Browsing Astronomy and Geoscience by Title
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Brkić, Zaharije (Belgrade , 1958)[more][less]
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Dačić, Miodrag (, 1998)[more][less]
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Ilić, Dragana (Belgrade , 2008)[more][less]
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Ćiprijanović, Aleksandra (, 2016)[more][less]
Abstract: Studying cosmic rays is very important for better understanding of high energy physical processes, since particles accelerated in space can reach energies far above what we can produce in accelerators on Earth, at the moment. Processes that produce cosmic rays are still unexplained to some extent, and models that have been proposed are awaiting confirmation. Presence of cosmic rays can be measured by detecting different products of interactions of these high energy particles with the interstellar medium through which they propagate. This thesis deals with the accretion shock as sources of cosmic rays in clusters of galaxies, as well as cosmic rays accelerated in supernova remnants inside galaxies. No matter which of these mechanisms is being considered, cosmic rays will collide with atoms and ions in the interstellar medium, and produce, among other things, gamma rays, neutrinos, as well as light elements, of which we will discuss lithium. In the thesis we primarily develop models that describe gamma rays produced by cosmic rays accelerated in shocks that can appear in different processes. We first examine accretion of new gas onto already virialized structures (for example in galaxy clusters). For the first time, we include the change of gamma-ray pro- duction with time, through the history of the universe, that reflects the evolution of accretion shocks which appear during large scale structure formation. Therefore, the models developed in this thesis describe the gamma rays from large scale struc- tures more realistically, compared to models which have previously been developed and which use single redshift approximation for the gamma-ray origin. Models are used to derive the gamma-ray flux of all unresolved galaxy clusters. These mode- led gamma rays are then compared to the isotropic diffuse gamma-ray background, measured by telescope Fermi-LAT. This leads to the conclusion that these cosmic rays have non-negligible contribution to the isotropic diffuse gamma-ray background (depending on the normalization, they can even explain the whole isotropic diffuse gamma-ray background) and that this population of cosmic rays has to be taken into consideration in addition to other components that are thought to be major contributors, like for example, unresolved normal galaxies or blazars. In the thesis, models of gamma-ray production in accretion shocks are also com- pared to observations of high-energy neutrinos detected by IceCube detector. Neu- trinos are used to normalize gamma-ray models, from which we conclude that if the accretion shocks are predominantly strong, neutrino background is more limiting to the possible gamma-ray emissivity of these objects, compared to the gamma-ray background we first used. Study of neutrinos as products of cosmic-ray interactions is very important, since neutrinos interact weakly with other particles, and therefore keep all of the information about the time they were produced and about cosmic rays that produced them. One part of the thesis deals with the production of cosmic rays in supernova remnants, in particular, the case of the Small Magellanic Cloud, which was detected in gamma rays. In this galaxy we also have the first measurements of the lithium abundances in the interstellar gas outside of the Milky Way. Since gamma rays and lithium are produced through interactions of cosmic rays with the interstellar medium, their same origin can be used to estimate the production of lithium and gamma rays by any cosmic-ray population. We show that galactic cosmic rays, which are considered to be dominant population of cosmic rays in the Small Magellanic Cloud, can only explain a very small part of the observed abundance of lithium, if we assume that the entire present gamma-ray emissivity that we observe also originates from the interaction of galactic cosmic rays with gas within the galaxy. This conclusion is interesting, because it leads to the possible existence of other sources of lithium in the Small Magellanic Cloud. Also, using the fact that gamma rays and lithium share the same origin, we estimate how much can irregular dwarf galaxies contribute to the diffuse gamma-ray background. Study of several different products of cosmic-ray interactions with the interstellar medium (gamma rays, neutrinos and lithium) on smaller scales (within the galaxy), as well as on the largest scales (galaxy clusters), showed that in addition to the galactic cosmic rays accelerated in supernova remnants, other still hypothetical co- smic rays (produced for example during accretion of gas on largest scales, or tidal interactions of galaxies) can have a non-negligible contribution to the measurements. URI: http://hdl.handle.net/123456789/4758 Files in this item: 1
AleksandraCiprijanovic.pdf ( 29.40Mb ) -
Martinović, Nemanja (Beograd , 2017)[more][less]
