Dissertations / Theses on the topic 'Astrophysics Technique'
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1
MacRae, John Hamish Kenneth. "The detection of very high energy cosmic gamma rays using the atmospheric Cerenkov technique." Thesis, Durham University, 1985. http://etheses.dur.ac.uk/7854/.
Abstract:
This thesis is concerned with the detection of very high energy cosmic gamma rays using the atmospheric Cerenkov technique. A general introduction to gamma ray detection techniques is followed by a detailed discussion of the principles of the atmospheric Cerenkov technique and the history of its use prior to this work. The design and operation of the University of Durham facility in Dugway, Utah is described in depth. Monte Carlo computer simulations have been developed to assist in both the understanding of the equipment and the analysis of the results for the Durham facility. The variation of the response of the array with zenith angle and detector threshold has been investigated and the aperture function of a single telescope calculated. The latter has been found to be a complicated function of both zenith angle and detector threshold. Computer simulations have also been developed to aid in the design of a camera to record two-dimensional Cerenkov light images from small extensive air showers, and to provide a means of testing analysis routines; these are discussed. The camera is located at the F.L. Whipple Observatory in Arizona. The techniques employed in the analysis of data recorded by the Dugway facility are discussed, and a computer package developed to aid in the routine aspects of the analysis is described. Results of observations from two sources, Cygnus X-3 and PSR0531, are presented, with particular reference to periodicities inherent in the sources and to bursts of gamma ray emission. The discussion of the results includes a review of the various models which have been proposed for Cygnus X-3.
2
Tisserand, Patrick. "Recherches d'astres sombres massifs du halo galactique par la technique des microlentilles gravitationnelles avec les caméras d'EROS-2." Phd thesis, Université de Nice Sophia-Antipolis, 2004. http://tel.archives-ouvertes.fr/tel-00008267.
Abstract:
Le problème de la masse manquante galactique prend sa source dans l'étude des courbes de rotation des galaxies spirales. Une des hypothèses privilégiées durant les vingt dernières années est la présence d'un halo entourant chaque galaxie composé d'objets compacts sombres (les machos) qui peuvent être détectés indirectement par le phénomène gravitationnel de microlentille. Le travail présenté dans cette thèse concerne l'analyse des courbes de lumière de plus de 55 millions d'étoiles des deux Nuages de Magellan, observées de juin 1996 à février 2003, avec les caméras de l'expérience EROS2. Deux études différentes ont été menées dans le but de détecter d'éventuels objets de 10^(-4) à 1000 masses solaires, complétant ainsi les contraintes déjà posées par EROS entre 10^(-7) et 10 masses solaires. Les candidats découverts sont en nombre insuffisant pour rendre compte de la masse manquante et ne permettent pas non plus de confirmer le signal présenté par le groupe MACHO.
3
Filip, Münz. "La simulation électronique de CELESTE : étude des biais et application à l'obsevation de la Nébuleuse de Crabe." Phd thesis, Université Paris-Diderot - Paris VII, 2003. http://tel.archives-ouvertes.fr/tel-00003334.
Abstract:
Cette thèse présente le domaine de l'astronomie gamma et les principales sources de notre Galaxie (pulsars et SNRs) et celles situées à des distances cosmologiques (noyaux actifs de galaxie). La technique Cerenkov permet aux observatoires au sol de détecter des rayons gamma d'énergie du GeV absorbés dans l'atmosphère après avoir développé une cascade électromagnétique. Le projet CELESTE a atteint le but d'un seuil bas de 30 GeV, utilisant une grande surface de collection de l'ancienne centrale solaire de Thémis (Pyrénées Orientales). Une étude des principaux éléments du dispositif - un système de déclenchement mixte analogique-digital et un échantillonnage à haute fréquence du signal enregistré en plusieurs points du champ - représente la partie essentielle de cette thèse. Le code développé pour la simulation de la chaîne électronique de l'expérience a été vérifié profondément en faisant des comparaisonsavec les données réelles.L'incertitude de la calibration en amplitude et de la mise-en-temps pour le déclenchement se traduit en erreurs sur la détermination du seuil en énergie et la surface effective. Les sources des biais éventuels sont étudiées (sur les différents points de la chaîne de simulation). Ces considérations sont appliquées aux observations de la Nébuleuse de Crabe, détectée par CELESTE déjà en 2000.
4
Kreckelbergh, Stephane. "Contrôle longitudinal et caractérisation optique du détecteur Virgo." Phd thesis, Université Paris Sud - Paris XI, 2005. http://tel.archives-ouvertes.fr/tel-00012159.
Abstract:
Le détecteur Virgo est constitué d'un interféromètre de Michelson avec des cavités Fabry-Perot de 3 km de long dans les bras et utilise la technique de recyclage de puissance. Il a pour but la détection directe des ondes gravitationnelles émises par des sources astrophysiques. Pour atteindre sa sensibilité, Virgo doit être emmené à son point de fonctionnement par des asservissements tant longitudinaux qu'angulaires. Pour cela, nous avons mis en place un algorithme de contrôle longitudinal ("lock") qui partant d'un interféromètre libre l'emmène à son point de fonctionnement. Pour arriver à ce résultat, nous utilisons la technique de Pound-Drever qui nous permet d'avoir un signal sensible à la variation de la position d'une cavité optique par rapport à la résonance.
Nous avons developpé deux algorithmes. Le premier s'inspire de celui utilisé par la collaboration LIGO. Nous arrivons au point de fonctionnement en contrôlant successivement les quatres longueurs caractéristiques de Virgo. L'application de cet algorithme sur l'instrument s'est soldé par un échec dont les causes sont liées aux différences entre Virgo et LIGO. Le deuxième algorithme nous permet de contrôler simultanément ces quatres longueurs en étant sur la mi frange de l'interféromètre de Michelson. Nous emmenons ensuite l'interféromètre en quelques minutes à son point de fonctionnement de manière déterministe.
Une autre partie de la thèse consiste en la mesure in situ des paramètres optiques nécessaires à la compréhension de l'instrument. Ceci nous a permis à la fois de faire accorder la simulation avec les données et de préparer l'algorithme d'acquisition du lock de Virgo.
Enfin, nous nous intéressons à l'impact de la technique d'Anderson utilisée pour le contrôle angulaire des miroirs sur le contrôle longitudinal des cavités optiques. Nous en montrons le mécanisme et évaluons son impact sur le lock de Virgo.
5
Platts, Emma. "Computational analysis techniques using fast radio bursts to probe astrophysics." Doctoral thesis, Faculty of Science, 2021. http://hdl.handle.net/11427/33921.
Abstract:
This thesis focuses on Fast Radio Bursts (FRBs) and presents computational techniques that can be used to understand these enigmatic events and the Universe around them. Chapter 1 provides a theoretical overview of FRBs; providing a foundation for the chapters that follow. Chapter 2 details current understandings by providing a review of FRB properties and progenitor theories. In Chapter 3, we implement non-parametric techniques to measure the elusive baryonic halo of the Milky Way. We show that even with a limited data set, FRBs and an appropriate set of statistical tools can provide reasonable constraints on the dispersion measure of the Milky Way halo. Further, we expect that a modest increase in data (from fewer than 100 FRB detections to over 1000) will significantly tighten constraints, demonstrating that the technique we present may offer a valuable complement to other analyses in the near future. In Chapter 4, we study the fine time-frequency structure of the most famous FRB: FRB 121102. Here, we use autocorrelation functions to maximise the structure of 11 pulses detected with the MeerKAT radio telescope. The study is motivated by the low time-resolution of MeerKAT data, which presents a challenge to more traditional techniques. The burst profiles that are unveiled offer unique insight into the local environment of the FRB, including a possible deviation from the expected cold plasma dispersion relationship. The pulse features and their possible physical mechanisms are critically discussed in a bid to uncover the nature and origin of these transients.
6
Lindgren, Sara. "Metallicity determination of M dwarfs." Doctoral thesis, Uppsala universitet, Observationell astrofysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-332102.
Abstract:
M dwarfs constitute around 70% of all stars in the local Galaxy. Their multitude together with their long main-sequence lifetimes make them important for studies of global properties of the Galaxy such as the initial mass function or the structure and kinematics of stellar populations. In addition, the exoplanet community is showing an increasing interest for those small, cold stars. However, very few M dwarfs are well characterized, and in the case of exoplanetary systems the stellar parameters have a direct influence on the derived planet properties. Stellar parameters of M dwarfs are difficult to determine because of their low surface temperatures that result in an optical spectrum dominated by molecular lines. Most previous works have therefore relied on empirical calibrations. High-resolution spectrographs operating in the infrared, a wavelength region less affected by molecular lines, have recently opened up a new window for the investigation of M dwarfs. In the two first papers of this thesis we have shown that we can determine the metallicity, and in some cases the effective temperature, using synthetic spectral fitting with improved accuracy. This method is time consuming and therefore not practical or even feasible for studies of large samples of M dwarfs. When comparing our results from the high-resolution studies with available photometric calibrations we find systematic differences. In the third paper we therefore used our sample to determine a new photometric metallicity calibration. Compared to previous calibrations our new photometric calibration shows improved statistical characteristics, and our calibration gives similar results as spectroscopic calibrations. In a comparison with theoretical calculations we find a good agreement of the shapes and slopes of iso-metallicity lines with our empirical relation. Applying the photometric calibration to a sample of M dwarfs with confirmed exoplanets we find a possible giant planet-metallicity correlation for M dwarfs.
7
Sajjad, Saeeda. "Développement d'outils de simulation et de reconstruction de gerbes de particules pour l'astronomie gamma avec les futurs imageurs Tcherenkov." Phd thesis, Université Montpellier II - Sciences et Techniques du Languedoc, 2007. http://tel.archives-ouvertes.fr/tel-00408835.
Abstract:
Le futur de l'astronomie gamma au sol repose sur l'utilisation de grands réseaux d'imageurs Tcherenkov atmosphériques (IACT) avec des capacités améliorées : seuil en énergie plus bas, meilleure sensibilité, meilleurs résolution et pouvoir de rejet. Pour concevoir ces systèmes et optimiser leurs caractéristiques, il est nécessaire de comprendre les gerbes atmosphériques et de disposer d'outils de simulation adaptés permettant d'évaluer les performances des réseaux. La première partie de cette thèse traite des gerbes atmosphériques, des propriétés de la lumière Tcherenkov qu'elles émettent et de leur simulation. La seconde partie présente les outils que nous avons développés pour la simulation des télescopes à imagerie Tcherenkov atmosphérique et les caractéristiques des images qu'ils obtiennent. La troisième partie de cette thèse contient une présentation des outils dévéloppés pour la reconstruction de la source, du pied de gerbe et de l'énergie ainsi que des propositions pour la séparation gamma-hadrons. Dans la dernière partie, ces outils sont utilisés pour étudier deux grands réseaux de télescopes à deux altitudes différentes et pour donner leurs performances pour la détection des rayons gamma.
