In this work the effect of rotation on oscillation frequencies have been inves- tigated . Rotation has been treated as a perturbation and detailed calculations were done on the infuence of second order rotation . We used an evolutionary model of DELTA-Scuti star V1162 Ori with a mass of 1:8 solar mass. The eigenfrequencies were calculated in two cases , the slow rotation case with vsini = 46km/s and the fast rotation case with vsini = 61.9km/s. Calculation were carried out by a modifed oscillation program and results were compared to observations .

W Ursae Majoris-type (W UMa-type) variable stars are contact eclipsing binary stars whose evolution is unknown. Modelling to determine the physical parameters of as many W UMa-type variable stars as possible might provide some insight as to how these contact binaries form and evolve. The All Sky Automated Survey (ASAS) has discovered over ve thousand of these systems. Using data from the ASAS and from the Wide Angle Search for Planets (SuperWASP) project, models of selected ASAS contact binaries are being created to determine their physical parameters. Some W UMa-type variable stars are known to undergo changes in orbital period. For selected ASAS contact binaries, a period analysis has been performed using SuperWASP data to determine if the systems are undergoing changes in orbital period. Results of the modelling and period analyses of selected systems are presented. / Thesis (M. Sc. (Astronomy))

D.Phil. / We propose solutions to two statistical problems using the frequency domain approach to time series analysis. In both problems the data at hand can be described by the well known signal plus noise model. The first problem addressed is the estimation of the underlying variance of a process for the use in a Shewhart or CUSUM control chart when the mean of the process may be changing. We propose an estimator for the underlying variance based on the periodogram of the observed data. Such estimators have properties which make them superior to some estimators currently used in Statistical Quality Control. We also present a CUSUM chart for monitoring the variance which is based upon the periodogram-based estimator for the variance. The second problem, stimulated by a specific problem in Variable Star Astronomy, is to test whether or not the mean of a bivariate time series is constant over the span of observations. We consider two periodogram-based tests for constancy of the mean, derive their asymptotic distributions under the null hypothesis and under local alternatives and show how consistent estimators for the unknown parameters in the proposed model can be found

<p>This article-based dissertation provides a review on the broad subject of magnetars-their characteristics, giant flares (GFs) and associated observations of X-ray, gamma-ray, and radio emissions and their proposed physical mechanisms. The primary purpose of this dissertation is to provide an extensive description of the two research projects I undertook during my tenure as a Graduate Research Assistant, under the guidance of my advisor. Broadly, my research was focused on building analytical models and running three-dimensional (3-D), high-resolution magnetohydrodynamic (MHD) simulations using the astrophysical PLUTO code to investigate the physical mechanisms behind high-energy (X-ray and gamma-ray) and radio emissions associated with magnetar GFs using observational constraints. This, in turn, aided in either validating or disfavoring existing theories behind such energetic explosions.</p><p>Chapter 1 provides a review on magnetars, their GFs and associated high-energy and radio emissions, largely based on excellent reviews by [1]–[5]. I summarize interesting observational features of magnetars, specifically those of soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs), along with known aspects of their X-ray and gamma-ray activity. I focus on the December 27, 2004 GF emitted by SGR 1806-20, the most energetic GF out of the three that occurred to date, describe its energetics and summarize existing theories behind the physical mechanisms that give rise to two emission characteristics associated with the GF - (i) quasi-periodic oscillations (QPOs) seen in the tail, and (ii) a radio afterglow detected a week after the GF. Lastly, I describe the methods I used to hypothesize the physical mechanisms behind QPOs and the radio emission and compare and contrast them with those suggested previously.</p><p>In chapter 2, I present a version of the research article in preparation and pending publication in the Monthly Notices of the Royal Astronomical Society. The work titled “Radio afterglow of magnetars’ giant flares”, undertaken under the supervision of Dr. Maxim Lyutikov and in collaboration with Dr. Maxim Barkov, explores the possible physical mechanisms behind the radio afterglow associated with the SGR 1806-20 GF using high-resolution 3-D MHD simulations.</p><p>In chapter 3, I present a version of the research article previously published by the Journal of Plasma Physics. The work titled “Tilting instability of magnetically confined spheromaks”, undertaken under the supervision of Dr. Maxim Lyutikov, in collaboration with Dr. Lorenzo Sironi and Dr. Maxim Barkov, investigates the tilting instability of a magnetically confined spheromak using 3-D MHD and relativistic particle-in-cell (PIC) simulations with an application to astrophysical plasmas, specifically to explain the QPOs arising in the tail of the SGR 1806-20 GF.</p><p>I summarize the main results and conclusions of the two research projects and describe future prospects in chapter 4, followed by appendices A and B which describe additional theoretical concepts and simulation results for a better understanding of the nature of radio afterglows associated with GFs, and structure of spheromaks. References are compiled after the appendices in order that they are first cited, followed by a brief autobiographical sketch, and a list of publications.<br></p>

