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)

Stars can be stretched and ripped apart by the super-massive black hole at the core of a galaxy. The remnant gaseous trail gradually circularizes in a ring of mass that spreads by the viscous forces into an accretion disc. In this thesis we have studied the spectral line profle time evolution of radiation refected by the accretion disc located around a super-massive black hole. We assume the central body to be a slowly rotating or non-rotating super-massive black hole with no charge, in the frst approximation represented by the Schwarzschild solution. In a sense of Shakura-Sunyaev standard accretion disc model with the kinematic viscosity parameter α ≈ 1 we allow the accretion disc evolution to be guided by the angular momentum transfer equation with the initial mass ring located at the tidal radius being the product of tidal disruption of a star passing by a super-massive black hole. During the simulations we keep varying the mass of the central body while we keep the mass and the radius of the star constant (M = 1M⊙ and R = 1R⊙), i.e. taking into account the solar-type stars only. We defer the prospects of the full analysis involving spin (and charge) of the central body for the future study as it will be necessary to use the equations for the redshift factor and the accretion disc evolution...

Using 8 micron infrared images from the Spitzer telescope we determine the half-light radius (the radius that contains half of the total light) for a sample of 49 nearby merging and merged pairs of galaxies. We compared this with other properties of the galaxies including a) the mass of X-ray emitting hot gas Mx(gas), b) the star formation rate (SFR), c) the large scale environment the galaxies reside in, and d) the chemical composition of the galaxies. Our goal is to better understand the processes that produce hot gas in galaxies

We present preliminary analysis of mid-infrared spectra of M-type and C-type Mira variables. Due to the brightness of this sample, it is straightforward to monitor changes with phase in the infrared spectral features of these regular pulsators. We have spectra of 25 Mira variables, taken with phase, using the Spitzer Infrared Spectrograph (IRS) high-resolution module. Each star has multiple spectra obtained over a one-year period from 2008-09. This is a rich, unique data set due to multiple observations of each star and the high signal-to-noise ratio from quick exposure times to prevent saturation of the IRS instrument. This paper focuses on the 17.6 and 33.2 micron lines shared by M-types and C-types. These are mostly emission lines that change with phase. We discuss preliminary physical diagnostics for the atmospheres based on the lines, as well as possible line identifcations such as fuorescence of metal species.

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The quality and quantity of polarimetric data being collected for stellar sources creates new opportunities for studying stellar properties and evolution, and also leads to new challenges for modeling and interpreting such data. Inspired by fresh prospects for detecting the Hanle effect to study photospheric magnetic fields, we have focused attention on purely geometrical aspects for polarimetric variability in the example of oblique magnetic rotators. In the case of axisymmetric fields, we highlight two key facts: (a) polarimetric lightcurves necessarily exhibit a certain time symmetry with rotation phase, and (b) variations in the polarization position angle can be modeled based on geometrical projection effects, independent of the photospheric magnetic field. These conclusions also have general applicability, such as to Thomson scattering and the transverse Zeeman effect. The authors gratefully acknowledge that funding for this work was provided by the National Science Foundation, grant AST-0807664.

Clumped structures in wind flows have substantially altered our interpretations of multiwavelength data for understanding mass loss from massive stars. Embedded wind shocks have long been the favored explanation for the hot plasma production and X-ray generation in massive star winds. This contribution reports on line profile shapes fromthe clump bowshock model and summarizes the temperature and emission measure distributions throughout the wind for this model with a focus on results that can be tested against observations.The authors acknowledge funding support for this work from a NASA grant(NNH09CF39C

The action of X-rays is commonly invoked to explain the wind properties of low-luminosity O-type stars. These stars have significantly smaller mass loss rates than predicted radiation-driven wind theories. In this respect they may resemble the first generation of supermassive stars in the early universe which presumably had weak winds due to their low metallicity. We present the high-resolution X-ray spectrum of a weak-wind star, mu Col, and discuss the potential for X-ray emission line strengths and profiles to discriminate among proposed mechanisms for the generation of X-rays in stellar winds, and in resolving the weak-wind problem.

Classical Cepheids and eclipsing binary systems are powerful probes for measuring stellar fundamental parameters and constraining stellar astrophysics. A Cepheid in an eclipsing binary system is even more powerful, constraining stellar physics, the distance scale and the Cepheid mass discrepancy. However, these systems are rare, only three have been discovered. One of these, OGLE-LMC-CEP1812, presents a new mystery: where the Cepheid component appears to be younger than its red giant companion. In this work, we present stellar evolution models and show that the Cepheid is actually product of a stellar merger during main sequence evolution that causes the Cepheid to be a rejuvenated star. This result raises new questions into the evolution of Cepheids and their connections to smaller-mass anomalous Cepheids.