8
Popescu, Marcel. "Techniques d'observation spectroscopique d'astéroïdes." Phd thesis, Observatoire de Paris, 2012. http://tel.archives-ouvertes.fr/tel-00785991.
Abstract:
L'objectif fondamental des sciences planétaires est la compréhension de la formation et de l'évolution du Système Solaire. Pour atteindre cet objectif les astéroïdes présentent un intérêt tout particulier pour la communauté scientifique. En effet, nous pouvons regarder la population astéroïdale comme une fenêtre vers le passée, par laquelle nous regardons les débuts de la formation du système planétaire. Ils sont les témoins des premiers moments de la formation des planètes gardant dans leur structure la complexité chimique de la nébuleuse primordiale. Pour cette raison, les études physiques et dynamiques de ces corps nous apportent des informations essentielles sur l'histoire et l'évolution de notre Système Solaire et plus généralement sur la formation des systèmes planétaires. Pendant ma thèse j'ai développé l'application Modelling for Asteroids (acronyme M4AST). M4AST est un service en libre service sur internet permettant la modélisation des surfaces d'astéroïdes en utilisant plusieurs approches théoriques. M4AST est composé d'une base de données contenant quelques 2500 spectres d'astéroïdes et d'une bibliothèque de routines permettant la modélisation et l'obtention de plusieurs paramètres minéralogiques. La base de données est accessible aussi bien par les biais des protocoles de l'Observatoire Virtuel (OV-Paris) que par sa propre interface. Le service est accessible depuis l'adresse http:// cardamine.imcce.fr/m4ast. M4AST permet plusieurs types d'analyses : classification taxonomique, modélisation de l'altération spatiale, comparaison avec les spectres des météorites et des minéraux terrestres, calculs des centres et des surfaces des bandes. J'ai participé à plus de 10 campagnes d'observations pour la caractérisation physique et dynamique des astéroïdes. Les observations spectroscopiques ont servi à la caractérisation minéralogique des surfaces d'astéroïdes. L'astrométrie a plutôt servi à la confirmation et la sécurisation de nouvelles découvertes d'astéroïdes. Pendant la thèse, j'ai observé et caractérisé les spectres en infrarouge proche de huit astéroïdes géocroiseurs : 1917, 8567, 16960, 164400, 188452, 2010 TD54, 5620, and 2001 SG286. Ces observations ont été obtenues avec le télescope IRTF et du spectrographe SpeX, en employant l'infrastructure CODAM de l'Observatoire de Paris. Pour chaque astéroïde j'ai proposé des solutions minéralogiques. Une révision de leur taxonomie a aussi été effectuée pour cinq astéroïdes de mon échantillon. Quatre des objets sont des objets à faible delta-V, qui sont des cibles souhaitables/possibles pour des missions spatiales. L'astéroïde (5620) Jasonwheeler montre un spectre similaire à ceux des météorites chondritiques. J'ai observé et modélisé six astéroïdes de la ceinture principale. (9147) Kourakuen, (854) Frostia, (10484) Hecht and (31569) 1999 FL18 montrent des caractéristiques des astéroïdes du type V; (1333) Cevenola, (3623) Chaplin sont du type taxonomique S. Quelques astéroïdes de cet échantillon sont particuliers : (854) Frostia est un astéroïde binaire, (10484) Hecht et (31569) 1999 FL18 ont des gémeaux dynamiques, (1333) Cevenola et (3623) Chaplin sont des objets avec des courbes de lumières à grandes amplitudes. La classification taxonomique, la comparaison avec les météorites, permettent l'établissement des solutions minéralogiques intéressantes et des ressemblances avec les météorites de la classe des howardites, eucrites et diogenites.
9
Adamakis, Sotiris. "Application of statistical analysis techniques to solar and stellar phenomena." Thesis, University of Central Lancashire, 2009. http://clok.uclan.ac.uk/20908/.
Abstract:
Currently, solar observers are investigating spectroscopic images of the Sun's outermost atmosphere (the corona), which are challenging long-held views on the density and temperature structure of this environment. The corona is "filled" with magnetic strands but determining their precise nature is not straightforward. One way of revealing the nature of the coronal heating mechanism is by comparing simple theoretical one dimensional hydrostatic loop models with observations of the temperature and/or density structure along these features. The most wellknown method for dealing with comparisons like that is the x2 approach. In this research we consider the restrictions imposed by this approach and present an alternative way for making model comparisons using Bayesian statistics. In order to quantify our beliefs we use Bayes factors and information criteria such as AIC and BIC. Three simulated data-sets are analysed in order to validate the procedure and assess the effects of varying error bar size. Another three datasets (Ugarte-Urra ci at., 2005; Priest ci at., 2000; Young ci al., 2007) are analysed using the method described above. For the Ugarte-Urra ci at. and Young ci al. data-sets, we conclude apex dominant heating is the likely heating candidate, whereas the Priest ci al. data-set implies basal heating. Note that these new results (regarding the Ugarte-Urra ci at. and Priest ci at. data-sets) are different from those obtained using the chi-squared statistic. The second research project involves extensive model comparison against solar flare plasma observed cooling curves. After a solar flare erupts, flare-loops form which cool over thousands of seconds. How the plasma cools over time is investigated. In this case, we test the adequacy of the zero-dimensional EBTEL (Enthalpy-Based Thermal Evolution of Loops) model as introduced by Klimchuk, Patsourakos, and Cargill (2008). An interesting approach here is to define the form of the non-thermal heating input to the system and compare it with the thermal heating input. For the data-set under investigation (Raftery et al., 2009) a Full-Gaussian energy profile is proposed. Also, from the data it is not possible to distinguish which of the thermal or non-thermal heat flux is more dominant, so both can be equally considered for temperature, density and pressure evolution of the system. Finally, the last part of this research is dedicated to recurrent nova outbursts. RS Ophiuehi is a nova produced by a white dwarf star and a red giant. In this case the white dwarf will steadily acerete gases on its surface from the red giant's outer atmosphere. About every twenty years, enough material will be accreted on the white dwarf's surface in order to produce an eruption. Over the past one hundred years at least five such outbursts have been observed. As another application of Bayesian model comparison techniques, curve fitting models are tested against light curves of RS Ophiuchi outbursts in order to decide upon the one that best describes the data. Furthermore, the magnitude of the star is analysed using wavclet analysis techniques. Ways of deriving the Cone of Influence are presented. An outcome of this analysis is that we can quantitatively confirm that an outburst occurred around November 26, 1945, which was not recorded due to the observational seasonal gaps. This was originally proposed by Oppenheimer and Mattei (1993) but was never accepted as a confirmed outburst. Also, this method reveals a pre-outburst signal in the light curve. For this, the way in which the wavelet analysis can be beneficial for future outburst predictions is presented.
10
Hills, Graeme Lawrence. "The application of polarimetric techniques to gamma ray astronomy." Thesis, University of Southampton, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.243187.
11
Baker, Paul Thomas. "Distinguishing signal from noise| New techniques for gravitational wave data analysis." Thesis, Montana State University, 2013. http://pqdtopen.proquest.com/#viewpdf?dispub=3596422.
Abstract:
The principal problem of gravitational wave detection is distinguishing true gravitational wave signals from non-Gaussian noise artifacts. We describe two methods to deal with the problem of non-Gaussian noise in the Laser Interferometer Gravitational Observatory (LIGO).
Perturbed black holes (BH) are known to vibrate at determinable quasi-normal mode frequencies. These vibrational modes are strongly excited during the inspiral and merger of binary BH systems. We will develop a template based search for gravitational waves from black hole ringdowns: the final stage of binary merger. Past searches for gravitational waves developed ad hoc detection statistics in an attempt to separate the expected gravitational wave signals from noise. We show how using the output of a multi-variate statistical classifier trained to directly probe the high dimensional parameter space of gravitational waves can improve a search over more traditional means. We conclude by placing preliminary upper limits on the rate of ringdown producing binary BH mergers.
LIGO data contains frequent, non-Gaussian, instrument artifacts or glitches. Current LIGO searches for un-modeled gravitational wave bursts are primarily limited by the presence of glitches in analyzed data. We describe the BayesWave algorithm, wherein we model gravitational wave signals and detector glitches simultaneously in the wavelet domain. Using bayesian model selection techniques and a reversible jump Markov chain Monte Carlo, we are able determine whether data is consistent with the presence of gravitational waves, detector glitches, or both. We demonstrate BayesWave's utility as a data quality tool by fitting glitches non-Gaussian LIGO data. Finally, we discuss how BayesWave can be extended into a full-fledged search for gravitational wave bursts.
12
Collin, Gabriel L. W. H. "Neutrinos, neurons and neutron stars : applications of new statistical and analysis techniques to particle and astrophysics." Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/118817.
Abstract:
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018.Cataloged from PDF version of thesis.
Includes bibliographical references (pages [181]-205) and index.
The IceCube detector opens a new window into our universe; valuable for both astronomy and particle physics. This thesis spans a wide range of topics that are bound together by a common theme: the development and application of new statistical and computational methods for analysing data from particle and astrophysics experiments. Sterile neutrinos are a hypothetical fourth kind of neutrino, which are motivated by anomalies observed in various short base-line neutrino experiments. These experiments have published results that are not mutually compatible. This thesis presents a global fit to many short base-line datasets with the addition of the recent IceCube sterile neutrino search, constraining the full 3+1 mixing matrix for the first time. The global fit strongly favours the sterile neutrino hypothesis, although significant tension still remains within the datasets. The origin of the observed astrophysical neutrino flux at IceCube remains elusive. Current methods, using a hot-spot model, have seen no significant clustering of events. This thesis presents a new test for point sources of neutrinos, based on the non-Poissonian Template Fitting technique. Constraints on population models for neutrino points sources are shown for the first time. Atmospheric neutrinos form a background for astrophysical analyses on IceCube, but also serve as the signal in particle physics analyses such as the sterile neutrino search. The first comprehensive study of the effect of global atmospheric temperature variations on atmospheric neutrino fluxes is provided. This thesis also presents two studies on using new computational methods for simulation and reconstruction on IceCube. Convolutional neural networks have been used to classify low-level waveform data, with the goal of identifying tau-neutrinos. Metropolis light transport, a rendering technique used in the CGI industry, has been extended to simulate the transport of light inside the IceCube experiment. Both show promising results, exceeding existing algorithms in their test cases.
by Gabriel L.W.H. Collin.
Ph. D.
13
Hinz, Philip Mark. "Nulling interferometry for studying other planetary systems: Techniques and observations." Diss., The University of Arizona, 2001. http://hdl.handle.net/10150/280541.
