18. 01. 2017
Dynamics of solar prominences
Abstract: Solar prominences are one of the manifestations of the solar activity. In filtergrams they appear as dark elongated structures on the background of solar photosphere (then called filaments) or as a bright objects above the solar limb. Using spectroscopic methods their Doppler velocity in the line-of-sight may be measured. This information may be further analyzed in two ways. In the first method oscillations of a prominence in the plane-of-the-sky may be detected. The results of analysis of oscillations along with modeling can bring us information of physical parameters in prominences, which is very valuable in so called prominence seismology - one of the branch of solar physics. The second way is to analyze Doppler velocity variation of so called knots in an observed prominence and to combine it with motion in the plane-of-the-sky. As a result full 3D knot trajectory may be constructed, which determines magnetic field topology. In the talk I will present results of observations of prominence dynamics with different telescopes of several prominences as well as synthetic models of oscillating prominence slab.
(Přednáška spojená s udělením Prémie Jana Friče)
14. 12. 2016
Abstract: The results of the interdisciplinary project "Sphaera octava" will be presented. Its goal was to study selected Latin treatises crucial for the development of astronomy from Late Antiquity through Middle Ages up to Early Modern Times. A particular attention was devoted to tracing the evolution of ideas on the fixed stars starting from the ancient description of constellations, through cataloging of stellar positions and magnitudes up to the breaking of the "eighth sphere" by telescopic observations. The texts like Hyginus's "Astronomy", Al Sufi's "Catalogue of stars", Galilei's "Sidereus nuncius" and others were translated into Czech and commented. In addition, the unique precession celestial globe from the collection of Nicolaus Cusanus was investigated.
14. 12. 2016
3-D Radiative Transfer in inhomogeneous winds of hot massive stars
Abstract: Traditionally, hot massive stars have been studied under the assumption that their winds are homogeneous and stationary. However, observations with the newest instruments, together with progress in model calculations, ultimately dictate a cardinal change of this paradigm: stellar winds are highly inhomogeneous. In this talk I'll present the new sophisticated 3-D models of radiation transfer in inhomogeneous expanding media which we developed in order to elucidate the physics of stellar winds and improve classical empiric mass-loss rate diagnostics. I'll also present applications of these new techniques to multi-wavelength observations of hot massive stars which yield consistent and robust stellar wind parameters.
14. 12. 2016
CME plasma diagnostics using space coronagraphs
Abstract: We will present results of a recent study of a hot prominence embedded in the core of large Coronal Mass Ejection (CME) as was observed by the SOHO satellite. Physical parameters of the prominence plasma were derived using SOHO/UVCS hydrogen Lα and Lβ line spectra and SOHO/LASCO visible-light images. We developed a diagnostic tool based on the non-LTE radiative transfer code which accounts for large velocities of the moving CME. We will also briefly discuss a complementary diagnostics based on the observed CIII line. The methods developed in this work will serve for future data analysis from the METIS coronagraph on board the Solar Orbiter mission (ESA).
14. 12. 2016
Very young asteroid pairs candidates
Abstract: Every asteroid pair consists of two components that share practically the same heliocentric orbit though they are not tied together any more. Shortly after their discovery 8 years ago many general properties of asteroid pairs were revealed (e.g. typical sizes, shapes, mass ratios of their components, correlation of the spin rate of primaries with the mass ratio, prevailing formation mechanism by rotational fission). Our knowledge about that population extended with additional data. Although initial results have been confirmed or nailed down, some details remain open. For example, how exactly asteroid pairs form? How frequently asteroid pairs form? With the estimated ages of known pairs (typically of the order of 10 4 to 10 6 years) and large uncertainties, answers to the questions above can hardly be found. It is believed that the study of the youngest pairs could shed some light on the problem. Recently, we have found a few candidates for very young asteroid pairs – perhaps younger than 10 4 years. But even their age determination is not easy. Large uncertainty or even ambiguity are caused due to several reasons, such as extremely slow separation velocity of pair components, components can repeatedly closely encounter each other several times, some of their physical properties, which are critical for orbital evolution, are usually unknown.
