Astronomy and Geoscience
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Čeki, Atila (Beograd , 2013)[more][less]
Abstract: The dissertation presents a program for modeling binary systems, Infinity, which combines the traditional principles of modeling (Roche geometry, von Zeipel’s law of temperature distribution and so forth) with a novel geometrical representation of the system. The study of binary stars has a long tradition in Serbia, mostly due to the research done by dr Gojko Đurašević. The program presented in the dissertation is based on his program for modeling of active close binary systems with an accretion disk (Đurašević, 1991) and brings several significant improvements that can be classified in two categories: The first group of improvements has to do with generalizing the model. First and foremost, the new program is applicable to systems with eccentric orbits, and is capable of computing and solving the inverse problem for radial velocities. It also features a new algorithm for determining which elements of the stellar surface are visible during the eclipse. The new algorithm allows for computing the light curves with greater precision using a method of ‘adaptive subdivision’ in the region of eclipse. The geometrical aspects of modeling the accretion disk are also improved and now include the possibility to create both conical and toroidal disks. The second group of improvements is related to the modeling of physical processes on the binary system components. The most significant change is in the way the emergent flux is calculated. Namely, the Đurašević program assumes that the elements of stellar surface produce blackbody radiation, while the new program assigns a one-dimensional Kurucz stellar atmosphere model calculated under the LTE assumption. Improvements are also made in the treatment of the influence of photometric filters on the calculated light curve, and the list of supported filters is extended. A binary system is described by the Roche model, which assumes that the motion and shape of the components are determined by the superposition of gravitational and centrifugal forces, while all the other interactions are neglected. The stars are treated as point masses (under the assumption of total central condensation), so their internal structure is not considered. If the system contains an accretion disk, it is assumed that its mass can be neglected, and that its presence doesn’t influence the dynamics of the system and the shape of the stars. The surface of the components is represented with a polygonal mesh. The stars are approximated with a geodesic, and the accretion disk with an axially-symmetric mesh. The new algorithm for solving the problem of visibility detection, proposed in the dissertation, is based on the so called inverse painter’s algorithm that originates from the field of computer graphics. The most important property of this algorithm is that, other than the polygonal meshes representing the components, it doesn’t require any other information about the geometry of the system. This makes the modeling program easily extendable to more exotic geometric configurations such as systems with eccentric or circumbinary disks, planetary and multiple systems, and so forth. It is assumed that within a single element of stellar surface all the physical quantities – local effective temperature, gravitational acceleration, emergent flux and radial velocity – are constant. Its radiation is affected by gravity darkening, limb darkening and the reflection effect. Gravity darkening is described using von Zeipel’s law, and the limb darkening can be calculated using one of the five available approximations (linear, logarithmic, square-root, quadratic and nonlinear fourth-order approximation). The reflection effect, as well as the temperature distribution along the accretion disk, are calculated using the approach from the Đurašević program (Đurašević, 1991). The local amount of radiation can additionally be affected by active regions that are represented with dark or bright circular spots. The emergent flux is computed as the sum of fluxes from all the surface elements visible in the given moment, and the radial velocity is calculated from the superposition from three kinds of motion: the proper motion of the system center of mass, the motion of the center of the star around the center of mass due to orbital revolution, and the motion of elementary surface around the center of the star due to rotation. Synthetic light and radial velocity curves obtained in this way can be directly compared to observations. The new program solves the inverse problem – estimating the optimal model parameters to fit the given observations – using the Nelder-Mead simplex algorithm to minimize the sum of squares of deviations between the observed and calculated light and radial velocity curves. The validity of the program and the proposed improvements is done by analyzing several binary systems and comparing the results with published solutions obtained with the Đurašević program. URI: http://hdl.handle.net/123456789/5839 Files in this item: 1
Disertacija4228.pdf ( 4.176Mb ) -
Latković, Olivera (Beograd , 2013)[more][less]