Abstract:
Nulling interferometry is an important technique in the quest for direct detection of extrasolar planets. It is central to NASA's plans for a Terrestrial Planet Finder (TPF) mission to detect and characterize Earth-like planets. This thesis presents the first experiments to demonstrate that the technique is a useful tool for ground-based observations as well. It demonstrates the ability of the technique to study faint, circumstellar environments otherwise not easily observed. In addition the observations and experiments allow more confident estimation of expected sensitivity to planetary systems around nearby stars. The old MMT was used for the first telescope experiments of stellar suppression via nulling. The stellar suppression achieved was sufficient to observe thermal emission from cool dust in the outflows around late-type stars. Based on the original MMT prototype, which worked at ambient temperature, I have constructed a cryogenic nulling interferometer for use with the renovated 6.5 m MMT. Features include the capability of sensing and correcting the phase between the two arms of the interferometer, achromatic tuning of the null using a unique symmetric beam-splitter, and compatibility with the deformable secondary of the MMT. The instrument has been used in a laboratory setup with an artificial source to demonstrate a high level of suppression. Commissioning of the instrument took place at the MMT in June 2000 using the fixed f/9 secondary. The instrument was aligned, phased, and used for science observations of 17 stars over five nights. The future impact of nulling with the MMT and the Large Binocular Telescope is sketched out. These telescopes will be sensitive to very faint levels of zodiacal dust, indicative of planetary companions and giving us clues as to the make up of planetary systems. Substellar companions down to near Jupiter mass will be detectable around the nearest stars for the LBT, allowing direct imaging of long-period giant planets. The detection of such companions will be complementary to the Doppler velocity searches, currently so successful in verifying the existence of planets, thus giving a balanced view of the prevalence and range of separations possible for giant planets around nearby stars.
14
Meli, Domenico Bertoloni. "The formation of Leibniz's techniques and ideas about planetary motion in the years 1688 to 1690." Thesis, University of Cambridge, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.293003.
15
Evans, Thomas. "Characterising exoplanet atmospheres : Bayesian techniques for transit lightcurves." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:d5f10fa0-3ea1-45b4-bf97-3f43555de0ed.
Abstract:
Precise measurements of transit lightcurves can be used to constrain the composition and structure of exoplanet atmospheres. Unfortunately, efforts to extract this information are usually hampered by the presence of correlated noise that is degenerate with the astrophysical signal of interest. A major theme of this thesis is the application of robust analysis methods to properly account for such degeneracies. In particular, I advocate the use of Bayesian inference for lightcurve fitting. For this study, the Bayesian framework is exploited by modelling lightcurves as Gaussian processes (GPs), which offer numerous advantages over traditional decorrelation methods. The main advantage is that GPs do not require a functional form to be specified for the poorly understood lightcurve systematics. Instead, the high-level properties of the signal covariance are parameterised, allowing complex correlations to be marginalised over relatively low-dimension parameter spaces. I use GP models to analyse transit and eclipse lightcurves for the hot Jupiters HD189733b, HD209458b, and HAT-P-32b. The work is spread over three separate projects. Firstly, I re-analyse the majority of the transits and eclipses that have been observed using the Spitzer Space Telescope Infrared Array Camera (IRAC) for HD189733b and HD20945b. The GP analyses generally produce uncertainties for inferred planet parameters that are factors of ~1-5 larger than those quoted in the literature. In a number of cases, I obtain results that are fundamentally different to those published previously, with significant implications for the understanding of the atmospheres. Secondly, I report an eclipse observation for HD189733b that was made using the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) over the 290-570nm wavelength range. Geometric albedos of Ag=0.37+12-13 and Ag=0.37+13-12 are measured in the wavelength ranges 290-450nm and 450-570nm, respectively. This represents the first ever multi-wavelength eclipse measurement made for an exoplanet at visible wavelengths. The nonzero albedo in the 290-450nm wavelength channel provides evidence for scattering in the atmosphere, possibly by haze/clouds or H2 molecules. The relatively low albedo in the 450-570nm wavelength channel is interpreted as being due to absorption by the wings of the Na 589nm doublet. Thirdly, I present two transit observations for HAT-P-32b made using the Nordic Optical Telescope (NOT) Andalucia Faint Object Spectrograph (ALFOSC) in multi-object spectroscopy mode over the 414-702nm wavelength range. A joint GP model is fit to the two white lightcurves produced by integrating the spectra over the full wavelength range. Spectroscopic lightcurves are also generated by binning into 32nm-wide wavelength channels, and preliminary lightcurve analyses are performed. The extracted transmission spectrum shows some evidence for absorption features, but this interpretation is currently very tentative. Further refinements to the data reduction and lightcurve analysis are suggested, which will allow the transmission spectrum to be evaluated more definitively.
16
Kamil, Mohamed. "Spectroscopy of proton unbound states in 32Cl." University of the Western Cape, 2019. http://hdl.handle.net/11394/6615.
Abstract:
>Magister Scientiae - MScThis project aimed to investigate proton unbound states in 32Cl using the 32S(3He; t) charge-exchange reaction. This research is relevant both in the context of nuclear structure and astrophysics. Excited states in 32Cl up to Ex 6 MeV were produced using a 50 MeV 3He++ beam from the K200 separated sector cyclotron at iThemba LABS. The triton ejectiles were mass analysed and detected at the focal plane of the K600 magnetic spectrometer. An additional segmented silicon detector array called CAKE was used to detect the unbound protons from states in 32Cl in conjunction with the tritons. In this work we looked for potential sources of isospin admixture that could explain the apparent violation of the Isobaric Multiplet Mass Equation (IMME) for the A = 32, T = 2 quintet. We also investigated the possibility of determining the 31S(p; ) reaction rate indirectly, via measurements of the partial proton widths of unbound states in 32Cl.
17
Fu, Nicole Christina. "Physical Properties of Massive, Star-Forming Galaxies When the Universe Was Only Two Billion Years Old." Thèse, Université d'Ottawa / University of Ottawa, 2011. http://hdl.handle.net/10393/19956.
Abstract:
Due to the finite speed of light and a vast, expanding universe, telescopes are just now receiving the light emitted by galaxies as they were forming in the very early universe. The light from these galaxies has been redshifted (stretched to longer, redder wavelengths) as a result of its journey through expanding space. Using sophisticated techniques and exceptional multi-wavelength optical and infrared data, we isolate a population of 378 galaxies in the process of formation when the Universe was only two billion years old. By matching the distinctive properties of the light spectra of these galaxies to models, the redshift, age, dust content, star formation rate and total stellar mass of each galaxy are determined. Comparing our results to similar surveys of galaxy populations at other redshifts, a picture emerges of the growth and evolution of massive, star-forming galaxies over the course of billions of years.
18
Mehdi, Benna. "Génération et inversion de données de propagation d'ondes radio à travers un noyau comètaire (Experience CONSERT - Mission Spatiale ROSETTA)." Phd thesis, Université Paul Sabatier - Toulouse III, 2002. http://tel.archives-ouvertes.fr/tel-00007990.
Abstract:
Ce memoire presente les modelisations directe et inverse entreprisent dans le cadre de la preparation de l'exploitation scientique de l'experience Consert de la mission spatiale Rosetta. Nous nous sommes interesses a la modelisation de la propagation d'ondes radio dans le cas simplifie d'un noyau cometaire a geometrie bidimentionnelle. A cette fin, une technique de trace de rayons (Ray Tracing Method, RTM) a ete developpee et validee par comparaison avec les resultats d'une methode spectrale (Pseudo Spectral Time Domain, PSTD). Cette technique a ete ensuite etendue au cas plus complexe d'une geometrie tridimentionnelle, aboutissant a la realisation d'un simulateur pour l'instrument Consert. Le probleme inverse a ete aborde dans le cas d'une geometrie bidimentionnelle a faibles perturbations de permittivite en utilisant la methode de regularisation de Tikhonov. Son etude a montre son caractere mal pose et fortement non lineaire, ainsi que le role important des informations a priori lors de la reconstruction d'image.
19
Coates, Adam Ross. "Methods for ultra-broadband correlator development focusing on high-speed digital sampling techniques." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:8ace9e68-d8e7-4f1d-b6c9-13853eecbd47.
Abstract:
In radio astronomy, a key limiting factor to observations made is the available bandwidth of the system. This thesis looks at two different approaches to building ultra-broadband correlators for use in radio astronomy. The first was a 2-20GHz double-sideband complex analogue correlator that was constructed before the work of this thesis. Characterisation tests are performed and a basic calibration is attempted. Both these sets of experiments show good results, with the basic calibration successfully being able to compensate for gain difference between the lags over a reduced bandwidth range used in the testing. The second approach was the investigation into different techniques for high-speed digital sampling, capable of providing equivalent bandwidths to the analogue system. The use of FPGA high-speed serial interfaces as direct 1-bit 3.125 GS/s samplers was investigated. Single-frequency sampling showed that a signal-to-noise ratio close to the theoretical maximum across the band was achieved (≈ 0.8 effective bits). Techniques were also identified to use multiple transceivers to generate a single interleaved stream at higher effective sampling rate. Two different methods were also explored for producing greater-than 1-bit sampling. A hysteresis approach was shown not to produce the desired results and a reference based sampler in the end was adopted. Finally, the interleaving and multi-bit techniques were combined to generate a single 1.5-bit 6.25 GS/s sampler. This was seen to have reduced signal-to-noise compared to the expected values. This was believed to be caused by the poor method of RF signal injection causing cross-talk between the channels and large amounts of loss. As a comparison to the direct sampling method, an external 1-bit high-speed Hittite comparator was also examined. The single-frequency experiment was repeated with a slightly higher signal-to-noise ratio found compared to the direct sampling method. This was again believed to be due to the RF environments used. From the sampling setups a four-input, six-baseline lag correlator was constructed using the direct sampling method. The entire correlator, as well as the sampling transceivers, was incorporated into a single Xilinx Virtex 5 FPGA. This was shown to have the expected response to single-frequency, broadband and noise signals. The thesis concludes with a characterisation of the RF devices used throughout the testing procedures. Several new devices were developed through the course of the experiments with the designs being documented. All the necessary components to construct IF chains for both the analogue and digital correlators described are present. This leads to simulations being made of complete IF chains, with the expected responses shown.
20
Long, Zachary. "Investigating the Physical Properties of Circumstellar Disks Using High Angular Resolution Observational Techniques." University of Cincinnati / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1535705067531455.
21
Robustini, Carolina. "Study of peacock jets observed above a sunspot light-bridge : results and techniques." Licentiate thesis, Stockholms universitet, Institutionen för astronomi, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-137687.
22
Robustini, Carolina. "Study of peacock jets observed above a sunspot light-bridge : results and techniques." Licentiate thesis, Stockholms universitet, Institutionen för astronomi, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-136516.
23
Grier, Jennifer Ann. "Determining the ages of impact events: Multidisciplinary studies using remote sensing and sample analysis techniques." Diss., The University of Arizona, 1999. http://hdl.handle.net/10150/284058.
