|
|
A study of dynamically bounded triple systems
Švrčková, Jana ; Harmanec, Petr (advisor) ; Korčáková, Daniela (referee)
Hierarchical triple stars consist of two stars orbiting each other on a close orbit and a third, more distant star. This is the only observed dynamically stable configuration of triple systems. The third star needs to be far enough from the close binary, otherwise the stars start moving chaotically which eventually results in ejecting one of the components out of the system. This condition is described by several criteria of dynamical stability. HD 152246 is a hierarchical triple star with an eccentricity seemingly too high to be dynamically stable. Using a recently observed set of spectra, I determined more precise and accurate orbital elements of this system. I showed that the eccentricity value is lower and the system does fulfill stability criteria. I also showed that the system is dynamically evolving - the short orbit inclination is changing in time. δ Cir is a triple system that consist of an eclipsing binary and a more distant third star. Moreover, there are some signs that the third star is also a binary. Using spectra of the system, I calculated a more precise mutual period of the eclipsing binary and the third star as well as its other orbital elements. I also combined spectroscopic and photometric data to determine more precise parameters of the eclipsing binary. 1
|
|
|
Observations and modeling of classical Be stars
Klement, Robert ; Korčáková, Daniela (advisor) ; Jones, Carol Evelyn (referee) ; Votruba, Viktor (referee)
The brightness and proximity of many classical Be stars makes them perfect laboratories for studying the physics of astrophysical disks. They are also among the most popular targets for optical/IR interferometers, which are able to fully resolve their circumstellar disks, to which much of the recent progress in our understanding of these enigmatic objects is owed. The current consensus is that classical Be stars eject material from the stellar surface into Keplerian orbits, thus forming a disk, whose subsequent evolution is governed by turbulent viscosity, which is the basis of the so-called viscous decretion disk (VDD) model. Among the main results of the present work is arguably the best-constrained model of a particular Be star β CMi. The VDD predictions were confronted also with radio observations, which allowed for the first determination of the physical extent of a Be disk. This result subsequently led to the detection of a binary companion, which is truncating the disk by tidal forces. Extending the sample to include five more targets led to revealing a similar outer disk structure in all of them. The range of explanations includes the most plausible scenario, in which the truncation of Be disks by (unseen) companions is much more common than previously thought.
|
|
|
Spectral analysis of the B[e] star FS CMa
Kříček, Radek ; Korčáková, Daniela (advisor) ; Mayer, Pavel (referee)
The thesis is dealing with spectral analysis of the B[e] star FS CMa, a prototype of FS CMa stars group. These objects are characterized by huge amount of matter in sourrounding shell which origin is not explained for many years. However, solving this issue could enrich stellar astrophysics by enhanced stellar evolution models. The goal of this thesis is to improve our understanding of FS CMa. We described spectral variability on various time scales and simulated spectra for different forms of the system in HDUST code. On the basis of new obtained spectra we confirmed long-term changes in variability of some spectral features and rapid variability of some spectral lines. We gained an estimate of the gas density. The simulations showed that spectral type of FS CMa is not certain and that the star could be surrounded by extended disk, many stellar radii wide. 1
|
|
|
Spectroscopic diagnosis of extended circumstellar regions about early type stars
Dvořáková, Nela ; Korčáková, Daniela (advisor) ; Zasche, Petr (referee)
Nela Dvořáková 19.7.2019 1 Abstract We study a representative of a diverse group of stars exhibiting the B[e] phe- nomenon, designated MWC 939. This object might be an important link be- tween the stage of the asymptotic giant branch and planetary nebula stage as it is one of a few such objects observed. Our focus is aimed at spectral analysis of the circumstellar matter around the central star. We assemble a line list for MWC 939 and observe variations of its spectrum. Electron density and temper- ature are estimated using nebular diagnostics of ratios of [S II] λλ 6716, 6731 ˚A and [O I] λλ 6300, 6364 ˚A and 5577 ˚A. These estimates are then used for detailed calculations of the structure of circumstellar envelope. 1
|
|
|
Spectral analysis of the B[e] star V743 Mon
Jeřábková, Tereza ; Korčáková, Daniela (advisor) ; Krtička, Jiří (referee)
This thesis deals with the study of the star V743 Mon. Stellar spectrum contains forbidden emission lines of [O I], strong Balmer emission lines and also infrared excess. Due to this, V743 Mon belongs to the group of B[e] objects. Observed spectra indicate very extended and rotating circumstellar envelope composed of gas and dust. We study time variability using data obtained during 2004-2012 at the Astronomical Institue of the Academy of Sciences of the Czech Republic in Ondřejov. We reduce and rectificate these spectra. We mesure relative intensities, equivalent widths and radial velocities for lines Hα, [O I] and Fe II. Graphical description contains also lines Si II and He I. The observed spectra show time variability on both, long- and short-term scales. Spectra from SAAO observatory are used to study the short time-scale changes. The discussion of problems connected with different instruments is also presented. 1
