cao. Tamb´em, encontramos que a os perfis de linha sao fortemente sens´veis ao incremento da taxa de perda de massa, aumentado a intensidade das linhas de emissao. Foram escolhidos dados espectrosc´opicos no UV de duas VCs Nova-like (NL) de baixa inclina¸cao, RW Sex e V3885 Sgr e dois sistemas de alta inclina¸cao, RW Tri e V347 Pup. Uma concordancia dos perfis em emissao dos modelos foi encontrada quando confrontados com os dados no caso de sistemas de alta inclina¸cao. Uma falta de fluxo nas linhas de alta ioniza¸cao Civ ¸¸1548,1551 and Nv ¸¸1238,1242, pode ser o sinal da influencia da boundary layer (BL) ou da influencia da irradia¸cao das regioes externas do vento pelo disco interno. Estas influencias seriam cruciais no caso de sistemas baixa inclina¸cao, mas sao menores no caso de sistemas de alta inclina¸cao. / We have developed a new spectral synthesis method for modeling the accretion disk of non-magnetic cataclysmic variables (CVs). The aim of this work is to analyze the continuum and line emission of disks in a wide ultraviolet (UV) spectral range. The disk is separated in concentric rings, and for each ring a wind plus disk atmosphere are calculated. The wind atmospheres are calculated consistently with a density given by Wade and Hubeny disk-atmosphere models at their base. The structure is calculated in the co-moving frame with a vertical velocity profile defined by the Eulers equation solution for the disk wind. We found that the resulting line and continuum behavior as a function of the orbital inclination is consistent with the observations. We also verify that the accretion rate changes the wind temperature, leading to corresponding trends in the intensity of lines. We found that the primary mass has a strong effect on the absorption profiles depth. It was verified that the lines profiles are strongly sensitive to the wind temperature structure and a rise of mass loss rate increases the line intensity. Selected UV data for two high orbital inclination nova-like (NL) CVs, RW Tri and V347 Pup, were confronted with synthetic spectra. The line widths and profiles are reasonably well reproduced by the models. A lack of flux in some high ionization lines (C iv ¸¸1548,1551 and Nv ¸¸1238,1242) may be the signature of the boundary layer (BL) effect and/or the irradiation of outer wind by inner disk. We also found that for high inclination systems the vertical wind structure is less important than for low inclination system models.

O estudo da envoltória de novas clássicas permite diagnosticar com precisão aceitável as condições físicas nos estágios avançados do fenômeno de nova. A Nova Velorum 1999, foi a mais brilhante em 24 anos, sendo um ótimo caso para este estudo. Trata-se de uma nova de neônio, formada em um sistema onde se acredita que a primária seja uma anã branca massiva de O-Ne-Mg. Neste trabalho seguimos a evolução espectral deste objeto nos primeiros três anos após sua erupção verificando um aumento da ionização aproximadamente 200 dias após o máximo visual. A partir de 565 dias após o máximo observamos um aumento do continuo azul que provavelmente reflete o restabelecimento do disco de acréscimo. Derivamos uma velocidade de ejeção do envelope da ordem de aproximadamente 1600 km/s e a escala de tempo de amortecimento no meio interestelar (aproximadamente 10 anos). Os principais resultados desta análise sugerem uma envoltória heterogênea sem simetria esférica. Estimamos também a densidade e temperatura eletrônica da envoltória bem como a evolução da temperatura da fonte central. Com estes dados estimamos limites inferiores para as abundâncias numéricas de He, N, O, Ne, S, Ar e Fe, confirmando ser uma nova de neônio. As abundâncias de oxigênio e neônio foram relativamente baixas se comparadas a outras novas de neônio, embora a abundância numérica deste último tenha sido a maior entre os elementos pesados desta nova. Outro fato relevante foi encontrar uma abundância de ferro bastante alta se comparada com o valor solar. / The study of the classical novae shells allows to diagnosis with accuracy the physical conditions of the advanced phases in nova phenomenum. Nova Velorum 1999 (V382 Vel) was the brightest nova in the last 24 years being a good candidate to this study. This object probably is a neon nova wich is formed in a system where, according to theoretical models, there is an O-Ne-Mg white dwarf primary. In this work we present the spectral evolution of Nova Velorum 1999 shell during the first 3 years after the visual maximum. We verified an increase in ionization about 200 days after the outburst. In addition, 565 days after the outburst an increase in the blue continuum was observed, which is probably due to the reestablishment of the accretion disk. A shell ejection velocity of 1600 km/s and a damping time-scale of about 10 years were observed. The main results suggested that the shell is inomogeneous and non-spherical. We also estimated shell eletronic temperatures and densities as well as the central source temperature. With these data we estimated lower limits to He, N, O, Ne, S, Ar and Fe numerical abundances. The results confirmed that this nova is a neon nova. The oxygen and neon abundance limits were relatively low when compared to other neon nova, but the neon abundances in this nova were high when compared with other heavy element abundances in this object. It was also found that iron is enhanced in the shell, when compared to solar value.

