|
|
Perturbed stellar motion in dense star clusters
Pavlík, Václav ; Šubr, Ladislav (advisor) ; Davies, B. Melvyn (referee) ; Portegies Zwart, Simon (referee)
Star clusters are thought to be the birthplaces of stars as well as the building blocks of galaxies. They typically consist of thousands to millions of stars bound together by self-gravity. These systems evolve on the scale of Myr to Gyr, there- fore, it is impossible for us to see any change in their global evolution even within hundreds of human lifetimes. Although the equations of motion of stars in a star cluster are simple New- tonian, it is impossible to predict precisely history of any star within them to any point in the future. Therefore, we may either compare the observations of different star clusters at different age, we may invent theoretical approaches and analytical predictions, or we must follow their evolution numerically (e.g. with direct N-body integrators) which is the main focus of my research and this thesis. First, we follow the evolution of star clusters in general while coming up with a novel method to estimate their characteristic timescale (i.e. the time of core collapse) based on global parameters. The core collapse is directly linked to the formation of hard binary stars, thus, we focus on their analysis as well. We also follow several recent observational results: (i) ALMA observations of the Serpens South star-forming region indicate that star clusters are born mass...
|
|
|
Chaos in motion around black holes
Suková, Petra ; Semerák, Oldřich (advisor) ; Šubr, Ladislav (referee)
The geodesic motion around Kerr black holes is regular, but this may change when an additional source is present, even if the space-time symmetry were not lowered. In this thesis we study the eect of a simple (static and axially symmetric) additional source on geodesic dynamics in the eld of a Schwarzschild black hole. In the static case the complete space-time can be described by an exact solution of Einstein equations thanks to a relatively simple superposition of the central-black-hole and the external-source metrics. Following the astrophysical motivation, we will specically consider, as the external source, the thin ring (linear source) of the Bach-Weyl type and innite thin discs (planar sources) of several types (inverted counter-rotating discs of Morgan and Morgan and the discs with power-law radial shape of density). The results may be relevant e.g. for a long-term behaviour of discrete sources (stars) in the eld of a very massive black hole in a galactic nucleus, surrounded by an accretion disc and/or by a massive toroid.
|
|
|
Symmetries and dynamics of star clusters
Haas, Jaroslav ; Šubr, Ladislav (advisor) ; Baumgardt, Holger (referee) ; Jungwiert, Bruno (referee)
We investigate the orbital evolution of an initially thin stellar disc around a supermassive black hole, considering various perturbative sources of gravity. By means of direct numerical N-body modelling, we first focus on the case when the disc is embedded in an extended spherically symmetric star cluster. We find that the gravitational influence of the disc triggers formation of macroscopic non- spherical substructure in the cluster which, subsequently, significantly affects the evolution of the disc itself. In another approximation, when the cluster is emu- lated by an analytic spherically symmetric potential, we further consider pertur- bative gravitational influence of a distant axisymmetric source. Using standard perturbation methods, we derive a simple semi-analytic model for such a config- uration. It turns out that the additional axisymmetric potential leads to mutual gravitational coupling of the individual orbits from the disc. Consequently, the dense parts of the disc can, for some period of time, evolve coherently. Finally, we apply some of our results to the young stellar disc which is observed in the innermost parsec of the Galactic Centre. 1
|
|
|
Neutron stars near a galactic centre
Zajaček, Michal ; Karas, Vladimír (advisor) ; Šubr, Ladislav (referee)
In this work we study the processes near the Galactic centre, which serves as a paradigm for low- luminosity galactic nuclei. The introductory part of the thesis is a brief review on the radio source Sagittarius A* in the Galactic centre and on its immediate surroundings. The main part of the thesis focuses on the hypothetical population of neutron stars that should be present in large numbers in this region. We analyse the predictions concerning the encounters of this observationally unexplored population with the ambient interstellar medium and we discuss the distribution of their interaction modes with respect to the parameters of the system. We find out that this distribution is strongly dependent on the density of the ambient medium, whereas only weakly dependent on its temperature. The effect of the prolongation of rotational period is negligible on the time-scale of about ten thousand years. In the second part, we predict the evolution of the high-eccentricity passages of clouds and dust- enshrouded stars (with pericentre distances at about 1000 Schwarzschild radii from the black hole). In all studied cases a major part of the matter is diverted from the original path. Powered by TCPDF (www.tcpdf.org)
|
|
|
Modelling the Orion Nebula Cluster
Pavlík, Václav ; Šubr, Ladislav (advisor)
Title: Modelling the Orion Nebula Cluster Author: Václav Pavlík Department: Astronomical Institute of the Charles University Supervisor: doc. RNDr. Ladislav Šubr, Ph.D. (Astronomical Institute of the Charles University) Abstract: Young star clusters are widely discussed from the point of view of their evolution and structure. In this work we focused our attention on studying a typical representative of these objects - the Orion Nebula Cluster (ONC, M 42) - based on the observational data, including their confrontation with N- body models from Šubr et al. (2012). These numerical models were inspired by the recently proposed evolutionary scenario, according to which the star clusters begin their evolution from very dense initial conditions. From the analysis of the X-ray sources we revealed that the ONC is likely to be rotationally symmetric in the inner area (� 0.7 pc). Further analysis including also optical and IR observational data led us to the conclusion that the ONC is elongated from the North-East to the South-West on large scales (up to 2 pc). We also compared radial profiles of different mass groups of stars and we discovered a possibly inverse mass segregation between stars with masses in the interval (1 ; 5) M⊙ and the stars less massive than 0.5 M⊙ in the range from 0.5 pc to 1.5 pc. This...