Abstract: The dissertation deals with modeling of stellar oscillations in close binary systems. This topic is импортант not only in for the study of binary stars, but also for asteroseismo-logy. One of the key problems of asetroseismology is the “mode identification” of stellar oscillations. Mode identification is the process of associating an oscillation frequency measured from photometric or spectroscopic observations with a spherical harmonic of degree and order which describes the spatial characteristics of the mode. The main obstacle for this crucial step between the collection and interpretation of data is the symmetry inherent to the patterns of oscillations that makes the periodic changes in the brightness and the radial velocities of surface elements cancel each other out, resulting in diminishing of observed amplitudes. Low sensitivity of both photometry and spectroscopy to high degree modes ( and in photometric and spectroscopic observations, respectively), is one of the consequences of this phenomenon, making the detection and identification of oscillations a difficult task. A method for mode identification by grid-fitting spherical harmonics to photometric light curves of pulsating stars inside eclipsing binary systems is developed in the dissertation. The eclipse of the pulsating star by its binary companion has visible effects on the observed properties of the oscillations because the eclipse spoils the symmetry of oscillation pattern, producing a modulation of frequency, amplitude and phase unique for each mode that can be recognized and fitted. Fitting, however, requires a model of a binary system with pulsating components. The only existing implementation of the method of fitting spherical harmonics to light curves uses a very simple model of the binary, with ideally spherical stars in circular orbits, and does not account for any of the proximity effects. Moreover, the oscillations are represented as sinusoidal perturbations of an arbitrary surface brightness distribution. This dissertation uses a much more detailed model, based on Roche geometry that includes all the important proximity effects. Modeling of stellar oscillations is improved by representing them as direct perturbations of local tempe- v rature and gravitational potential and including the effects of tidal distortion and rotation on the frequencies and spatial properties of the modes. The scientific goal of the dissertation is to develop a reliable method – the method of fitting spherical harmonics – for mode identification on pulsating stars in close binary systems, in order to enable asteroseismological studies of their interiors. The basic assumption is that a close binary system can be represented using the Roche model, which is based on the approximation of total central condensation. In other words, it is assumed that the contribution of outer layers of the star to its total mass can be neglected. With this approximation, it is possible to treat the gravitational interaction of the components as the interaction of point masses. Within the Roche model, the shape of the star is determined by the Roche potential that corresponds to the superposition of the gravitational and the centrifugal force. The temperature distribution across the surface is determined by the von Zeipel law that defines the dependence of flux on the effective gravitational acceleration. Stellar oscillations are then modeled as perturbations of the local values of the Roche potential and temperature. It is assumed that the oscillations are adiabatic, of small amplitudes, and that they can be represented using spherical harmonics. It is also assumed that the symmetry axis of the oscillations will coincide with the dominant geometrical axis (usually with the rotation axis, or in the case of tidally distorted stars, with the line connecting the centers of the components). The proposed method of mode identification on pulsating components of close binary systems is implemented as a part of the modeling program Infinity. Infinity is based on the Roche model and accounts for all the phenomena that influence the formation of light and radial velocity curves, from limb darkening and surface temperature distribution caused by departures from spherical shape, to the reflection effect, irregularities in surface brightness and the presence of the accretion disc. Stellar oscillations can be modeled on one or both components of the system. The method is verified by means of tests using synthetic data. In other words, the results of fitting are compared to the stellar and orbital parameters that were used to generate vi the synthetic light curves, or the fake “observations”. The tests encompass both favorable and unfavorable configurations (total and partial eclipses, approximately spherical and tidally distorted stars, modes of low and high degrees and so forth). The results are finally compared to the results of a similar technique of mode identification – the method of eclipse mapping. From the obtained results, we can conclude that the proposed method of mode identification is reliable (i.e. provides correct identification) for approximately spherical stars in detached binary systems. The results for tidally distorted stars are encouraging, but show that before an attempt is made to apply the method on real observation of that nature, it will be necessary to include the simultaneous fitting of frequency, amplitude and phase for each trial mode into the procedure of mode identification, and to try to solve the problem of ambiguous identifications. This will be the focus of my future work. URI: http://hdl.handle.net/123456789/5837 Files in this item: 1
Disertacija4284.pdf ( 5.184Mb ) -
Vučetić, Milica (Beograd , 2017)[more][less]
Abstract: In this dissertation we discuss in uence of H emission from supernova remnants (SNRs) on H derived star formation rates (SFRs). We present the detection of 16 optical SNR candidates in a part of nearby spiral galaxy IC342, and two more poten- tial SNRs in NGC 185 galaxy. The candidates were detected by applying [S ii]/H ratio criterion on observations made with the 2 m telescope at Rozhen National Astronomical Observatory in Bulgaria. Also, in this dissertation we present the most up-to-date list of nearby galaxies with optically detected SNRs. When deri- ving H star formation rates, assumption is that the radiation is coming from the ionized gas surrounding hot and young stars { H ii regions. In this case, emission from SNRs contaminates H ux used to derive SFR. We found that the contribu- tion of SNRs' ux to the total H ux, for 18 galaxies in our sample of galaxies with optical SNRs, is 5 5%. Due to the observational selection effects, the SNR contamination of SFRs derived herein represents only a lower limit. M83 is the galaxy with the most detected optical SNRs and in this galaxy SNRs account for 9 per cent of the total H emission. We expect that fraction similar to this would be close to the real contribution of SNR emission to the total H emission in spiral galaxies. Using two dwarf galaxies as an example, Holmberg IX and NGC 185, we show that the contamination of H ux by SNRs, or other sources, can be much larger, up to ten times higher than total H ux coming from H ii regions of the observed galaxy. URI: http://hdl.handle.net/123456789/5832 Files in this item: 1