A nova is a cataclysmic variable star consisting of a white dwarf and a main sequence, subgiant or red giant companion. In a nova explosion, the white dwarf undergoes a thermonuclear runaway on its surface, as a result of build-up of accreted material from its companion. This leads to a large expulsion of matter from the WD surface, as well as a dramatic increase in the optical magnitude of the system. In this thesis, 5GHz C-band e-MERLIN observations of two novae (V959 Mon and V339 Del) and one symbiotic star (AG Pegasi) are presented. V959 Mon is a classical nova which was discovered in June 2012 by the Fermi Large Area Telescope as a gamma-ray source. High resolution e-MERLIN images of V959 Mon were made at six epochs following the June 2012 outburst, between 90 and 615 days after the Fermi discovery. The first four e-MERLIN observations revealed a morphology which was aspherical and expanding in the east-west direction. However, in the last two epochs, V959 Mon's ejecta morphology changed from being elongated east-west to being elongated north-south. Constant velocity models fit to measurements of V959 Mon's angular size at each epoch indicated that the expansion rate of the north-south component was significantly slower than that of the east-west component. The e-MERLIN observations of V339 Del and AG Pegasi were not as well resolved as the observations of V959 Mon, and only limited analysis of their ejecta structure was possible. In order to understand the complicated morphology seen in the e-MERLIN observations of V959 Mon, radio emission models of nova ejecta were constructed, and e-MERLIN observations of them were simulated. When constructing the models, two possible explanations for V959 Mon's morphology were explored. Firstly, the possibility was considered that the elongation observed in V959 Mon's ejecta, which changed over time from east- west to north-south, was spurious, resulting from incomplete sampling of the uv plane by e-MERLIN. To investigate this, an expanding spherical shell of ejecta was simulated, the extent to which its shape was distorted in simulated e-MERLIN images of it was investigated. Secondly, emission from an ejecta model featuring two components - one fast-moving component in the east-west direction, and another, more slow-moving component in the north-south direction - was simulated, in order to determine whether such an ejecta structure could result in the morphology seen in the e-MERLIN observations of V959 Mon. It was found that the spherical, Hubble flow model simulations were a reasonably good fit to V959 Mon's light curve, but could not explain the east-west elongation seen in V959 Mon's ejecta at the early epochs. Simulated observations of the two-component model were morphologically similar to V959 Mon, but produced light curves which were much fainter than V959 Mon's, indicating that a more in-depth analysis of two-component models of nova ejecta is necessary.