Abstract:
The determination of the timing of impact events and the ages of cratered planetary surfaces is a complex and challenging undertaking. A powerful approach to this endeavor is a multidisciplinary study; understanding and using data from both rock samples and remote sensing. Shocked material (especially melt material) found in meteorites like Orvinio provides critical material dating impact craters. Orvinio, in spite of a complex degassing history shows evidence for multiple impacts at 4.2 Ga, 7.5 Ma, and possibly 330 Ma. Correlating impact histories for inner solar system bodies and the asteroid belt will constrain the genesis of impactor populations. Determining the recent cratering history of the Earth, however, is complicated by surface processes which erode and destroy impact craters. The Gardnos impact structure, for example, while possessing samples suitable for dating, has suffered substantial post-impact degassing due to metamorphism in the Caledonian orogeny ∼385 Ma. We must therefore look to the Moon to unravel the recent cratering history of the Earth-Moon system The Clementine mission data set provides an excellent resource for research into the bright rayed craters on the lunar surface. Studies of large rayed craters using the OMAT (optical maturity parameter) technique of Lucey and colleagues has revealed much information on the maturation of the crater ejecta. Profiles of OMAT values for the ejecta of large craters as a group show no evidence for an increase in the cratering rate during the Copernican era as advocated by Shoemaker. Future studies of both remote sensing and sample data will allow a better understanding of meteorite parent body impact histories and their implications for widespread epochs of increased impactor flux; the age-size correlation in lunar craters; the calibration of a large crater relative age scheme based on optical maturity with implications for the nature of the impactor flux in recent history; the nature of the recent small impactors on the lunar surface; implications for impact hazards on Earth today; and the best target sites for future landings and sample acquisition on the lunar surface.
24
Testov, Dmitry. "Effect of shell closure N = 50 and N = 82 on the structure of very neutron-rich nuclei produced at ALTO : measurements of neutron emission probabilities and half lives of nuclei at astrophysical r-processes path." Phd thesis, Université Paris Sud - Paris XI, 2014. http://tel.archives-ouvertes.fr/tel-01059803.
Abstract:
Nowadays we are all witnesses of a competition of facilities at different countries to study unknown regions of neutron rich nuclei. Much efforts are devoted to understand the role of neutron excess and its influence on nuclei in vicinity of closed neutron shells. One of the means to investigate nuclear structure is in beta-decay. Once a nucleus is proven to exist, its beta-decay properties, such as T1/2 and Pn (probability of beta-delayed neutron emission), which are relatively easy to measure, can provide the first hints on the nuclear structure. On the r-process site, "waiting points"(nuclei on closed neutron shells) has significant effects on the r-process dynamics and the abundance distribution. The actual side and the astrophysical conditions under which the nuclear synthesis takes place are still not certainly known - since r-process nuclei are difficult to produce and to study experimentally, input parameters for r-process calculations are mostly derived from theoretical models. As it has been seen lately, most of the theories have failed to reproduce newly measured data sets near shell closures. With new experimental data already (or shortly) available theoretical approaches can be adjusted. Since a beta-delayed neutron emission becomes strong if not dominating decaying channel for nuclei far stability, a proper neutron detector to study their properties is indispensable. To conduct the appropriate investigations, in the frame of the present thesis, in close collaboration with JINR (Dubna) a new detection system was constructed. It consists of 80 ³He-filled counters, 4π beta detector and a HPGe in order to measure simultaneously beta, gamma, neutron activity. The development of such a detection system system, currently installed at ALTO ISOL facility, was the first objective of the thesis. Then, during two experimental campaigns conducted to investigate beta decay properties of neutron rich nuclei in the neighborhood of N=50, N=82 the workability of the newly produced detection system was proven. In the vicinity of ⁷⁸Ni: half- lives and probability of beta-delayed neutron emission for ⁸º,⁸²,⁸³,⁸⁴Ga were measured. We were the first to observe the structure of ⁸¹,⁸² Ge via beta neutron gated gamma spectra. Thanks to the neutron detection channel the absolute intensities of beta decay were proposed for the first time. In the vicinity of ¹³²Sn the half lives of ¹²³Ag, ¹²⁴Ag, ¹²⁵Ag and ¹²⁷In, ¹²⁸In was measured. For the first time the beta delayed neutron emission was observed for ¹²⁶Cd, its Pn value also measured. Based on the data obtained we come to the conclusion that to figure out the relative contribution of allowed and forbidden decays more theoretical efforts should be done crossing the N=50 shell. Whereas in the vicinity of N=82 shell more experimental challenge are required.
25
Morris, Chad Michael. "Detection Techniques of Radio Emission from Ultra High Energy Cosmic Rays." The Ohio State University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=osu1254506832.
26
Corn, Philip B. "Measurement of the ⁸Li(d,n)⁹Be and ⁸Li(a,n)¹¹B reaction cross sections at astrophysical energies by radioactive beam techniques /." The Ohio State University, 1989. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487671108308862.
27
Ramiaramanantsoa, Tahina, Anthony F. J. Moffat, Robert Harmon, and Richard Ignace. "BRITE-Constellation High-Precision Time-Dependent Photometry of the Early O-Type Supergiant ζ Puppis Unveils the Photospheric Drivers of Its Small- and Large-Scale Wind Structures." Digital Commons @ East Tennessee State University, 2017. https://dc.etsu.edu/etsu-works/2724.
Abstract:
From 5.5 months of dual-band optical photometric monitoring at the 1 mmag level, BRITE-Constellation has revealed two simultaneous types of variability in the O4I(n)fp star ζ Puppis: one single periodic non-sinusoidal component superimposed on a stochastic component. The monoperiodic component is the 1.78-d signal previously detected by Coriolis/Solar Mass Ejection Imager, but this time along with a prominent first harmonic. The shape of this signal changes over time, a behaviour that is incompatible with stellar oscillations but consistent with rotational modulation arising from evolving bright surface inhomogeneities. By means of a constrained non-linear light-curve inversion algorithm, we mapped the locations of the bright surface spots and traced their evolution. Our simultaneous ground-based multisite spectroscopic monitoring of the star unveiled cyclical modulation of its He IIλ4686 wind emission line with the 1.78-d rotation period, showing signatures of corotating interaction regions that turn out to be driven by the bright photospheric spots observed by BRITE. Traces of wind clumps are also observed in the He II λ4686 line and are correlated with the amplitudes of the stochastic component of the light variations probed by BRITE at the photosphere, suggesting that the BRITE observations additionally unveiled the photospheric drivers of wind clumps in ζ Pup and that the clumping phenomenon starts at the very base of the wind. The origins of both the bright surface inhomogeneities and the stochastic light variations remain unknown, but a subsurface convective zone might play an important role in the generation of these two types of photospheric variability.
28
Safsten, Emily Diane. "Using Advanced PSF Subtraction Techniques on Archival Data of Herbig Ae/Be Stars to Search for New Candidate Companions." BYU ScholarsArchive, 2017. https://scholarsarchive.byu.edu/etd/6967.
Abstract:
Herbig Ae/Be (HAeBe) stars are intermediate mass (2-10 solar mass) pre-main sequence stars with circumstellar disks. Observing planets within these young disks would greatly aid in understanding planet formation processes and timescales particularly around massive stars. So far, only one planet, HD 100546 b, has been confirmed to orbit a HAeBe star. With over 250 HAeBe stars known, and several observed to have disks with structures thought to be related to planet formation, it seems likely that there are as yet undiscovered planetary companions within the circumstellar disks of some of these young stars. Direct detection of a low-luminosity companion near a star requires high contrast imaging, often with the use of a coronagraph, and the subtraction of the central star's point spread function (PSF). Several processing algorithms have been developed in recent years to improve PSF subtraction and enhance the signal-to-noise of sources close to the star. However, many HAeBe stars were observed via direct imaging before these algorithms came out. We used the PSF subtraction program PynPoint to reprocess archival images of HAeBe stars from the Advanced Camera for Surveys on the Hubble Space Telescope to increase the likelihood of detecting a planet in their disks. We believe we have recovered the known planet around HD 100546 and possibly its candidate second companion. We also detect new candidate sources in the vicinities of HD 141569 and HD 163296. Further observations are needed to confirm the reality of these detections and also establish their association with the host stars.
29
Young, Joshua Michael. "Synthesis, Modification, and Analysis of Silicate Cosmic Dust Analogues Using Ion-Beam Techniques." Thesis, University of North Texas, 2020. https://digital.library.unt.edu/ark:/67531/metadc1707354/.
Abstract:
Silicates analogous to cosmic dust were synthesized, modified, and analyzed utilizing ion-beam techniques with Rutherford backscattering spectrometry (RBS) and x-ray diffraction (XRD). Silicate dust is a common constituent in interstellar space, with an estimated 50% of dust produced in the stellar winds of M class Asymptotic Giant Branch (AGB) stars. Silicate dust acts as a surface upon which other chemicals may form (water ice for example), increasing significance in the cosmochemistry field, as well as laboratory astrophysics. Silicate formation in the stellar winds of AGB stars was simulated in the laboratory environment. Three sequential ion implantations of Fe-, MgH2-, and O- with thermal annealing were used to synthesize a mixture appropriate to silicate dust in the surface layers of a p-type Si substrate. Post implantation He+ irradiation was shown to preferentially induce crystalline formation in the analogue prior to thermal annealing. This effect is believed to originate in the ion-electron interaction in the Si substrate. The effects of ionization and ion energy loss due to electronic stopping forces is believed to precipitate nucleation in the amorphous media. For annealing temperatures of 1273 K, predominant quartz formation was found in the substrate, whereas lower annealing temperatures of 1000 K formed enstatite without post-implantation He+ irradiation, and olivine with He+ irradiation. Post annealed crystalline phase modification was investigated via x-ray diffraction and elemental compositions were investigated utilizing RBS. Finally, the interdiffusion of Fe and Mg at temperatures of 900-1100 K was investigated with RBS, and activation energies for interdiffusion were extracted for the transition from amorphous to crystalline phase in the silicate analogues. Fe had an interdiffusion energy of 1.8 eV and Mg 1.5eV. The produced analogues have similar properties to those inferred from infrared spectroscopy of the stellar winds of M-class AGB stars with an oxygen-rich outflow. This work established a method of silicate production using ion beam modifications, explored He+ irradiation effects in the annealed structures, and derived interdiffusion activation energies for Fe and Mg in the amorphous structure. Grain sizes were <100 nm with the observed formation of quartz, enstatite, and olivine.
30
Anderson, Brian Eric. "Derivation of Moving-Coil Loudspeaker Parameters Using Plane Wave Tube Techniques." BYU ScholarsArchive, 2004. https://scholarsarchive.byu.edu/etd/17.
