Astronomický ústav

Astronomický ústav Nalezeno 248 záznamů.  předchozí11 - 20dalšíkonec  přejít na záznam: Hledání trvalo 0.00 vteřin. 
Probing dark energy through perfect fluid thermodynamics
Lukes-Gerakopoulos, Georgios ; Acquaviva, G. ; Markakis, K.
We demonstrate that the thermodynamics of a perfect fluid describing baryonic matter can, in certain limits, lead to an equation of state similar to that of dark energy. We keep the cosmic fluid equation of state quite general by just demanding that the speed of sound is positive and less than the speed of light. In this framework, we discuss some propositions by looking at the asymptotic behaviour of the cosmic fluid.
Emergence of magnetic null points in electro-vacuum magnetospheres of compact objects: The case of a plunging neutron star
Kopáček, Ondřej ; Tahamtan, T. ; Karas, Vladimír
We study the possible emergence of magnetic null points which are astrophysically relevant for the processes of magnetic reconnection. While the magnetic reconnection occurs in the presence of plasma and may lead to violent mass ejection, we show here that strong gravitation of the supermassive black hole may actively support the process by suit-ably entangling the field lines even in the electro-vacuum description. In this contribution we further discuss the case of a dipole-type magnetic field of the neutron staron the plunging trajectory to the supermassive black hole. While we have previously shown that given model in principle admits the formation of magnetic null points, here we explore whether and where the null points appear for the astrophysically relevant values of the parameters.
Dynamical analysis approaches in spatially curved FRW spacetimes
Kerachian, M. ; Acquaviva, G. ; Lukes-Gerakopoulos, Georgios
We summarize two agnostic approaches in the framework of spatially curved Friedmann-Robertson-Walker (FRW) cosmologies discussed in detail in (Kerachian et al., 2020, 2019). The first case concerns the dynamics of a fluid with an unspecified barotropic equation of state (EoS), for which the only assumption made is the non-negativity of the fluid’s energy density. The second case concerns the dynamics of a non-minimally coupled real scalar field with unspecified positive potential. For each of these models, we define a new set of dimensionless variables and a new evolution parameter. In the framework of these agnostic setups, we are able to identify several general features, like symmetries, invariant subsetsand critical points, and provide their cosmological interpretation.
Magnetically ejected disks: Equatorial outflows near vertically magnetized black hole
Karas, Vladimír ; Sapountzis, K. ; Janiuk, A.
We show an example of an equatorial outflow driven by a large scale magnetic field. We initiate our computations with a spherically symmetric distribution of gas, which flows on to the domain from a large distance, r R+. After the flow settles in a steady (Bondi) solution, we impose an axially symmetric configuration of a uniform (Wald) magnetic field aligned with the rotation axis of the black hole. Then we evolve the initial configuration numerically by employing the MHD code that approaches the force-free limit of a perfectly conducting fluid. We observe how the magnetic lines of force start accreting with the plasma while an equatorial intermittent outflow develops and goes on ejecting some material away from the black hole.
Propagation of waves in polytropic disks
Horák, Jiří
We derive an analytic dispersion relation for waves in three-dimensional polytropic disks. The problem can be separated to two one-dimensional problems of radial and vertical wave propagation. For the vertical problem, we use and generalize first-order perturbation method for waves near the vertical resonance introduced previously by several authors. Based on comparison of the analytical solutions with numerical integration, we find a surprisingly large region of validity of our dispersion relation including both p-mode and g-mode oscillations.
Automation of processing and photometric data analysis for transiting exoplanets observed with ESO NIR instrument HAWK-I
Blažek, M. ; Kabáth, Petr ; Klocová, Tereza ; Skarka, Marek
Nowadays, when amount of data still increases, it is necessary to automatise their processing. State-of-the-art instruments are capable to produce even tens of thousands of images during a single night. One of them is HAWK-I that is a part of Very Large Telescope of European Southern Observatory. This instrument works in near-infrared band. In my Master thesis, I dealt with developing a pipeline to process data obtained by the instrument. It is written in Python programming language using commands of IRAF astronomical software and it is developed directly for “Fast Photometry Mode” of HAWK-I. In this mode, a large number of data has been obtained during secondary eclipses of exoplanets by their host star. The pipeline was tested by a data set from sorting of the images to making a light curve. The data of WASP-18 system contained almost 40 000 images observed by using a filter centered at 2.09 μm wavelength and there is a plan to process other data sets. A goal of processing of WASP-18 and the other data sets is consecutive analysis of exoplanetary atmospheres of the observed systems.