03. 10. 2016
Radioastronomy in the Czech Republic: Tradition, current state and perspectives
Abstract: It is now a bit more than 80 years from the early days that radio astronomy has begun opening a new window to Universe - it is thus still quite a young branch of science. Nevertheless, during its relatively short history it changed our view to many astrophysical objects and processes a lot. The recent years see a new revolution in this fascinating field. Namely thanks to rapid progress in computing technology, really large interferometric arrays based on aperture synthesis start to appear from millimetre to kilometre wavelength range - ALMA,LOFAR or SKA can serve as most notoriously known examples. Czech astronomy does not stay aside of this development: Solar radio spectrograph - the second device of its kind in Europe - started its operation in Ondřejov already in 1967. Meanwhile it underwent several modernizations and dozens of original research papers is based on data acquired by this radio-telescope. A big milestone in development of the Czech radio-astronomy came with our participation in ESO. Also thanks to international recognition of radio spectroscopy in Ondřejov, one of the seven nodes of European ALMA Regional Centre (EU ARC) has been established and hosted at our Institute. Since this year it became one of the National Research Infrastructures in CR. In addition to standard user services it acts as the European leader in development and operation of specific Solar ALMA Observing Mode. Progress in IT brought revolution also to solar radio spectroscopy. Modern FFT-based spectrographs reach temporal and frequency resolutions that reveal a new world of fine structures and corresponding plasma processes in the solar corona. This kind of data represent important contribution to our understanding dynamics of upper solar atmosphere, pretty complementary to high-resolution interferometric ALMA images of the lower layers. The contribution aims to review the current state of (namely solar) radio astronomy in the Czech Republic and indicate directions of its future development.
03. 10. 2016
The structure of large meteoroids and small asteroids from observations of superbolides
Abstract: Superbolides are extremely bright meteors produced by entries of meter-sized bodies into terrestrial atmosphere. They do not occur frequently and good observations of these events are quite rare. I will present detailed analysis of a superbolide, which occurred over Romania on January 7, 2015. The trajectory, velocity, and orbit were determined using two all-sky photographs from a station of the European Fireball Network (EN) in Slovakia and five casual video records from Romania, which were carefully calibrated. Bolide light curve was measured by EN radiometers. The orbit was asteroidal with low inclination and aphelion inside Jupiter's orbit. The atmospheric behavior was, however, not typical for an steroidal body. The peak brightness of absolute magnitude of -21 was reached at a quite large height of 42.8 km and the brightness then decreased quickly. A comparison was made with three other superbolides for which we have radiometric light curves: ordinary chondrite fall Košice, carbonaceous chondrite fall Maribo, and cometary Taurid bolide of October 31, 2015. The Romanian superbolide was not similar to any of these and represents probably a new type of asteroidal material, which is not represented in meteorite collections.
03. 10. 2016
The evolution of low-metallicity massive stars
Abstract: Massive star evolution taking place in astrophysical environments consisting almost entirely of hydrogen and helium – in other words, low-metallicity environments – is responsible for some of the most intriguing and energetic cosmic phenomena, including supernovae, gamma- ray bursts and gravitational waves. Therefore, to model rotating and non-rotating massive stars with low-metallicity is of crutial importance.I present a large set of evolutionary models of massive stars (betweeen 9-600 solar masses) with and without rotation. The initial metal composition of the models is appropriate for the low-metallicity dwarf galaxy I Zwicky 18. To test the models, I compare the predicted flux of ionizing photons to that observed in this galaxy, and find that, due to the predicted presence of hot stars with weak winds at this metallicity, the models can self-consistently explain both the high ionizing flux and the low number of Wolf–Rayet stars in I Zwicky 18. Moreover, some of our models are predicted to explode as long-duration gamma-ray bursts. Thus, we speculate that the high ionizing flux observed can be a signpost for upcoming gamma-ray bursts in dwarf galaxies. The models are also applied to explain anomalous abundance patterns observed in galactic globular clusters. In particular, I show that low-metallcity supergiants might have played an important role in the pollution of the second generation of globular cluster stars. As an outlook, I suggest that the models be applied in the context of the early Universe, since the majority of the first few generations of stars were also massive and metal-poor.