|
|
|
A study of spectral and photometric variations of the Be star V960 Tau = 120 Tau
Švrčková, Jana ; Harmanec, Petr (advisor) ; Korčáková, Daniela (referee)
The thesis is focused on analysis of spectra and photometry of Be star 120 Tau. A period of 142,4 d was found in the spectroscopy dat, which might be related to a binary character of this object. This period has to be confirmed by a set new observations. Both positive and negative correlation were present in the color-color diagram of U − B vs B − V . The explanation of this fact is the inclination of the rotational axis, which is near 60◦ . Spectral type, radius and an interval of possible rotational periods were also determined. Two more periods were found in the photometry data - 1,0375 d a 10,91 d. The shorter one is probably related to the rotation of the star. Other short periods listed in other works were not confirmed. The object's behavior is very complicated, more observations are needed to explain it. 1
|
|
|
Spectral analysis of flares on AD Leo
Wollmann, Jiří ; Heinzel, Petr (advisor) ; Korčáková, Daniela (referee)
Solar flare is a process which releases magnetic energy stored in the solar corona due to the effect of magnetic reconnection. Similar activity has been observed on other stellar types, namely type K and M stars, which usually have vast and strong magnetic field. Often during flares on M stars we observe asymmetry of profiles of some spectral lines with enhanced red wing. The cause of these asymmetries is not well understood. The aim of this thesis is to analyse spectra of AD Leo, which is frequently flaring dMe star, and to model radiation of the Hα line coming from flare loops analogically to flare loops on the Sun. The light curves of selected spectral lines rise sharply during initial phases of flares and gradually fall back to preflare state. The light curves of continua surrounding these selected spectral lines rise sharply in the blue part of the spectrum during flares. The continua in the red part exhibit only small rise. Simulations of Hα radiation coming from flare loops yield asymmetric profiles with highly enhanced red wings. 1
|
|
|
A new study of time variability of the Be star LQ And
Lipták, Jozef ; Harmanec, Petr (advisor) ; Korčáková, Daniela (referee)
We contradict binary nature of LQ And. Radial velocities of Hα emision are bound to changes less than 2 km/s. We were not able to detect any periodic variability in radial velocities of core and wings of Hα emision nor of line He I 6678 Å. We discus effect of telluric line movement on precision of our measurement. Detected period of 0,415 d in intensity ratio IV /IR needs aditional confirmation. The star overcome striking increase in intensity of the emission accompained by increase in overall brightness and shortening of photometric period from 0,3094 d to 0,3083 d. Our estimated interval of rotational periods of the star lies in between this photometric period and double of it, which shows that rotation might not be responsible for the photometric changes. 1
|
|
|
Distance to the Magellanic clouds
Kulich, Matúš ; Zasche, Petr (advisor) ; Korčáková, Daniela (referee)
Twelve eclipsing binary systems from the region of the Large Magellanic Cloud have been studied in this thesis in order to calculate the distance. The photometric data come from the OGLE survey carried out at the Las Campanas Observatory in Chile and the spectroscopic data come from other papers where they had already been partially ana- lyzed for other purposes. From these data, the light curves and the radial velocity curves of these systems have been constructed and analyzed using the PHOEBE soft- ware, in which the physical parameters and the absolute magnitudes of their individual components have been determined by the method of synthetic curves. By comparing them to the observed apparent magnitudes in I-band deduced from the light curves and taking into account the bolometric corrections and the V-I indices, the distance modu- lus of the Large Magellanic Cloud could be determined to be 18.59 ± 0.4 mag, which fairly corresponds to the quite familiar results determined by using this and other meth- ods in various research papers. However, in order to determine the distance modulus more accurately, it would be necessary to use accurate, much more complex models of stellar atmospheres and not simply rely on the general relations of the main-se- quence stellar parameters, which exhibit quite a large variance.
|
|
|
A study of the long-term brightness and colour variations of the Be star 88 Herculis
Vitovský, Kristián ; Harmanec, Petr (advisor) ; Korčáková, Daniela (referee)
Photometric observations of the Be star 88 Herculis from 12 different sources were collected covering a period from 1968 to 2019 for the purpose of studying variations on the timescale of years to decades. All the used data was either available in or transformed into the Johnson Standard system. Since systematic photometric observations began in the 1970's the star underwent three long-term variation cycles that differ in strength and length. A quantitative description of each cycle is given. The data suggests that the star is also changing on an even longer timescale in both luminosity and colour. The photometric data was compared to the evolution of the Hα emission line and discussed in connection to possible physical models. It is shown that he object exhibits behavior that corresponds to what is described in literature as the Negative correlation. 1
|
guest ::