Abstract: Within this dissertation results from the analysis of formation, abundance and evolution of the dwarf galaxies from cosmological simulations will be presented. We will present results from first cosmological simulation which was performed and analyzed in its entirety in Serbia. From the obtained results it was shown how the large scale structure formed, primarily the formation of clusters of galaxies. Since the clusters of galaxies are dominant structures in the universe, which are relatively slowly forming in relation to the existence of the universe, using the Illustris cosmological simulation, with included hydrodynamics, it will be shown that a significant part of the clusters of galaxies are not in the state of dynamic equilibrium, as some authors claim, therefore their formation process continues. The obtained results will be used to divide the clusters of galaxies into three types: interacting clusters, clusters interacting with smaller groups (weakly interacting clusters), and clusters of galaxies that are not interacting. As the dwarf galaxies constitute largest number of galaxies in the universe, notably largest number in the clusters of galaxies, this raises the question how cluster’s interactions affect the structure and abundance of a diverse type of dwarf galaxies within them. After a brief review of today’s known types of dwarf galaxies, we will show that there is susceptibility of the abundance of dwarf irregular galaxies in relation to whether the parent cluster of galaxies is in interaction or not. As it is assumed that the dwarf irregular galaxies are predecessors for the formation of the most numerous types of dwarf galaxies, this could point to the dominant channel of delivering these galaxies to clusters of galaxies where they are expected to undergo morphological transformation. In the end, the discovery of two formation channels for compact elliptical galaxies in the clusters of galaxies will be presented. The formation of this type of dwarf galaxies has been the subject of discussion due to limitations in observations and their compact nature from which it could not be concluded which process leads to their formation. It has been shown here that they occur either by tidal stripping of large spiral galaxies after their spiraling into the cluster of galaxies or forming directly inside clusters from the clouds of gas which is most probably formed by the accretion of cold gas from the environment. URI: http://hdl.handle.net/123456789/4505 Files in this item: 1
nm_dis1.pdf ( 25.52Mb ) -
Smole, Majda (Beograd , 2017)[more][less]
Abstract: Formation mechanism of supermassive black holes (SMBHs) observed in the early Universe is still not fully understood. The goal of this thesis is to nd under what conditions black hole (BH) remnants of Population III stars can form SMBH with mass 109 M by redshift z = 7. We use Millennium and Millennium-II N-body cosmological simulations to investigate BH growth on cosmological scales. In order to exploit both high mass resolution in the Millennium-II simulation and large box size in the Millennium simulation, we develop a method to combine these two simulations together. BHs can grow through mergers with other BHs and through episodes of gas accretion triggered by major mergers of dark matter haloes. As a constraint in our model, we use observed BH mass function at redshift z = 6. We nd that BH seeds with masses 100 M could grow to SMBHs in distant quasars if e ective Eddington ratio is xed at fEdd = 3:7 and each accretion episode is limited to 50 Myr. During a BH merger asymmetric emission of gravitational radiation can lead to BH kick. Gravitational wave recoil can completely eject BH from it's host if the kick velocity is larger than the escape velocity from the galaxy. Since gravitational wave recoil could a ect SMBH growth through mergers, recoiling BHs are investigated in di erent models of host galaxies. BH trajectories are investigated in static and evolving dark matter halo potential described by NFW and Einasto density distributions. We nd that evolution of dark matter haloes clearly impact their capability to retain recoiling BHs since escape velocities are lower for smaller haloes at high redshifts. If the Einasto pro le is considered, then a larger number of complete BHs ejections is expected compared to NFW potential. Further, we construct analytical and numerical host galaxy models whose components are dark matter halo, bulge and disc. If baryonic component of a galaxy is included escape velocity is higher compared to a purely dark matter halo potential. Major (1:1) and minor (1:10) galaxy remnants are modeled separately. In numerical models BHs are ejected from their host centre before galaxy merger is completed, so escape velocities are generally lower in numerical models compared to analytical models where galaxy potential is unperturbed. Even though BHs could occasionally escape the most massive hosts, our model is not considerably sensitive to the gravitational wave recoil except for mergers of equal mass BHs in the least massive haloes at high redshifts where kick velocities of Vk . 100 km=s could permanently eject BHs from their hosts. URI: http://hdl.handle.net/123456789/4502 Files in this item: 1
Majda_Smole_disertacija.pdf ( 4.149Mb ) -
Pavlović, Rade (Faculty of Mathematics, University of Belgrade , 2008)[more][less]
Abstract: We investigate the ful¯llment of conditions for application of Nekhoroshev theorem to real dynamical system, such as the motion of asteroids under the perturbation by major planets. We have in particular analyzed the regions of the phase space where asteroid families of Koronis and Veritas are located. The results obtained for a number of selected family members indicate that in these regions even stricter conditions (convexity, quasi-convexity and 3- jet), than the condition of steepness required by the theorem, are fullled. Consequently, the theorem of Nekhoroshev can be applied to members of Koronis family and to the members of Veritas family with regular motion. URI: http://hdl.handle.net/123456789/717 Files in this item: 1
phdRadePavlovic.pdf ( 3.135Mb ) -
Milisavljević, R. Slaviša (Belgrade , 2013)[more][less]