03. 10. 2016
Dust in the wind
Abstract: The growing evidence pointing at core-collapse supernovae as large dust producers makes young massive stellar clusters (SSCs) ideal laboratories to study the evolution of dust immersed into a hot plasma. Dust grains by supernovae are heated by means of random collisions with gas particles and the absorption of UV photons which influence their spectral energy distributions (SEDs) in the infrared. I will present theoretical SEDs which include the effects of thermal sputtering and stochastic dust temperature fluctuations. These results are compared to observed SEDs, in particular to the case of the low-metallicity blue compact dwarf I Zw 18.
28. 06. 2016
Long-term activity of soft X-ray transients
Abstract: We discuss the long-term (years) activity of soft X-ray transients. These systems contain a compact object (neutron star, black hole) accreting matter from their low-mass, lobe filling companion. We show how this activity manifests itself in the X-ray spectral region. We pay special attention to the intense outbursts of such systems. We present examples of the light curves of such objects obtained by monitors of X-ray emission (ASM/RXTE and BAT/Swift). These monitors are important instruments for observing the activity on long timescales (even of more than ten years). We show that the X-ray light curves of X-ray transients obtained by the monitors contain various very useful features which can be used for the astrophysical analyzes.
28. 06. 2016
Spectroscopy and modeling of Ellerman bursts
Abstract: Observations of Ellerman bursts (EBs) show them as a small brightenings well observed in different chromospheric lines, both in the optical and UV spectral ranges (H I, Ca II, Mg II). EBs are manifestation of small reconnection processes occurred in the lower solar atmosphere and they can contribute to the chromospheric heating. H-alpha line profiles of EBs exhibit enhanced emission at line-wings even up to a few angstroms from the line core. On the contrary, Ca II and Mg II lines emitted by EBs can be strongly enhanced also in the line cores. We used new UV (IRIS) spectroscopic observations to find models of different EBs observed in the new magnetic flux emergence area. Obtained models of EBs showed that they can be described by one or two component 'hot-spot’ thermal structure. We found that EBs can be formed in an extended domain of altitudes in the photosphere and/or the chromosphere (400 to 750 km). Our results are consistent with the theory of heating by Joule dissipation in the atmosphere produced by magnetic field reconnection during flux emergence. I will also discuss the new EBs diagnostics possibilities based on the new radio observations in millimeter range obtained in future by ALMA instrument.
28. 06. 2016
New regime of fragmentation of gaseous layers
Abstract: Massive stars strongly influence their surrounding interstellar medium mainly via photo-ionisation, stellar winds and supernova explosions. These processes are able to sweep up significant amounts of the surrounding gas into cold and dense shells. The shells fragment due to their self-gravity and if they form sufficiently massive stars, the whole process can repeat itself leading to propagating star formation. To circumvent numerical difficulties when modelling this process, we approximate a part of such a shell by an isothermal layer and investigate its fragmentation. The layer is self-gravitating and is confined by external pressure. The importance of self-gravity relative to external pressure is a free parameter in our models. In linear regime of fragmentation, we compare our numerical dispersion relation with analytical estimates found in literature. We follow the fragmentation in the non-linear regime and find that the layer fragments in two qualitatively different ways that were not reported previously. If the external pressure dominates, the layer firstly breaks into gravitationally unbound fragments which merge until they form gravitationally bound objects. On the other hand, if self-gravity dominates, the layer monotonically collapses to bound objects. We use this simple model to provide estimates for fragmenting time and mass of fragments formed in a shell powered by an HII region.