Abstract:
Small-signal moving-coil loudspeaker driver parameters are traditionally derived through electrical impedance measurement techniques. These parameters are commonly called Thiele/Small parameters, after Neville Thiele and Richard Small who are credited with developing industry-standard loudspeaker modeling techniques. However, because loudspeaker drivers are electro-mechano-acoustical transducers, it should be possible to measure their parameters in physical domains other than the electrical domain. A method of measuring loudspeaker parameters from the acoustical domain will be developed. The technique uses a plane wave tube to measure acoustical properties of a baffled driver under test. Quantities such as the transmission loss through the driver are measured for a driver placed in the tube using the two-microphone transfer-function technique. Models have been developed to curve fit the resulting data, from which small-signal loudspeaker parameters are subsequently derived. This thesis discusses the acoustical measurement theory, apparatus, and system modeling methods (via equivalent circuits). It also compares measured parameters to those derived using electrical techniques. Parameters derived from both approaches are compared with reference values to establish bias errors. Sequential measurements are also compared to reveal random errors in the derivation processes.
31
Fraser, Eric M. "Deep-UV Light Emitting Diodes: An Experimental Investigation of Characterization and Optimization Techniques." Scholarship @ Claremont, 2005. http://scholarship.claremont.edu/pomona_theses/11.
Abstract:
Light emitting diodes (LEDs) and laser diodes (LDs) have many advantages over conventional light sources. Current commercial LEDs span the spectrum from IR to near- UV. There are a variety of applications for devices that extend into the deep-UV, including biological agent detection and optical storage. The nitride material system is a set of semiconducting compounds that have wavelengths that span a broad range, from yellow to deep-UV. AlGaN has a direct bandgap that extends into the deep-UV range; we will try to grow device-quality material, deposited epitaxially using metalorganic chemical vapor deposition on sapphire substrates.
32
Hicks, Stacy Jo. "Characterization of Eight Potentially Hazardous Near Earth Asteroids: Rotation Period Analysis and Structure Modeling Via Light Curve Inversion Techniques." TopSCHOLAR®, 2018. https://digitalcommons.wku.edu/theses/3060.
Abstract:
The term “homeland security”, seems to have become synonymous with terrorism in the minds of the general public. However, there are other threats to the security of the United States homeland that can be just as, if not more, devastating than terrorism. Included among these other threats is the potential of an asteroid collision with Earth. Historically, asteroid impact events have been responsible for the devastation of our planet and many of the mass extinction events encountered throughout the geologic record. Knowledge of physical parameters such as structure and rotational dynamics of the asteroid are critical parameters in developing interception and deflection techniques, as well as assessing the risk associated with these bodies and mitigation planning in the event of impact. This thesis encompasses the study of eight potentially hazardous asteroids identified in conjunction with NASA’s OSIRIS REX Mission and observed via the Target Asteroid Project, along with observations from the Robotically Controlled Telescope, and the Asteroid Light Curve Database of Photometry. Photometric data was extracted from all observations. Rotation periods of each target were confirmed using Lomb-Scargle time series analysis, with possible secondary periods indicated in the cases of Hathor (2.2169 hours), Bede (161.1501 hours), and Phaethon (4.5563 hours). Shape models for 2002 FG7, 2004 JN13, and Icarus were produced using light curve inversion techniques These are believed to be the first such models for these asteroids.
33
Galliano, Frédéric. "Étude Multi-Longueurs d'Onde de Galaxies Naines Proches: Propriétés des Milieux Interstellaires de Faible Métallicité." Phd thesis, Université Paris Sud - Paris XI, 2004. http://tel.archives-ouvertes.fr/tel-00122609.
Abstract:
Cette thèse est consacrée à l'observation multi-longueurs d'onde et à la modélisation des poussières, dans les galaxies naines locales de faible métallicité. Les principales motivations d'un tel projet sont : (i) l'étude des propriétés des poussières - composition, distribution de taille, etc. - dans des milieux interstellaires de métallicité non solaire ; (ii) l'étude de distributions spectrales d'énergie globales de galaxies naines qui, de par leur précocité chimique, présentent des analogies avec les galaxies primordiales que l'on ne peut pas observer ; et (iii) l'obtention d'informations sur l'évolution chimique de ces galaxies, par l'intermédiaire du rapport gaz-sur-poussière.Je commence par une étude détaillée des spectres de ces galaxies, en infrarouge moyen, obtenus avec ISO. Les principales caractéristiques spectrales que nous dégageons sont : (i) la faiblesse de l'émission par les bandes aromatiques, par rapport à ce qui est observé dans les galaxies à flambée de formation d'étoiles de métallicité normale ; (ii) la similarité avec les spectres de régions HII Galactiques - un continuum raide de très petits grains et des raies ioniques proéminentes. Ensuite, nous étudions les spectres d'un échantillon d'objets très variés - galaxies spirales, à flambée de formation d'étoiles, naines et régions HII - afin de construire un diagramme de rapports de bandes. Les corrélations entre les rapports 6.2/11.3, 7.7/11.3 et 8.6/11.3 sont, pour la première fois
établies sur un échantillon aussi diversifié. Elles montrent que les galaxies naines occupent une région particulière de ce diagramme, différente de celle occupée par les régions HII Galactiques, suggérant une différence de structure des PAHs - ionisation, hydrogénation, taille, etc.
La deuxième étape de ce projet est la modélisation de distributions spectrales d'énergie de quatre galaxies naines (He 2-10, II Zw 40, NGC 1140, NGC 1569), de l'ultraviolet au millimétrique. J'ai, pour cela, ajouter à nos propres observations, des données de la littérature. La modélisation se fait de manière cohérente en utilisant des contraintes sur l'émission des poussières, sur le rayonnement stellaire et sur les raies du gaz ionisé. Nous synthétisons les distributions spectrales d'énergie de ces galaxies, ainsi que les courbes d'extinction correspondantes. Les propriétés que nous sommes capables de dégager sont que : (i) l'émission est dominée par des grains de petite taille (3-4 nm) chauffés stochastiquement ; (ii) la courbe d'extinction synthétisée présente des analogies avec celle des nuages de Magellan, au niveau de sa pente, et l'intensité de la bande à 2175 angströms est plus faible dans trois galaxies sur quatre ; (iii) toutes nos distributions spectrales d'énergie ont un excès d'émission submillimétrique que nous attribuons à une composante de poussière très froide, enfouie dans des cœurs denses, contenant une fraction importante de la masse totale de poussière.
Ce manuscrit contient, par ailleurs : (i) une présentation des motivations de l'étude des distributions spectrales d'énergie de galaxies ; (ii) une description générale de la manière dont les propriétés optiques des poussières sont modélisées ; (iii) une description détaillée de la réduction des données ISOCAM ; et (iv) une description détaillée de l'observation et de la réduction des données, avec les caméras de bolomètres submillimétriques, SCUBA et MAMBO.
34
Byrum, Laura E. "Analysis of GaN/AlxGa1−xN Heterojunction Dual-Band Photodetectors Using Capacitance Profiling Techniques." Digital Archive @ GSU, 2009. http://digitalarchive.gsu.edu/phy_astr_theses/6.
Abstract:
Capacitance-voltage-frequency measurements on n+-GaN/AlxGa1−xN UV/IR dual-band detectors are reported. The presence of shallow Si-donor, deep Si-donor, and C-donor/N-vacancy defect states were found to significantly alter the electrical characteristics of the detectors. The barrier Al fraction was found to change the position of the interface defect states relative to the Fermi level. The sample with Al fraction of 0.1 shows a distinct capacitance-step and hysteresis, which is attributed to C-donor/N-vacancy electron trap states located above the Fermi level (200 meV) at the heterointerface; whereas, the sample with Al fraction of 0.026 shows negative capacitance and dispersion, indicating C-donor/N-vacancy and deep Si-donor defect states located below the Fermi level (88 meV). When an i-GaN buffer layer was added to the structure, an anomalous high-frequency capacitance peak was observed and attributed to resonance scattering due to hybridization of localized Si-donor states in the band gap with conduction band states at the i-GaN/n+-GaN interface.
35
Flitti, Farid. "Techniques de réduction de données et analyse d'images multispectrales astronomiques par arbres de Markov." Phd thesis, Université Louis Pasteur - Strasbourg I, 2005. http://tel.archives-ouvertes.fr/tel-00156963.
Abstract:
Le développement de nouveaux capteurs multispectraux en imagerie astronomique permet l'acquisition de données d'une grande richesse. Néanmoins, la classification d'images multidimensionnelles se heurte souvent au phénomène de Hughes : l'augmentation de la dimensionnalité s'accompagne d'un accroissement du nombre de paramètres du modèle et donc inévitablement une baisse de précision de leur estimation entrainant une dégradation de la qualité de la segmentation. Il est donc impératif d'écarter l'information redondante afin de réaliser des opérations de segmentation ou de classification robustes. Dans le cadre de cette thèse, nous avons propose deux méthodes de réduction de la dimensionnalité pour des images multispectrales : 1) le regroupement de bandes suivis de projections locales ; 2) la réduction des cubes radio par un modèle de mélange de gaussiennes. Nous avons également propose un schéma de réduction/segmentation jointe base sur la régularisation du mélange d'analyseurs en composantes principales probabilistes (MACPP). En se qui concerne la tâche de segmentation, nous avons choisie une approche bayésienne s'appuyant sur des modèles hiérarchiques récents a base d'arbres de Markov cache et couple. Ces modèles permettent en effet un calcul rapide et exact des probabilités a posteriori. Pour le terme d'attache aux données, nous avons utilisée la loi gaussienne multidimensionnelle classique, la loi gaussienne généralisée multidimensionnelles formulée grâce à la théorie des copules et la vraisemblance par rapport au modèle de l'ACP probabiliste (dans le cadre de la MACPP régularisée). L'apport majeur de ce travail consiste donc a proposer différents modèles markoviens hiérarchiques de segmentation adaptés aux données multidimensionnelles multirésolutions. Leur exploitation pour des données issues d'une analyse par ondelettes adaptée au contexte astronomique nous a permis de développer des techniques de débruitage et de fusion d'images astronomiques multispectrales nouvelles. Tous les algorithmes sont non supervises et ont été valides sur des images synthétiques et réelles.
36
Soriano, Melanie. "Astérosismologie d'étoiles de séquence principale ou évoluées, en relation avec l'expérience spatiale CoRoT, et les instruments au sol HARPS et SOPHIE." Phd thesis, Université Paul Sabatier - Toulouse III, 2009. http://tel.archives-ouvertes.fr/tel-00438082.