Classical nova explosions are stellar explosions that take place in close binary systems with an energy release only exceeded by gamma-ray bursts and supernova explosions. Matter from the white dwarf flows through the inner lagrangian point and spirals in towards the white dwarf for about 10^4-10^5 years, forming an accretion disk around it. Ultimately, part of this hydrogen-rich matter piles-up on top of the compact object and becomes partially degenerate due to the high densities attained. Consequently, temperature is allowed to rise, but the envelope does not experience any expansion. Actually, this is the key mechanism that controls the subsequent phases and powers a thermonuclear runaway, which is followed by an ejection of part of the accreted envelope. The ejecta are enriched with the products from the nuclear processes, presenting a final metallicity much above solar. This model, introduced in the early 70s, is a solid theory that can account for the gross scenario of nova explosions. Nevertheless, the theory relies on the fact that a mixing episode with matter from the white dwarf core has to take place at the core-envelope interface to successfully account for the high metallicities inferred from observations. During the past 40 years, theoreticians have performed many one-dimensional simulations, which can reproduce the abundances in the ejecta and other important observational properties. However, these calculations performed in spherical symmetry cannot study the mixing process, since they exclude a suite of very important multi-dimensional effects, such as convection. Therefore, multi-dimensional calculations are required to shed light into the mixing episode. In this thesis we have performed two- and three- dimensional simulations of CO novae to study the mixing mechanisms operating at the core-envelope interface, how convection sets in and how the deflagration spreads over the domain, by means of the Eulerian, parallelized, hydrodynamical FLASH code. The two-dimensional results show how convection sets in at the innermost envelope layers, after the appearance of temperature fluctuations that arise from the interface. Convection, in turn, powers the formation of kelvin-Helmholtz instabilities, which efficiently dredge-up 12C from the core and carry it into the envelope, reproducing correctly the high metallicity found in the ejecta. This result solves the controversy generated by the two existing two-dimensional calculations up-to-date. We have also realized a sensitivity study to analyze the impact of some initial parameters, such as the temperature perturbation, resolution of the simulations and the size of the computational domain. The results point out that these parameters have a negligible impact on the degree of mixing and, therefore, the calculations are not affected by numerical artifacts. Although two-dimensional calculations can quantitatively reproduce the mixing episode, they cannot describe correctly the convective pattern due to conservation of vorticity, which translates into recombination of the convective cells. Therefore, we have extended the work to three dimensions and performed the first three-dimesional model of mixing in classical novae up-to-date. These calculations can successfully reproduce the intermittency present in turbulent convection, with an energy cascade into smaller scales which clearly fulfills the Kolmogorov theory, while the thermonuclear runaway continues propagating with almost spherical symmetry. Mixing proceeds through the filamentary structure powered by robust kelvin-Helmholtz instabilitites that arise from the interface, resulting in a CNO enhancement which agrees with observations. This convective profile also generates density contrasts that could be the origin of the inhomogeneous distribution of chemical species. / Les explosions de noves tenen lloc en un sistema estel.lar binari, on un dels estels ha arribat a la fi de la seva vida convertit en una nana blanca. En sistemes binaris molt propers, l'estel acompanyant cedeix part del seu gas (material ric en hidrogen), el qual s'arremolina al voltant de la nana blanca durant prop de 10^4 - 10^5 anys. Una fracció d'aquest material acaba apilant-se a la superfície de l'objecte compacte i esdevé parcialment degenerat com a conseqüència de l'elevada densitat. Aquest fet és clau en el procés, ja que permet que la temperatura augmenti sense que es produeixi una expansió de l'embolcall, desencadenant un allau termonuclear i finalment, l'ejecció de