Abstract: The problem of the minimal mutual distances for two confocal elliptical orbits (local minima), in the literature known as the proximity calculation for minor planets and recognised recently as Minimal Orbit Intersection Distance – MOID, occupies a very important place in astronomical studies, not only because of the prediction of possible collisions of asteroids and other celestial bodies, but also because of the fact that by analysing the behaviour of asteroids during their encounters it is possible to determine their masses, changes of orbital elements and other important characteristics. Dealing with this problem in this thesis the author has analysed the distance function for two elliptical confocal orbits of minor planets combining analytical and numerical methods for proximity calculation. A survey of all relevant results in this field from the middle of the XIX century till our days indicates that the problem has been transformed from looking for a solution of two transcendental equations by applying various methods and approximations of long duration towards efficient and rapid solutions of vector equations of the system which describes the problem. In the thesis a simple and efficient analytic-numerical method has been developed, presented and applied. It finds out all the minima and maxima in the distance function and, indirectly, makes it possible to determine also the inflection points. The method is essentially based on Simovljevic’s (1974) graphical interpretation and on transcendental equations developed by Lazovic (1993). The present method has been examined on almost three million pairs of real elliptical asteroid orbits and its possibilities and the computation results have been compared to the algebraic solutions given by Gronchi (2005). The case of a pair of confocal orbits with four proximities found by Gronchi (2002), who applied the method of random samples and carried out numerous simulations with different values of orbital elements, gave the motivation to try here to find out such a pair among the real pairs of asteroid orbits. Thanks to the efficacy of the method developed in the thesis two such pairs have been found and their parameters are presented. In addition to the one meantioned above a further analysis of distance function through simulations including more than 20 million different pairs of asteroid orbits has resulted in several additional interesting solutions of the distance function. The results are given in the form of tables and plots showing the diversity of solutions for the distance function. URI: http://hdl.handle.net/123456789/2490 Files in this item: 1
phdSlavisaMilisavljevic.pdf ( 3.815Mb ) -
Simovljević, Jovan (None)[more][less]
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Pejović, Nadežda (Belgrade)[more][less]
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Kostić, Petar (b , 2023)[more][less]
Abstract: The models of radio synchrotron emission of supernova remnants (SNRs) imply uniform density ahead of shock wave, so the evolution of luminosity is usu- ally studied in such an environment, most often through the surface-brightness-to- diameter dependence, the Σ–D relation. This field aims to better understand the SNR evolution, the emission models, but also the methods for determining their distance. It is not an easy task because of a very large scatter in the Σ–D Milky Way sample. The dissertation puts a different perspective at the Σ–D relation (usually treated as power-law function), assuming that non-uniform environment around the stars considerably affects its shape and slope, that may vary during the SNR expansion. It makes the ambient density structure an important factor whose impact must be investigated. The numerical code for hydrodynamic (HD) simulations and the emission model were developed. The 3D HD simulations were performed in different non-uniform environments, including low-density bubbles and a variety of clumpy models. Based on the simulation results, a semi-analytical 3D spherically-symmetric model of HD and Σ–D evolution of SNRs in clumpy medium was developed, which is used to generate large Σ–D samples. The results show that after entering the clumpy medium the SNR brightness enhances, but afterward the Σ–D slope steepens, shortening the brightness evolu- tion lifetime. Despite the evident increase in slope in clumpy medium, the Galactic sample average slope flattens at ≈ 13–50 pc. After analyzing the generated SNR samples in clumpy medium it is concluded that the significant flattening and scatter in Galactic sample originates in sporadic emission jumps of individual SNRs in a limited diameter interval. The additional analyses of selection effects are needed to investigate these issues. URI: http://hdl.handle.net/123456789/5606 Files in this item: 1
Kostic_Petar_disertacija.pdf ( 1.947Mb ) -
Damljanović, Goran (Faculty of Mathematics, University of Belgrade , 2007)[more][less]
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Stojanović, Milan (Beograd , 2017)[more][less]
Abstract: The goal of this dissertation is to determine values of local dynamical constants. This goal is achieved through examination of multiple samples of selected stars near the Sun. The selection is done by using planar and vertical eccentricities as sampling criteria. The solution for calculating eccentricities is given. In the next step a large sample of stars is selected by defining upper limits for eccentricities and vertical amplitude. Then nested subsamples are formed in two ways: in the first one upper eccentricity limit is subjected to decreasing, in the other one this is the case with upper amplitude of oscillations perpendicular to the plane. The values of the local dynamical constants are deduced by analysing this material. URI: http://hdl.handle.net/123456789/4498 Files in this item: 1
Stojanovic_Milan_teza.pdf ( 8.577Mb ) -
Sadžakov, Sofija (Belgrade)[more][less]
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Stalevski, T. Marko (Belgrade, Gent , 2012)[more][less]
URI: http://hdl.handle.net/123456789/2487 Files in this item: 1
Marko_Stalevski_doktorska_disertacija.pdf ( 6.024Mb ) -
Popović, Georgije (Belgrade)[more][less]
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Kubičela, Aleksandar (Belgrade , 1973)[more][less]
URI: http://hdl.handle.net/123456789/120 Files in this item: 1
phdAleksandarKubicela.pdf ( 30.99Mb ) -
Komarčić, Lazar (Belgrade , 1902)[more][less]
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Valjarević, Aleksandar (Niš , 2012)[more][less]
Abstract: The territory of Kosovo and Metohia has alwaus represented the hidrology interesting area of study, in which they compared the parameters of water drainage and their physical properties. Generalization is one of the methods for these porposes and may be used and the results can be applied to various forms of digital maps. URI: http://hdl.handle.net/123456789/3118 Files in this item: 1
ValjarevicAleksandarDD.pdf ( 39.32Mb ) -
Mitić, Ljubiša (Belgrade)[more][less]
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