Abstract:
Le travail présenté dans cette thèse porte sur l'astérosismologie des étoiles de type solaire, et plus particulièrement des étoiles centrales de systèmes planétaires. L'analyse des ondes se propageant dans ces étoiles permet de mieux contraindre leur structure interne. Les deux premiers chapitres décrivent la théorie des oscillations stellaires et les outils numériques utilisés. La troisième partie traite de HD 52265, étoile cible de la mission CoRoT possédant une planète. Nous avons calculé des modèles préliminaires de cette étoile, en tenant compte des contraintes spectroscopiques, et nous avons avons fait des prédictions astérosismiques. Au cours de cette étude, nous avons mis en évidence une signature sismique caractéristique du cœur de l'étoile. Ce phénomène est induit par un fort gradient d'hélium au centre de l'étoile. Nous avons étudié cet effet dans le cas général des étoiles de type solaire, et nous avons montré qu'il se produit systématiquement, à la fin de la phase de séquence principale ou au début de la branche des sous-géantes. Cette signature caractéristique peut être utilisée pour contraindre le cœur de l'étoile. Le cinquième chapitre est consacré à l'étoile 51 Peg. Nous avons observé cette étoile avec le spectrographe SOPHIE à l'Observatoire de Haute Provence en 2007 et nous avons détecté ses oscillations. L'analyse des données a conduit à l'identification de 21 modes de pulsation. Enfin, nous présentons une nouvelle analyse sismique de l'étoile centrale de système planétaire µ Arae, observée et analysée par Bazot et al. en 2004. L'astérosismologie couplée avec la spectroscopie nous a permis de déterminer l'abondance d'hélium de l'étoile, ainsi que ses paramètres: masse, âge, rayon, taille du cœur convectif et extension possible due à de l'overshooting. Ces résultats illustrent que l'astérosismologie est un outil puissant pour apporter des contraintes sur la structure interne des étoiles.
37
Chaty, Sylvain. "Étude multi-longueur d'onde du microquasar GRS 1915+105 et de sources binaires de haute énergie de la Galaxie." Phd thesis, Université Paris Sud - Paris XI, 1998. http://tel.archives-ouvertes.fr/tel-00003559.
Abstract:
Cette thèse est consacrée à l'étude des microquasars, des sources binaires de haute énergie appartenant à notre Galaxie, analogues morphologiquement aux quasars, mais sur des échelles de longueur et de masse considérablement inférieures. La première partie concerne l'un des meilleurs représentants des microquasars: GRS 1915+105, découvert par le télescope WATCH/GRANAT, et qui présente des mouvements en apparence superluminiques. Nous avons établi la nature galactique de cette source, située à une distance de 12.5 +/- 1.5 kpc, ainsi que son environnement, à partir des observations millimétriques. Nos observations de cette source dans le domaine infrarouge ont montré qu'elle subissait des variations importantes, de 1 à 2 magnitudes, sur des intervalles de temps variables. Il apparaît que GRS 1915+105 est un système binaire lumineux et de grande masse, dont l'objet compact, certainement un trou noir, est entouré d'un disque d'accrétion optiquement épais. Grâce à un ensemble de campagnes d'observations simultanées à plusieurs longueurs d'onde, nous avons mis en évidence le lien entre les phénomènes d'accrétion et d'éjection de matière se produisant autour de l'objet compact. Ces observations, en établissant le modèle du microquasar, montrent que les analogies morphologiques entre quasars et microquasars sont sous-tendues par des analogies dynamiques et physiques. Enfin, grâce à des observations millimétriques, nous avons analysé l'effet d'éventuelles interactions entre les éjections énergétiques de cette source et le milieu interstellaire environnant. Dans la deuxième partie de cette thèse, j'expose une étude traitant de plusieurs sources de haute énergie, pour lesquelles nous avons identifié et/ou étudié les contreparties infrarouge et radio. Il ressort de cette étude que ce sont les objets compacts, trous noirs ou étoiles à neutrons, dans des systèmes binaires de faible masse, qui subissent les plus grandes variations de luminosité infrarouge.
38
Escobar, María Eliana. "Astérosismologie des étoiles de type solaire, avec ou sans planètes, abondance des éléments et phénomènes de transport." Phd thesis, Université Paul Sabatier - Toulouse III, 2013. http://tel.archives-ouvertes.fr/tel-00909055.
Abstract:
Depuis plusieurs décennies, les observations des oscillations stellaires et la recherche d'exoplanètes se sont développées en parallèle, en utilisant les mêmes méthodes et les mêmes instruments : la méthode de vitesse radiale, à partir d'instruments au sol comme SOPHIE à l'OHP ou HARPS au Chili, et la méthode photométrique, à partir d'instruments spatiaux, comme CoRoT et Kepler. L'intérêt d'étudier les oscillations des étoiles centrales de systèmes planétaires est apparu dès le début de ces observations. La caractérisation des planètes nécessite une très bonne connaissance de l'étoile centrale et particulièrement de ses paramètres globaux comme la masse, le rayon, la température. L'étude de la différence entre les étoiles possédant des planètes et celles qui n'en ont pas, peut apporter des informations précieuses pour mieux comprendre la formation des systèmes planétaires. Dans cette thèse, nous avons choisi d'étudier précisément trois étoiles centrales de systèmes planétaires de type solaire : (i) HD 52265, la seule étoile cible principale de CoRoT, observée pendant plusieurs mois consécutifs avec une précision inégalée; (ii) 94 Cet, une étoile centrale de système planétaire dont les paramètres spectroscopiques ressemblent à ceux de Virginis, étoile sans planète détectée, elle-même bien étudiée par ailleurs. 94 Cet a été observée avec le spectromètre HARPS à La Silla, Chili; (iii) 51 Pegasi, étoile "mythique", hôte de la première exoplanète observée en 1995 par Michel Mayor et Didier Queloz. Cette étoile a été observée avec le spectromètre SOPHIE à l'Observatoire d'Haute Provence. Dans tous les cas, nous avons comparé les fréquences observées et leurs combinaisons, avec celles calculées pour des modèles obtenus avec le Toulouse-Geneva Evolution Code. Les fréquences ont été calculées avec le code PULSE, de Montréal. Divers aspects physiques ont été testés, en particulier la diffusion atomique incluant les forces radiatives sur les éléments lourds. Nous avons obtenu des résultats intéressants pour ces trois étoiles, pour lesquelles les approches sont différentes. Dans les trois cas nous déduisons les paramètres extérieurs et des informations sur la structure. Ce travail comprend à la fois un aspect observationnel et de modélisation. C'est donc une approche assez complète de l'astérosismologie et de ses techniques.
39
Nguyen, Catherine. "Development of squeezing techniques for quantum noise reduction in gravitational-wave detectors." Thesis, Université Paris Cité, 2021. http://www.theses.fr/2021UNIP7129.
Abstract:
Le bruit quantique est une des limitations principales des détecteurs interférométriques d'ondes gravitationnelles, comme Virgo et LIGO. Réduire le bruit quantique a un impact direct sur la portée scientifique des futurs détecteurs d'ondes gravitationnelles (Advanced Virgo +, Advanced LIGO+, Einstein Telescope, Cosmic Explorer). L'origine du bruit quantique réside dans la nature quantique de la lumière, et en particulier dans les fluctuations du vide qui entrent par la sortie de l'interféromètre. Actuellement, l'injection d'états de vide comprimé (squeezing indépendant de la fréquence) dans Virgo et LIGO permet de réduire le bruit quantique dans la bande spectrale de détection correspondante à une des deux composantes de ce bruit, le bruit de photons, ou shot noise, pour des fréquences supérieures à environ 100 Hz. La pression de radiation, l'autre composante, se manifeste quant à elle à de plus basses fréquences. Le shot noise émane de l'incertitude sur la phase tandis que la pression de radiation, de l'incertitude sur l'amplitude. Le principe d'incertitude d'Heisenberg impose que la réduction du shot noise grâce à l'injection d'états du vide comprimé sur la phase, se traduise nécessairement par une augmentation de la pression de radiation. Cet état comprimé peut être représenté par une ellipse, illustrant l'état comprimé du vide dans l'espace phase-amplitude, où les incertitudes sur la phase et l'amplitude sont inégales. Cependant, cet effet a commencé à dégrader la sensibilité des interféromètres Virgo et LIGO, durant la prise de données appelée O3. Afin de réduire le bruit quantique sur toute la bande spectrale de détection (et donc aussi à basse fréquence), il est nécessaire d'introduire dans l'interféromètre un squeezing dépendant de la fréquence, c'est-à-dire un état du vide comprimé, tantôt sur l'amplitude et tantôt sur la phase, permettant de réduire à la fois la pression de radiation et le shot noise. Pour Advanced Virgo+ et Advanced LIGO+ (les projets d'améliorations en cours, pour les détecteurs actuels, appelés Advanced Virgo et Advanced LIGO), l'ajout d'une cavité de filtrage quantique suspendue de 300 mètres et avec une très grande finesse, permettra de réaliser ce squeezing dépendant de la fréquence. Ma thèse porte sur le développement de techniques de squeezing pour la réduction du bruit quantique dans les futurs détecteurs d'ondes gravitationnelles. J'ai d'abord contribué à un travail expérimental sur l'automatisation et l'amélioration d'une source de squeezing indépendant de la fréquence et situé sur le site de Virgo, à Pise. Ce travail préparatoire a été réalisé pour la conception d'un banc de démonstration pour l'étude d'une technique de squeezing dépendant de la fréquence, alternative à celle proposée ci-dessus et basée sur l'intrication quantique (de type Einstein-Podolsky-Rosen). Les fondements théoriques de ce squeezing EPR ayant été proposés en 2017, cette technique présente des avantages pour les futurs détecteurs d'ondes gravitationnelles, notamment liés à l'absence de cavité de filtrage. Dans ce cadre, j'ai participé au design optique complet de cette expérience, qui pourra être implémentée sur le détecteur Virgo. J'ai conçu, réalisé et testé dans le laboratoire optique de l'APC, une cavité Fabry-Perot monolithique (de type étalon) nécessaire pour la séparation et la détection de deux faisceaux intriqués. Plus précisément, j'ai effectué des mesures de caractérisation optique et sur la stabilisation thermique de cette cavité, permettant de conclure sur les performances de cet étalonQuantum noise is one of the main limitations for interferometric gravitational-wave (GW) detectors as Virgo and LIGO. Reducing quantum noise has a direct impact on the science reach of future GW detectors (Advanced Virgo +, Advanced LIGO+, Einstein Telescope, Cosmic Explorer). Quantum noise originates from the quantum nature of light, especially from the vacuum fluctuations entering by the interferometer detection stage. The current injection of vacuum squeezed states (frequency-independent squeezing) into Virgo and LIGO leads to the quantum noise reduction in the spectral detection region corresponding to one of the two components of quantum noise. This so-called quantum shot noise is present at frequencies higher than 100 Hz. The other quantum noise component, the so-called quantum radiation pressure noise, manifests itself at lower frequencies. Shot noise arises from the uncertainty on the phase, while the latter arises from the uncertainty on the amplitude. Heisenberg's uncertainty principles induce that the shot noise reduction, thanks to the injection of vacuum squeezed states, results in a radiation pressure noise increase. This squeezed state of light can be depicted with an ellipse, representing the squeezed states in a phase-amplitude space, with inequal uncertainties for the phase and the amplitude. Nonetheless, during the data-taking period called O3, this subsequent noise increase started to degrade the Virgo and LIGO interferometers' sensitivities. To achieve a broadband reduction of quantum noise, it is necessary to inject a frequency-dependent squeezing inside the interferometer, i.e., injecting vacuum squeezed states in a frequency-dependent way, which will have a smaller uncertainty accordingly to the concerned quantum noise component. For the next upgrade of the current detectors Advanced Virgo and Advanced LIGO, called Advanced Virgo+ and Advanced LIGO+, frequency-dependent squeezing is obtained by adding a suspended 300-meter filter cavity, with very high finesse. My thesis engages in the development of squeezing techniques for quantum noise reduction in future GW detectors. First, I contributed to an experimental work based on the automation and the improvement of a frequency-independent squeezed vacuum source located on the Virgo site, at Pisa. This was a preparatory work for the conception of a table-top experiment to study a frequency-dependent squeezing technique, alternative compared to the one proposed previously and based on Einstein-Podolsky-Rosen entanglement. The theory being brought forward in 2017, this technique offers significant advantages for future GW detectors, due to the absence of an external cost-intensive filter cavity. In this framework, I participated to the realization of a complete optical design for this experimental demonstrator, that can be implemented into the detector Virgo. I designed, realized, and tested a monolithic Fabry-Perot cavity (a solid etalon), at the optical laboratory of APC, necessary for the separation and detection of two entangled beams. More precisely, this cavity was optically characterized and its thermal stabilization was evaluated, which allowed to check its performances
40
Galicher, Raphaël. "Étude de techniques d'imagerie à haut contraste basées sur la cohérence." Phd thesis, Université Paris-Diderot - Paris VII, 2009. http://tel.archives-ouvertes.fr/tel-00436293.