matèria. El material ejectat està enriquit amb els isòtops processats en les reaccions nuclears, presentant una metal.licitat molt superior a la solar. Aquest model, presentat a principis dels anys 70, és una teoria sòlida que explica raonablement l'explosió de noves. No obstant, la teoria rau en el fet que s'ha de produir un procés de barreja entre el material de la nana blanca i el material de les capes més internes de l'embolcall per poder explicar l'alta metal.licitat que s'observa en el material ejectat. Durant els últims 40 anys, s'han fet molts estudis en una dimensió que aconsegueixen reproduir correctament les abundàncies del material ejectat i altres importants propietats observacionals, però que no poden explicar com es produeix el procés de barreja, ja que aquests càlculs amb simetria esfèrica exlouen tota una sèrie d'importants fenòmens multidimensionals. Per tant, per estudiar aquests aspectes de la teoria es requereixen estudis multidimensionals. En aquesta tesi hem realitzat simulacions en dues i tres dimensions de noves de CO per estudiar els mecanismes de barreja que es produeixen a la interfície del nucli de la nana blanca i l'embolcall, com s'estableix la convecció i com es propaga el front deflagratiu, mitjançant el codi hidrodinàmic FLASH, que és Eulerià i està paral.lelitzat. Els resultats en dues dimensions mostren com es genera convecció a les capes més internes de l'embolcall, després de la formació de fluctuacions de temperatura a la interfície. La convecció, al seu torn, origina inestabilitats Kelvin-Helmholtz que transporten eficientment 12C del nucli cap a l'embolcall, aconseguint reproduir correctament el grau de metal.licitat observat. Aquest resultat resol la controvèrsia generada pels dos estudis en dues dimensions realitzats fins ara. També hem realitzat un estudi per analitzar l'impacte dels paràmetres inicials tals com la perturbació inicial, la resolució de les simulacions o les dimensions del domini computacional. Els resultats indiquen que cap d'aquests paràmetres influeix en el grau de barreja final i, per tant, que els càlculs no estan condicionats per aspectes numèrics. Finalment, hem presentat el primer model tridimensional de barreja de noves fet fins ara. Aquest càlcul és necessari, ja que les simulacions bidimensionals, tot i que quantitativament reprodueixen la barreja esperada, no poden representar el patró convectiu correctament, degut a la conservació de la vorticitat, fent que les cel.les convectives esdevinguin cada cop més grans. El nostre càlcul aconsegueix reproduir el comportament intermitent de la turbulència, amb una cascada d'energia que flueix cap a escales cada cop més petites, tal i com prediu la teoria de Kolmogorov, alhora que el front convectiu avança pràcticament amb simetria esfèrica. La barreja procedeix a través de l'estructura filamentosa originada per l'aparició de potents inestabilitats Kelvin-Helmholtz a la interfície, obtenint-se una metal.licitat final a l'embolcall que concorda amb els valors observacionals. Aquest patró convectiu també genera contrastos de densitat que podrien ser l'origen de la distribució inhomogènia que presenten les espècies químiques.

In this work present the broad-band noise structure in the 2-60 keV data of Cataclysmic Variables (CVs) with Anomalous X-Ray Pulsars (AXPs) and Soft Gamma Repeaters (SGRs). We analyzed Rossi X-ray Timing Explorer (RXTE) PCA data and derived time series from 27 CVs, 4 AXPs and 1 SGR using the RXTE archive. In general, CVs of different types all show broad band noise which can be fitted with power laws, using exponentional cut-offs, and Lorentzians in a similar way to power spectral (noise) characteristics of X-ray Binaries (XRBs). In general terms the power spectra show a power law index of (-)1.2-2. A rather large scale flattening of the power spectra exits in nonmagnetic systems in the low to very low frequency range. We observe that in low and high states/outbursts the noise in the high frequency range and low frequency range is changed. CVs show considerably low frequency noise. In addition, we recovered several possible QPOs in the X-ray wavelengths from CVs mainly from Intermediate Polar systems. AXP and SGR sources which are thought to be powered by either magnetic decay or accretion show band limited noise in their low frequencies. We also correlated their equal time interval noise characteristic with their burst states and discovered that in the two AXPs (1E 2259+586, 1E 1048.1-5937) noise correlates with their bursts.