28. 06. 2016
Stubenberg meteorite fall - demonstration of capabilities of the Czech fireball network
Abstract: Instrumental observations of fireballs, especially those that can produce meteorites, are of great scientific interest and importance because meteorites provide us with a surviving physical record of the formation of our solar system and a direct link to their parent asteroids. Last such event occurred over Upper Austria in the late evening of March 6, 2016 and terminated exactly over Upper Austria-Bavaria border. This -15 magnitude bolide terminated its light very deeply in the atmosphere and heavily fragmented in the last third of its luminous flight, so the fall of a larger number of meteorites of different sizes was almost certain. In spite of a quite bad weather situation over whole Central Europe, this bolide was well recorded optically by 7 Digital Autonomous Fireball Observatories (DAFO) in the Czech part of the European Fireball Network. Apart from these optical records almost all DAFOs in our network provided us with high resolution light curves. It enabled us to get very reliable and complex information about position, dynamics, photometry, absolute timing and initial orbit of this extraordinary event. This case very illustratively demonstrates the capability of our recently modernized fireball network because only scientifically useful records of this extraordinary case were taken by our automated stations. Thanks to it we were also able to predict the possible impact area already second day after the bolide and meteorites could be recovered in this predicted area so soon. The first results based on the analysis of the available records will be presented. Recovery of meteorites and their analysis will be also shortly mentioned.
14. 03. 2016
On the direct detection of gravitational waves
Abstract: Current information about recent announcement of the first in history detection of gravitational waves.
14. 03. 2016
The past activity of the Galactic centre
Abstract: Despite the low level of current activity in the Galactic centre, X-ray reflection from molecular clouds in the Sgr A* region indicates that the supermassive black hole was orders of magnitude brighter just a few hundred years ago than it is currently. We investigate the idea of the Galactic centre minispiral as the origin of gaseous material for the enhanced past activity of Sgr A*. Collisions between clumps of gas in the minispiral can reduce their angular momentum and set some of the clumps on a plunging trajectory towards the supermassive black hole. We demonstrate that the amount of material contained in the minispiral is sufficient to sustain the luminosity of Sgr A* at the required level. We also study the possibility of the formation of thermal instability in the two-phase minispiral region, enhancing the accretion of clouds by the central black hole.
14. 03. 2016
Evolution of binary near-Earth asteroids
Abstract: To date, about 50 binary near-Earth asteroids, 20 Mars-crossing and 80 small Main Belt binary asteroids are known, most of which were observed within our programme. For three of them we have data spanning more than 14 years, which allow us to study their long-term evolution. Orbits of natural satellites of asteroids are subject to tidal evolution and the so-called BYORP effect, causing shrinking or expanding of orbit due to solar radiation pressure. If the BYORP effect is removing angular momentum from the orbit of the satellite, a counterbalance of BYORP and mutual tides can result in a long-term stable solution. I will show the observational evidence of two Binary asteroids with tidally locked satellite and one with long-term evolution of the satellite's orbit. The observations of long-term evolution have important implications for asteroid geophysics. From the equilibrium between mutual tides and the BYORP effect we can derive a rigidity of the main body, allowing us to reveal whether it has monolithic structure or it is the so-called 'rubble-pile'.
14. 03. 2016
High-resolution fine-structure synthetic imaging of an entire prominence using 3D whole-prominence fine structure modelling
Abstract: The newly developed 3D whole-prominence fine structure (WPFS) model (Gunár & Mackay 2015) allows us for the first time to simulate entire prominences/filaments including their numerous fine structures. This model combines a 3D magnetic field configuration of an entire prominence obtained from non-linear force-free field simulations, with a detailed description of the prominence plasma. The plasma is located in magnetic dips in hydrostatic equilibrium and is distributed along hundreds of fine structures within the 3D magnetic model. The prominence plasma has realistic density and temperature distributions including the prominence-corona transition region. To produce the high-resolution synthetic H-alpha images of the WPFS model we use a novel fast approximate radiative transfer visualization technique (Heinzel et al. 2015). This allows us for the first time to produce images of the prominences in emission on the solar limb and filaments in absorption against the solar disk using a single model. The prominence plasma and magnetic field are described in the WPFS model on scales that allow us to produce synthetic images with resolution matching that of the state-of-the-art Hinode/SOT observations, or indeed that of the upcoming solar observatories, such as DKIST or Solar-C. Moreover, to complement the prominence/filament synthetic images we have consistent information about the magnetic field and plasma parameters everywhere in the modelled prominences. This allows us to investigate the apparent puzzling nature of the observed prominence and filament fine structures. We can also study the connections between the local configuration of the magnetic field and the observable structure of the finest prominence/filament features. In addition, we are able to investigate the prominence evolution. We can consistently study the influence of the varying photospheric flux distribution on the prominence magnetic field configuration and its effect on the observable prominence plasma.