Abstract:
Depuis 1995, environ 350 exoplanètes ont été détectées mais seule une dizaine l'a été directement pour plusieurs raisons. D'abord, les projections de l'étoile hôte et de sa planète sur la sphère céleste sont très proches -- quelques fractions de secondes d'arc. Ceci impose un diamètre minimum pour le télescope et un système compensant les perturbations atmosphériques. Ensuite, le flux lumineux de l'étoile hôte est entre un million et dix milliards de fois plus fort que celui de la planète. Pour réduire ce flux stellaire sans affecter le flux planétaire, nous utilisons un coronographe dont les performances sont limitées par les défauts optiques qui doivent être compensés ou estimés. Pendant ma thèse, j'ai étudié sur des plans théoriques et expérimentaux deux techniques: le coronographe à quatre quadrants à étages multiples (MFQPM) pour atténuer le flux stellaire et la self coherent camera (SCC) qui minimise l'impact des aberrations optiques en utilisant l'incohérence entre lumières stellaires et planétaires. J'ai montré en laboratoire qu'un prototype non optimisé du MFQPM fournissait une extinction achromatique de l'étoile centrale de mille à dix milles entre 550 et 750nm. Puis, j'ai montré par simulations que la SCC s'associe aisément avec un coronographe de type Lyot et qu'en utilisant ses deux modes de fonctionnement -- analyseur de surface d'onde en plan focal et imagerie différentielle --, des planètes de type Terre pouvaient être détectées de l'espace sous des conditions réalistes. J'ai également mesuré un défaut réel de phase via la SCC sur le banc d'Imagerie Très Haute Dynamique que j'ai développé à l'Observatoire de Paris.
41
Burke, Christopher J. "Survey for transiting extrasolar planets in stellar systems stellar and planetary content of the Open Cluster NGC 1245 /." Columbus, Ohio : Ohio State University, 2005. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1132168623.
42
Joner, Michael Deloss. "High-Quality Broadband BVRI Photometry of Benchmark Open Clusters." BYU ScholarsArchive, 2011. https://scholarsarchive.byu.edu/etd/2495.
Abstract:
Photometric techniques are often used to observe stars and it can be demonstrated that fundamental stellar properties can be observationally determined using calibrated sets of photometric data. Many of the most powerful techniques utilized to calibrate stellar photometry employ the use of stars in clusters since the individual stars are believed to have many common properties such as age, composition, and approximate distance. Broadband photometric Johnson/Cousins BVRI observations are presented for several nearby open clusters. The new photometry has been tested for consistency relative to archival work and shown to be both accurate and precise. The careful use of a regular routine when making photometric observations, along with the monitoring of instrumental systems and the use of various quality control techniques when making observations or performing data reductions, will enhance an observer's ability to produce high-quality photometric measurements. This work contains a condensed review of the history of photometry, along with a brief description of several popular photometric systems that are often utilized in the field of stellar astrophysics. Publications written by Taylor or produced during the early Taylor and Joner collaboration are deemed especially relevant to the current work. A synopsis of seven archival publications is offered, along with a review of notable reports of VRI photometric observations for the nearby Hyades open star cluster. The body of this present work consists of four publications that appeared between the years 2005 and 2008, along with a soon to be submitted manuscript for a fifth publication. Each of these papers deals specifically with high-quality broadband photometry of open clusters with new data being presented for the Hyades, Coma, NGC 752, Praesepe, and M67. It is concluded that the VRI photometry produced during the Taylor and Joner collaborative investigations forms a high-quality data set that has been: 1) stable for a period of more than 25 years; 2) monitored and tested several times for consistency relative to the broadband Cousins system, and 3) shown to have well-understood transformations to other versions of broadband photometric systems. Further work is suggested for: 1) the transformation relationships for the reddest stars available for use as standards; 2) the standardization of more fields for use with CCD detectors; 3) a further investigation of transformations of blue color indices for observations done using CCD detectors with enhanced UV sensitivity, and 4) a continuation of work on methods to produce high-quality observations of assorted star clusters (both open and globular) with CCD-based instrumentation and intermediate-band photometric systems.
43
St-Louis, Nicole, Patrick Tremblay, and Richard Ignace. "Polarization Light Curve Modeling of Corotating Interaction Regions in the Wind of the Wolf-Rayet Star Wr 6." Digital Commons @ East Tennessee State University, 2018. https://dc.etsu.edu/etsu-works/2690.
Abstract:
The intriguing WN4b star WR 6 has been known to display epoch-dependent spectroscopic, photometric and polarimetric variability for several decades. In this paper, we set out to verify if a simplified analytical model in which corotating interaction regions (CIRs) threading an otherwise spherical wind is able to reproduce the many broad-band continuum light curves from the literature with a reasonable set of parameters. We modified the optically thin model developed by Ignace, St-Louis & Proulx-Giraldeau to approximately account for multiple scattering and used it to fit 13 separate data sets of this star. By including two CIRs in the wind, we obtained reasonable fits for all data sets with coherent values for the inclination of the rotation axis (i0 = 166°) and for its orientation in the plane of the sky, although in the latter case we obtained two equally acceptable values (ψ = 63° and 152°) from the polarimetry. Additional line profile variation simulations using the Sobolev approximation for the line transfer allowed us to eliminate the ψ = 152° solution. With the adopted configuration (i0 = 166° and ψ = 63°), we were able to reproduce all data sets relatively well with two CIRs located near the stellar equator and always separated by ∼90° in longitude. The epoch dependence comes from the fact that these CIRs migrate along the surface of the star. Density contrasts smaller than a factor of 2 and large opening angles for the CIR (β⪆35∘) were found to best reproduce the type of spectroscopic variability reported in the literature.
44
Stevance, H. F., Richard Igance, P. A. Crowther, J. R. Maund, B. Davies, and G. Rate. "Probing the Rotational Velocity of Galactic WO Stars with Spectropolarimetry." Digital Commons @ East Tennessee State University, 2018. https://dc.etsu.edu/etsu-works/5506.
Abstract:
Oxygen sequence Wolf-Rayet stars (WO) are thought to be the final evolution phase of some high-mass stars, as such they may be the progenitors of Type Ic SNe as well as potential progenitors of broad-lined Ic and long gamma-ray bursts. We present the first spectropolarimetric observations of the Galactic WO stars WR93b and WR102 obtained with FORS1 on the Very Large Telescope. We find no sign of a line effect, which could be expected if these stars were rapid rotators. We also place constraints on the amplitude of a potentially undetected line effect. This allows us to derive upper limits on the possible intrinsic continuum polarization and find Pcont < 0.077 per cent and Pcont < 0.057 per cent for WR93b and WR102, respectively. Furthermore, we derive upper limits on the rotation of our WO stars by considering our results in the context of the wind compression effect. We estimate that for an edge-on case the rotational velocity of WR93b is vrot < 324 km s−1 while for WR102 vrot < 234 km s−1. These correspond to values of vrot/vcrit < 19 per cent and j) < 18.0 cm2 s−1 for WR93b and 2 s−1 for WR102. The upper limits found on vrot/vcrit and log(j) for our WO stars are therefore similar to the estimates calculated for Galactic Wolf-Rayet (WR) stars that do show a line effect. Therefore, although the presence of a line effect in a single WR star is indicative of fast rotation, the absence of a line effect does not rule out significant rotation, even when considering the edge-on scenario.
45
Mate, Sujay. "Développement d'un simulateur du ciel pour les instruments à grand champ de vue X-gamma en orbite terrestre basse : application à l'évaluation des performances du spectro-imageur SVOM-ECLAIRs." Thesis, Toulouse 3, 2021. http://www.theses.fr/2021TOU30031.