In this work, the CHANDRA ACIS-S3 data of the old classical nova RR Pic (1925) is presented. The source is detected with a count rate of 0.067 (+-0.002) c/s in the 0.3-5.0 keV energy range. The orbital period of the binary system in X-ray wavelengths is detected. X-ray spectrum of RR Pic can be represented by a composite model of bremsstrahlung with a photoelectric absorption, two absorption lines centered around 1.1-1.4 keV and 5 Gaussian emission lines centered at around 0.3-1.1 keV . The best t bremsstrahlung temperature derived from the model ranges from 0.99 to 1.60 keV and the unabsorbed X-ray flux is found to be 2.51 (+0.39 -1.21) erg/cm2/s in the 0.3-5.0 keV range.The absorption lines correspond to several transitions of Fe, Ne and Na. The emission lines correspond to various transitions of S, N, O, C, Ne and Fe / and observations with better spectral resolution are needed to determine the exact transitions. The source spectrum is better explained with photoionized plasma model rather than the cooling flow model. It has also been found that the neutral Hydrogen column density differs for orbital minimum and orbital maximum spectra with values 0.25 (+0.23 -0:18)*10^22 atoms/cm2 and 0.64 (+0.13 -0.14)*10^22 atoms/cm2 respectively at 3 sigma confidence level. The difference of neutral Hydrogen column density between the maximum and minimum phase spectra indicates existence of a warm absorbing region on the disc at the location of the impact zone.

It has recently been discovered that some, if not all, classical novae emit GeV gamma-rays during outburst, but the mechanisms involved in the production ofgamma-rays are still not well understood. We present here a comprehensive multiwavelength data set-from radio to X-rays-for the most gamma-ray-luminous classical nova to date, V1324 Sco. Using this data set, we show that V1324 Sco is a canonical dusty Fe II-type nova, with a maximum ejecta velocity of 2600 km s(-1) and an ejecta mass of a few x 10(-5) M-circle dot. There is also evidence for complex shock interactions, including a double-peaked radio light curve which shows high brightness temperatures at early times. To explore why V1324. Sco was so gamma-ray luminous, we present a model of the nova ejecta featuring strong internal shocks and find that higher gamma-ray luminosities result from higher ejecta velocities and/or mass-loss rates. Comparison of V1324. Sco with other gamma-ray-detected novae does not show clear signatures of either, and we conclude that a larger sample of similarly well-observed novae is needed to understand the origin and variation of gamma-rays in novae.

This thesis is primarily concerned with a description of four types of stochastic algorithms, namely the genetic algorithm, the continuous parameter genetic algorithm, the particle swarm algorithm and the differential evolution algorithm. Each of these techniques is presented in sufficient detail to allow the layman to develop her own program upon examining the text. All four algorithms are applied to the optimization of a certain set of unconstrained problems known as the extended Dixon-Szegö test set. An algorithm's performance at optimizing a set of problems such as these is often used as a benchmark for judging its efficacy. Although the same thing is done here, an argument is presented that shows that no such general benchmarking is possible. Indeed, it is asserted that drawing general comparisons between stochastic algorithms on the basis of any performance criterion is a meaningless pursuit unless the scope of such comparative statements is limited to specific sets of optimization problems. The idea is a result of the no free lunch theorems proposed by Wolpert and Macready. Two methods of presenting the results of an optimization run are discussed. They are used to show that judging an optimizer's performance is largely a subjective undertaking, despite the apparently objective performance measures which are commonly used when results are published. An important theme of this thesis is the observation that a simple paradigm shift can result in a different decision regarding which algorithm is best suited to a certain task. Hence, an effort is made to present the proper interpretation of the results of such tests (from the author's point of view). Additionally, the four abovementioned algorithms are used in a modelling environment designed to determine the structure of a Magnetic Cataclysmic Variable. This 'real world' modelling problem contrasts starkly with the well defined test set and highlights some of the issues that designers must face in the optimization of physical systems. The particle swarm optimizer will be shown to be the algorithm capable of achieving the best results for this modelling problem if an unbiased <font face="symbol">c</font>2 performance measure is used. However, the solution it generates is clearly not physically acceptable. Even though this drawback is not directly attributable to the optimizer, it is at least indicative of the fact that there are practical considerations which complicate the issue of algorithm selection. / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2006. / Mechanical and Aeronautical Engineering / unrestricted