|
|
|
Perturbed stellar motion in dense star clusters
Pavlík, Václav ; Šubr, Ladislav (advisor) ; Davies, B. Melvyn (referee) ; Portegies Zwart, Simon (referee)
Star clusters are thought to be the birthplaces of stars as well as the building blocks of galaxies. They typically consist of thousands to millions of stars bound together by self-gravity. These systems evolve on the scale of Myr to Gyr, there- fore, it is impossible for us to see any change in their global evolution even within hundreds of human lifetimes. Although the equations of motion of stars in a star cluster are simple New- tonian, it is impossible to predict precisely history of any star within them to any point in the future. Therefore, we may either compare the observations of different star clusters at different age, we may invent theoretical approaches and analytical predictions, or we must follow their evolution numerically (e.g. with direct N-body integrators) which is the main focus of my research and this thesis. First, we follow the evolution of star clusters in general while coming up with a novel method to estimate their characteristic timescale (i.e. the time of core collapse) based on global parameters. The core collapse is directly linked to the formation of hard binary stars, thus, we focus on their analysis as well. We also follow several recent observational results: (i) ALMA observations of the Serpens South star-forming region indicate that star clusters are born mass...
|
|
|
Spatial distribution of the young stars observed in the centre of the Milky Way
Kepčija, Petr ; Haas, Jaroslav (advisor) ; Šubr, Ladislav (referee)
Galactic centre of Milky way is one of its most exotic parts. Right in the centre there lies a black hole, which has a mass of several million masses of our Sun. In the vicinity of this black hole there was recently found a young star cluster. The origin of these stars remains a mystery. Their spatial arrangement in a disc can give us a clue. The result of this thesis is a thorough analysis of the existence of two discs of young stars using the data from VLT telescopes in Chile and Keck telescopes in Hawaii. The analysis confirms the existence of one disc and shows the existence of the other disc seems to be very probable. 1
|
|
|
Modelling the Orion Nebula Cluster
Pavlík, Václav ; Šubr, Ladislav (advisor)
Title: Modelling the Orion Nebula Cluster Author: Václav Pavlík Department: Astronomical Institute of the Charles University Supervisor: doc. RNDr. Ladislav Šubr, Ph.D. (Astronomical Institute of the Charles University) Abstract: Young star clusters are widely discussed from the point of view of their evolution and structure. In this work we focused our attention on studying a typical representative of these objects - the Orion Nebula Cluster (ONC, M 42) - based on the observational data, including their confrontation with N- body models from Šubr et al. (2012). These numerical models were inspired by the recently proposed evolutionary scenario, according to which the star clusters begin their evolution from very dense initial conditions. From the analysis of the X-ray sources we revealed that the ONC is likely to be rotationally symmetric in the inner area (� 0.7 pc). Further analysis including also optical and IR observational data led us to the conclusion that the ONC is elongated from the North-East to the South-West on large scales (up to 2 pc). We also compared radial profiles of different mass groups of stars and we discovered a possibly inverse mass segregation between stars with masses in the interval (1 ; 5) M⊙ and the stars less massive than 0.5 M⊙ in the range from 0.5 pc to 1.5 pc. This...
|
|
|
Modelování Velké mlhoviny v Orionu
Pavlík, Václav ; Šubr, Ladislav (advisor) ; Jungwiert, Bruno (referee)
Title: Modelling the Orion Nebula Cluster Author: Václav Pavlík Department: Astronomical Institute of the Charles University Supervisor: doc. RNDr. Ladislav Šubr, Ph.D. (Astronomical Institute of the Charles University) Abstract: Young star clusters are widely discussed from the point of view of their evolution and structure. In this work we focused our attention on studying a typical representative of these objects - the Orion Nebula Cluster (ONC, M 42) - based on the observational data, including their confrontation with N- body models from Šubr et al. (2012). These numerical models were inspired by the recently proposed evolutionary scenario, according to which the star clusters begin their evolution from very dense initial conditions. From the analysis of the X-ray sources we revealed that the ONC is likely to be rotationally symmetric in the inner area (� 0.7 pc). Further analysis including also optical and IR observational data led us to the conclusion that the ONC is elongated from the North-East to the South-West on large scales (up to 2 pc). We also compared radial profiles of different mass groups of stars and we discovered a possibly inverse mass segregation between stars with masses in the interval (1 ; 5) M⊙ and the stars less massive than 0.5 M⊙ in the range from 0.5 pc to 1.5 pc. This...
|
|
|
Neutron stars near a galactic centre
Zajaček, Michal ; Karas, Vladimír (advisor) ; Šubr, Ladislav (referee)
In this work we study the processes near the Galactic centre, which serves as a paradigm for low- luminosity galactic nuclei. The introductory part of the thesis is a brief review on the radio source Sagittarius A* in the Galactic centre and on its immediate surroundings. The main part of the thesis focuses on the hypothetical population of neutron stars that should be present in large numbers in this region. We analyse the predictions concerning the encounters of this observationally unexplored population with the ambient interstellar medium and we discuss the distribution of their interaction modes with respect to the parameters of the system. We find out that this distribution is strongly dependent on the density of the ambient medium, whereas only weakly dependent on its temperature. The effect of the prolongation of rotational period is negligible on the time-scale of about ten thousand years. In the second part, we predict the evolution of the high-eccentricity passages of clouds and dust- enshrouded stars (with pericentre distances at about 1000 Schwarzschild radii from the black hole). In all studied cases a major part of the matter is diverted from the original path. Powered by TCPDF (www.tcpdf.org)
|
guest ::