Abstract:
Les sursauts gamma (Gamma-Ray Bursts ou GRBs) sont les explosions les plus lumineuses de l'univers. On les observe sous la forme de bouffées de rayons X/ƴ (d'une durée de quelques millisecondes à quelques dizaines de secondes) suivies d'une émission rémanente (généralement à de plus grandes longueurs d'onde). Ils résultent soit de la fusion de deux objets compacts (une paire d'étoiles à neutrons ou une étoile à neutrons et un trou noir), soit de l'effondrement du noyau d'une étoile massive (>15 masse solaire). Les GRBs sont d'excellents candidats pour étudier la physique aux énergies et densités extrêmes et un outil astrophysique pour sonder l'histoire de l'univers car ils sont observés à tous les âges de celui-ci. La mission spatiale Sino-Française SVOM (lancement prévu en juin 2022) a pour objectif la détection et l'étude des GRBs à l'aide d'instruments spatiaux et terrestres dédiés afin d'obtenir une couverture multi-longueurs d'onde. Le principal instrument à bord du satellite SVOM est ECLAIRs, un imageur à masque codé à grand champ de vue (~2 sr) fonctionnant dans la bande d'énergie de 4 à 150 keV. ECLAIRs détectera et localisera les GRBs (ainsi que d'autres sources transitoires à hautes énergies) en temps quasi réel grâce à son " trigger " embarqué. Le bruit de fond d'ECLAIRs est élevé et variable en raison du grand champ de vue et de la stratégie de pointage de SVOM qui amène la Terre à transiter dans le champ de vue. Une nouvelle méthode (appelée "Particle Interaction Recycling Approach" ou PIRA en anglais), basée sur des simulations de Monte-Carlo (GEANT4), a été développée pour estimer précisément et rapidement le bruit de fond variable. Les simulations du bruit de fond sont complétées avec des sources X et des sursauts gamma afin de générer des scénarios d'observation complets. Le bruit de fond variable d'ECLAIRs pose des problèmes pour la détection des GRBs et affecte la sensibilité de l'instrument. Nous avons évalué les performances du "trigger" embarqué, notamment l'impact du bruit de fond sur la détection des sources transitoires et sa sensibilité aux caractéristiques des GRBs (durée, profil temporel, forme spectrale, position dans le champ de vue). ECLAIRs enverra au sol tous les photons détectés. De plus, la disponibilité d'une plus grande puissance de calcul et une meilleure connaissance du contexte (par exemple, les variations du bruit de fond, les sources dans le champ de vue, etc.) au sol, nous ont conduits à développer un "trigger" sol pour surmonter les difficultés rencontrées par le "trigger" embarqué. Ainsi, nous proposons un algorithme basé sur des transformées en ondelettes pour détecter les GRBs dans le cadre du "trigger" sol. Les travaux de cette thèse, à savoir le développement de PIRA, l'évaluation des performances et le développement d'un nouvel algorithme de détection de sursauts, fournissent une base solide pour construire un "trigger" sol efficace, qui complétera le "trigger" embarqué et améliorera les performances globales de la mission SVOMGamma-Ray Bursts (GRBs) are the most luminous explosions in the universe. They are observed as bright flashes of gamma/X-rays (lasting a few milliseconds to a few tens of seconds) followed by an "afterglow" emission (usually at longer wavelengths). They are produced either due to the merger of two compact objects (a pair of neutron stars or a neutron star and a black hole) or due to the core collapse of a massive star (> 15 solar mass). GRBs are excellent candidates to study physics at extreme energies and densities. They also constitute important astrophysical tools to probe the history of the universe as they are observed at all epochs. The upcoming (June 2022) Sino-French mission SVOM (Space-based multi-band astronomical Variable Objects Monitor) aims to detect and study GRBs using dedicated space and ground based instruments to obtain multi-wavelength coverage. The primary instrument onboard SVOM spacecraft is ECLAIRs, a wide-field (~ 2 sr) coded-mask imager sensitive in the 4 - 150 keV energy range. ECLAIRs will detect and localise GRBs (and other high energy transients) in near real time using an onboard trigger. ECLAIRs will encounter a high and variable background due to the wide field-of-view (FoV) and the pointing strategy of SVOM which makes the Earth transit through the FoV. A new method (called Particle Interaction Recycling Approach or PIRA), based on Monte-Carlo simulations (GEANT4), was developed to estimate the variable background accurately and rapidly. The simulations of the background are complemented with simulations of X-ray sources and gamma-ray bursts to generate complete observation scenarios. The variable background of ECLAIRs poses challenges to detect GRBs and affects the sensitivity of the instrument. We use the simulated data to evaluate the performance of the onboard trigger, in particular, the impact of the variable background and its sensitivity to the GRB characteristics (duration, temporal profile, spectral shape,position in the FoV). ECLAIRs will send all detected photons to the ground. In addition, the availability of a larger computational power and the better knowledge of the context (e.g. background variations, sources in the FoV, etc.) on the ground motivates us to develop an "offline trigger" to overcome the challenges faced by the onboard trigger. An algorithm based on wavelet transforms is proposed to detect GRBs as part of the offline trigger. The work in this thesis, i.e. the development of PIRA, instrument's performance evaluation and development of a trigger method, provides a sound basis to build an effective offline trigger that will complement the onboard trigger and improve the overall performance of the SVOM mission
46
Shrestha, Manisha, Hilding R. Neilson, Jennifer L. Hoffman, and Richard Ignace. "Polarization Simulations of Stellar Wind Bow Shocks. I. The Case of Electron Scattering." Digital Commons @ East Tennessee State University, 2018. https://dc.etsu.edu/etsu-works/2691.
Abstract:
Bow shocks and related density enhancements produced by the winds of massive stars moving through the interstellar medium provide important information regarding the motions of the stars, the properties of their stellar winds, and the characteristics of the local medium. Since bow-shock nebulae are aspherical structures, light scattering within them produces a net polarization signal even if the region is spatially unresolved. Scattering opacity arising from free electrons and dust leads to a distribution of polarized intensity across the bow-shock structure. That polarization encodes information about the shape, composition, opacity, density, and ionization state of the material within the structure. In this paper, we use the Monte Carlo radiative transfer code SLIP to investigate the polarization created when photons scatter in a bow-shock-shaped region of enhanced density surrounding a stellar source. We present results for electron scattering, and investigate the polarization behaviour as a function of optical depth, temperature, and source of photons for two different cases: pure scattering and scattering with absorption. In both regimes, we consider resolved and unresolved cases. We discuss the implications of these results as well as their possible use along with observational data to constrain the properties of observed bow-shock systems. In different situations and under certain assumptions, our simulations can constrain viewing angle, optical depth and temperature of the
47
Papakonstantinou, Nikolaos. "Investigation of variable Ap Stars in TESS continuous viewing zone." Thesis, Uppsala universitet, Observationell astrofysik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-441349.
48
Hubrig, S., K. Scholz, Wolf-Rainer Hamann, M. Schöller, Richard Ignace, I. Ilyin, K. G. Gayley, and Lidia M. Oskinova. "Searching for a Magnetic Field in Wolf-Rayet Stars Using FORS 2 Spectropolarimetry." Digital Commons @ East Tennessee State University, 2016. https://dc.etsu.edu/etsu-works/2694.
Abstract:
To investigate if magnetic fields are present in Wolf–Rayet stars, we selected a few stars in the Galaxy and one in the Large Magellanic Cloud (LMC). We acquired low-resolution spectropolarimetric observations with the European Southern Observatory FORS 2 (FOcal Reducer low dispersion Spectrograph) instrument during two different observing runs. During the first run in visitor mode, we observed the LMC Wolf–Rayet star BAT99 7 and the stars WR 6, WR 7, WR 18, and WR 23 in our Galaxy. The second run in service mode was focused on monitoring the star WR 6. Linear polarization was recorded immediately after the observations of circular polarization. During our visitor observing run, the magnetic field for the cyclically variable star WR 6 was measured at a significance level of 3.3σ (〈Bz〉 = 258 ± 78 G). Among the other targets, the highest value for the longitudinal magnetic field, 〈Bz〉 = 327 ± 141 G, was measured in the LMC star BAT99 7. Spectropolarimetric monitoring of the star WR 6 revealed a sinusoidal nature of the 〈Bz〉 variations with the known rotation period of 3.77 d, significantly adding to the confidence in the detection. The presence of the rotation-modulated magnetic variability is also indicated in our frequency periodogram. The reported field magnitude suffers from significant systematic uncertainties at the factor of 2 level, in addition to the quoted statistical uncertainties, owing to the theoretical approach used to characterize it. Linear polarization measurements showed no line effect in the stars, apart from WR 6. BAT99 7, WR 7, and WR 23 do not show variability of the linear polarization over two nights.
49
Neilson, Hilding, Richard Ignace, Beverly Smith, Gary Henson, and Alyssa Adams. "Evidence of a Mira-like tail and bow shock about the semi-regular variable V CVn from four decades of polarization measurements." Digital Commons @ East Tennessee State University, 2014. https://dc.etsu.edu/etsu-works/6233.
Abstract:
Polarization is a powerful tool for understanding stellar atmospheres and circumstellar environments. Mira and semi-regular variable stars have been observed for decades and some are known to be polarimetrically variable, however, the semi-regular variable V Canes Venatici displays an unusually large, unexplained amount of polarization. We present ten years of optical polarization observations obtained with the HPOL instrument, supplemented by published observations spanning a total interval of about forty years for V CVn. We find that V CVn shows large polarization variations ranging from 1 - 6%. We also find that for the past forty years the position angle measured for V CVn has been virtually constant suggesting a long-term, stable, asymmetric structure about the star. We suggest that this asymmetry is caused by the presence of a stellar wind bow shock and tail, consistent with the star's large space velocity.
50
Lai, Olivier. "L'optique adaptative du Telescope Canada France Hawaii et son utilisation pour l'etude des coeurs de galaxies a flambee d'etoiles." Phd thesis, Université Paris-Diderot - Paris VII, 1996. http://tel.archives-ouvertes.fr/tel-00003538.
Abstract:
L'optique adaptative est une technique qui permet de corriger en temps reel des defauts introduits par la turbulence atmospherique sur les images produites par un telescope. Les instruments ont maintenant atteint une maturite suffisante pour que des systemes de seconde generation apparaissent. Le Telescope Canada France Hawaii s'est equipe d'une optique adaptative a mesure et controle de courbure. Son integration et csa caracterisation en laboratoire et sur le ciel constituent la permiere partie de cette these. La philosophie et la methodologie d'integration sont exposes, ainsi que les resultats des tests. Ces derniers permettent d'estimer les performances de l'instrument dans de nombreuses conditions d'observations; elles sont excellentes et s'accordent avec les previsions. Une etude des galaxies a flambee d'etoiles constitue la seconde partie de la these. En effet, l'optique adaptative est un outil de choix pour l'observation de ces objets pour deux raisons; tout d'abord les sursauts de formation d'etoiles ont lieu dans des regions tres riches en poussieres, dont la profondeur optique est moindre dans le domaine infrarouge que dans celui du visible; ce domaine est aussi celui ou la correction apportee par l'optique adaptative est la plus efficace. De plus ces objets sont lointains, et il est necessaire de les observer a haute resolution angulaire pour pouvoir saisir les details de leur morphologie, et c'est precisment le but de l'optique adaptative: augmenter la resolution en s'affranchissant du flou residuel introduit par la turbulence atmospherique. Des resultats obtenus par les deux instruments d'optique adaptative ouverts a la communaute astrophysique sont presentes: onze galaxies (NGC 863, NGC 1068, NGC 1365, NGC 5033, NGC 7469, NGC 7714m Markarian 231, Markarian 565 et Arp 299) ont ete observes, et chaque cas est replace dans le contexte du modele unifie des noyaux actifs de galaxie. Par ailleurs, certains phenomenes sont mis en evidence ou trouvent confirmation (barres internes, super-amas d'etoiles, entoulement de bras spiraux).