Disertacija-5.pdf ( 26.28Mb ) -
Vidojević, Sonja (, 2012)[more][less]
Abstract: Interplanetary electron beams, produced by solar flares, are unstable in the solar wind and generate Langmuir waves at the local plasma frequency, fp. These waves are then converted into the so-called type III radio bursts which are freely propagating electromagnetic emissions at fp or its harmonic. The type IIIs are therefore observed as drifting emissions from high to low frequencies, in the kilometric wavelengths range. Since the first theoretical explanation by Ginzburg and Zhelezniakov (1958a), several refined models have attempted to describe in details the physical processes at the origin of type III bursts. The mechanisms of “Bump-on-tail” instabilities, Langmuir waves generations, conversion of these Langmuir waves into radio emissions throughout nonlinear wave-wave interactions etc, have been studied in detail. Of particular interest from the observational point of view are the so called in situ type III bursts for which the electron beam, at the origin of the emission and traveling along open interplanetary magnetic field lines, is observed directly in situ by a spacecraft, together with the local Langmuir waves and the resulting radio emissions. Until now only a few of these in situ type IIIs have been reported in the literature. The first research study performed in this thesis was to examine the first 16 years of radio, waves and particles data recorded by the Wind spacecraft in the Solar Wind and to look for in situ type IIIs. Applying rigorous and careful criteria, this examination has yielded to a data set of 36 high-quality events. With such a numerous data set, which is statistically representative of the studied phenomenon, it is now possible to constrain obser- vationally and with a better confidence the type III generation models. After having built our statistical dataset, we have studied, for each of the events, the precise shapes of the Langmuir wave power distributions, observed in the spectral domain. We have fitted these observed distributions by a Pearson’s system of probability distributions and have shown that the probability distributions of the logarithm of the Langmuir waves power spectral density belong to three “main” types of Pearson’s probability distributions: type I, type IV and type VI. In addition we have modeled the effects of the instrumental integration time of the Wind radio receivers on the observed Langmuir wave power distributions. By combining our observations with our models we have shown that it was not possible to con- clude definitively, that the distribution of the Langmuir waves energy in the real temporal domain is lognormal, as it is predicted in some theories as the Stochastic Growth Theory by Robinson (1992). In the last part of the thesis, we have shown how our high-quality data set of 36 in situ type III events can be used for further studies that could allow to constrain the theoretical models even better. For instance we have investigated the correlation between the Langmuir waves power and the energy of impulsive electron or with the power of the radio emissions themselves. URI: http://hdl.handle.net/123456789/5802 Files in this item: 1
Disertacija_Sonja.pdf ( 4.444Mb ) -
Pavlović, Mrina (Beograd , 2022)[more][less]
Abstract: The studyofgalaxiesthroughhighredshiftsarekeytounderstandingthe evolutionofgalaxiesthroughcosmictimes.Assuchobjectsareverydifficultto observedirectly,theyaremainlyexaminedusingempiricallyderivedtoolssuchas the numerouscorrelationsbetweentheirdifferentparametriccharacteristics,one of thembeingthelinearrelationshipbetweenfar-infraredandradioemissionin star-forming galaxies,namedtheFarInfrared-Radio(FIR)Correlation.Although the correlationwasconsideredtobestableintermsoflinearity,recentworks, whichincludegalaxiesathighredshifts(0 < z< 6), showedalargedeviation from thecorrelationinthesesystems.Thegoalofthisdoctoraldissertationis an examinationofthephysicalprocessesthatleadtothiskindofbehavior.As a possiblecauseofthisevolution,wewillassumeforthefirsttime,andexamine interactionsbetweengalaxies(collisionsandcloseapproaches).Interactionsbe- tweengalaxiesleadtotheformationofshockwavesonlargescalesthatcanlead to changesintherelationshipbetweeninfraredandradioemissions.Ourhypoth- esis wastestedinseveralstadiumsandthemainresultsareasfollows:1.We developedmodelsoftheevolutionoftheFIRcorrelationwithredshiftasfunctions of thegalaxyinteractionrate.Wetestedthemodelsonasampleofgalaxieswith an alreadydeterminedmorphologyseparatelyfordiscgalaxiesandforgalaxies that haverecentlybeenorarecurrentlyinteracting-irregulargalaxies. 2. Inasmallsampleof34galaxiesthatwetookfrompaperMiettinenetal.(2017), it wasshownthatthereisanindicationthattheinteractionbetweengalaxiescan beresponsiblefortheevolutionofthecorrelationwiththeredshift. 3. Thenextanalysiswasperformedonamuchlargersampleofstar-forming galaxies takenfromCOSMOSfield,wherewedidnotfindanyevolutionofcorre- lation withtheredshift.Also,itwasshownthatthemeanvalueofthecorrelation parameter islowerinirregulargalaxiesthanindiskgalaxies. Although recentobservationsindicatedanevolutionoftheFIRcorrelation with redshift,theresultsofthisresearchfailedtoreproducethatevolutionand showedthattheFIRcorrelationisstablewithredshift.However,itwasalso shownthatduetotheinteractionofgalaxies,theevolutionoftheFIRcorrelation is possibleiftherepresentationofinteractingsystemsinthesampleishigher. URI: http://hdl.handle.net/123456789/5795 Files in this item: 1
Disertacija_15613.pdf ( 8.624Mb )