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The detection of variable stars in open clusters is an essential component of testing stellar structure and evolution theories. The ability to detect low-amplitude variability among cluster members is directly related to the quality of the photometric results. Point Spread Function (PSF) fitting is the best method available for measuring accurate magnitudes within crowded fields of stars, while high-precision differential photometry is the preferred technique for removing the effects of atmospheric extinction and variable seeing. In the search for new variable stars among hundreds or thousands of stars, the Robust Median Statistic (RoMS) is proven more effective for finding low-amplitude variables than the traditional error curve approach. A reputable computer program called DAOPHOT was used to perform PSF fitting, whereas programs, CLUSTER and RoMS, were created to carry out high-precision differential photometry and calculate the RoMS, respectively, on the open clusters NGC 225, NGC 559, NGC 6811, NGC 6940, NGC 7142, and NGC 7160. Twenty-two new variables and eighty-seven suspected variable stars were discovered, and time-series data of the new variables are presented.

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There exists many astronomical objects that vary in brightness. Objects such as variable stars like the Cepheids that periodically expands and contracts their outer layers, or the active galactic nuclei (AGN) where accretion of matter into a black hole generates a often varying brightness. Several candidates for being such variable objects have been identified as a result of the Vanishing and Appearing Stuff during a Century of Observations (VASCO) project. These candidates were then narrowed down to a handful that showed variability towards the infrared part of the spectrum. This bachelor’s thesis then aims to look further into these candidates using various databases and catalogues taking data from several sky surveys (SDSS, 2MASS etc). This is done to get better overview of the objects lightcurve over a bigger part of the spectrum, to establish whether the variability is real or a result from errors and to form a hypothesis of what kind of objects they could be. The result obtained from the data from the surveys points towards all the objects being real variable objects. The hypothesis is that all the objects are AGN’s that vary in brightness. / Det existerar många olika astronomiska objekt som varierar i ljusstyrka. Allt från variabla stjärnor som Cepheiderna som periodvis expanderar och kontraherar dess yttre skikt, till aktiva galaxkärnor där ackretion av materia in i ett svart hål genererar en ofta varierande ljusstyrka. Ett flertal kandidater för just sådana varierande objekt har identifierats som ett resultat av VASCO projektet. Dessa kandidater har sedan skalats ner till en handfull mängd kandidater som visade variation mot den infraröda delen av spektrumet. Detta kandidatarbete siktar på att vidare undersöka dessa kandidater genom att använda diverse astronomiska databaser och kataloger för att få data från flera kartläggningsprojekt (t.ex. SDSS, 2MASS etc). Detta är gjort för att få en bättre överblick över objektens ljuskurvor över en större del av spektrumet, att fastställa ifall objekten är riktiga variabla objekt eller uppstått på grund av diverse fel, samt att framställa en hypotes för vad det är för typ av objekt de kan vara. Resultaten från undersökningarna pekar på att alla objekten är riktiga variabla objekt. Hypotesen är att alla av objekten är aktiva galaxkärnor som varierar i ljusstyrka.