Z CVn - Still mysterious
Skarka, Marek ; Liška, J. ; Dřevěný, R. ; Sódor, A. ; Kolenberg, B.
We comment on short- and long-term pulsation period variations of Z CVn, a classical RR Lyrae star with the Blazhko effect. Z CVn shows cyclic-like O-C diagram that can be interpreted as a consequence of binarity throught the light travel time effect. We show that this hypothesis is false and that the observed long-term period variations must be caused by some effect that is intrinsic to the star. We also show that the Blazhko period is not simply anti-correlated with the long-term period variations as was suggested by previous authors.
Flows along arch filaments observed in the GRIS `very fast spectroscopic mode'
Gonzalez Manrique, S. J. ; Denker, C. ; Kuckein, C. ; Pastor Yabar, A. ; Collados Vera, M. ; Verma, M. ; Balthasar, H. ; Diercke, A. ; Fischer, C.E. ; Gömöry, P. ; Bello González, N. ; Schlichenmaier, R. ; Cubas Armas, M. ; Berkefeld, T. ; Feller, A. ; Hoch, S. ; Hofmann, A. ; Lagg, A. ; Nicklas, H. ; Orozco Suárez, D. ; Schmidt, D. ; Schmid, W. ; Sigwarth, M. ; Sobotka, Michal ; Solanki, S.K. ; Soltau, D. ; Staude, J. ; Strassmeier, K.G. ; Volkmer, R. ; von der Lühe, O. ; Waldmann, T.A.
A new generation of solar instruments provides improved spectral, spatial, and temporal resolution, thus facilitating a better understanding of dynamic processes on the Sun. High-resolution observations often reveal multiple-component spectral line profiles, which provides information about the chromospheric velocity and magnetic fine structure. We observed an emerging flux region, including two small pores and an arch filament system, on 2015 April 17 with the "very fast spectroscopic mode" of the GREGOR Infrared Spectrograph (GRIS) situated at the 1.5-meter GREGOR solar telescope at Observatorio del Teide, Tenerife, Spain. We discuss this method of obtaining fast (one per minute) spectral scans of the solar surface and its potential to follow dynamic processes on the Sun. We demonstrate the performance of the "very fast spectroscopic mode" by tracking chromospheric high-velocity features in the arch filament system.
Identification of features in solar ALMA images and comparison with solar atmospheric models
Skokić, Ivica ; Brajša, R. ; Sudar, D. ; Kuhar, M. ; Benz, A. O.
Various solar features were identified in the full-disc single-dish ALMA images taken at wavelength of 1.2 and 2.8 mm and compared with H.alpha., Ca II K and EUV images and magnetograms. Assuming thermal bremsstrahlung as the dominant radiation mechanism, several models were constructed to calculate the brightness temperature of various solar features (quiet and active solar regions, filaments and coronal holes) in a wavelength range from 0.3 to 10 mm. It is found that model predictions are in a qualitative agreement with ALMA images where active regions appear brighter, while filaments and coronal holes are not discernible from the quiet Sun level. It is also found that in ALMA images regions around magnetic neutral lines appear darker than the quiet Sun. Models predict a critical frequency below which filaments should appear in absorption and above it in emission.
ALMA Development Plan Study: Solar Research with ALMA. Final report
Brajša, R. ; Skokić, Ivica ; Bárta, Miroslav ; Bastian, T.S. ; Shimojo, M. ; White, S. M. ; Iwai, K.
The final report of the research/development project Solar Research with ALMA: Development study. The project has been demanded by ESO to the Czech node of the European ALMA Regional Center. The main goal of the project is development of the specific Solar Observing Mode for ALMA observatory. The report on demand of ESO summarizes in detail results of the WP3: Optimum strategies for project preparation, execution and post-processing. This summary of the second term of the project is precedded by a brief annoation of results described in the prevoius two mid-term reports. Main result: Based on the test (Science Verification) solar observations, which we took part in Chile in December 2014 and 2015, it has been shown that suggested procedures for solar observations with ALMA, including subsequent calibration and imaging, do work and can be used for regular ALMA science program. The report has been reviewed by international expert panel nominated by ESO.\n\n\n\n

Astronomický ústav : Nalezeno 248 záznamů.   předchozí11 - 20dalšíkonec  přejít na záznam:
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