14. 03. 2016
Study of Gamma-Ray Bursts With Robotic Telescopes
Abstract: I will talk about gamma-ray bursts (GRBs), observed as brief flashes of gamma radiation of cosmic origin. The phenomena, now considered one of the most energetic phenomena in the universe, was discovered in 1967. Nowadays, with specialized satellites, GRBs are detected and localized automatically, and triggers are generated and sent to ground-based robotic telescopes. This way, a multi wavelength studies, employing data ranging from gamma-rays through X-rays and optical to radio wavelengths, permit a complex view onto these events, since the discovery of the afterglows - the emission on other wavelengths in 1997. I will provide a very brief introduction into the GRB physics, followed by a few examples of studies of individual gamma-ray bursts. Then I will briefly present an effort done to summarize all the successful GRB follow-ups performed by BOOTES-1B and BOOTES-2 during the past decade. The last few minutes will be dedicated to the Compact Low Resolution Spectrograph (COLORES), a lightweight FOSC instrument we developed and have been successfully operating at BOOTES-2 since 2012.
16. 12. 2015
Initial mass function and feedback in dwarf galaxies
Abstract: The stellar initial mass function (IMF) is a crucial quantity for the evolution of galaxies. It determines the fraction of stars in various mass ranges; hence it affects various crucial feedback mechanisms (e.g. from supernovae and radiation by massive stars). In the last years, systematic IMF variations in galaxies, particularly in low-mass ones, have been reported. I will review in this seminar what is know about IMF variations in dwarf galaxies and I will show how these variations affect the chemical evolution of galaxies. I will describe in particular the so-called integrated galactic IMF (IGIMF) theory, according to which the IMF is steeper in dwarf galaxies and flatter in larger ones. I will show which evidences support this theory and which evidenced do not support it. However, the IMF does not only change the rate of metal production. It changes also the dynamics and thermodynamics of gas, which in turn affect star formation and metal circulation. I will then review our efforts to implemented IMF variations in chemo-dynamical simulations of dwarf-galaxy evolution and I will show the multiple ways in which IMF variations can affect the dynamical and chemical evolution of galaxies. I will finish my presentation with some new ideas and modelling of the induced star formation in supershells and its connection with the formation and evolution of globular clusters.
16. 12. 2015
Recent meteor shower outbursts and storms - models and observations
Abstract: A number of the meteor shower outbursts and storms occurred in recent years starting with several Leonid storms around 2000. The methods of the meteoroid streams modelling became better and more precise. An increasing number of the observing systems enabled better coverage of such events. The observers provide modellers with an important feedback on precision of their models. Here we present comparison of several observational results with the model predictions. As the main source of the data the video observations of our team are used.
16. 12. 2015
Space Instrumentation Development at the Astronomical Institute
Abstract: The Solar Physics Department studies is not only focused on the basic research and ground observations of the active phenomena from the solar atmosphere throughout the whole heliosphere but also participates in development of remote/in-situ space experiments on several future ESA missions. Currently the largest involvement inheres in the Solar Orbiter mission (namely the RPW, STIX, and METIS experiments) with a planned launch in 2018. Another two contributions are related to the ASPIICS sensor on mission Proba-3 and RPWI instrument on mission JUICE. Here the current status of all the space hardware activities will be presented.
16. 12. 2015
Astroinformatics - The Key to New Astronomical Discoveries
Abstract: The current astronomy is flooded by the data. Some instruments have been already producing petabyte-scaled data flows, the reduction and analysis of which require supercomputing clusters. Astronomy becomes the data-driven science where the major part of budget of big projects will not go into telescope and instrumentation but to the data processing infrastructure. Astronomy, as well as other science disciplines has begun to recognize informatics, advanced statistics and machine learning as a key part of its methodology and new astronomical discoveries are expected from knowledge extraction of federated astronomical databases and archives. The new ways of multi-dimensional data analysis are being tested using wider range of 3D visualization devices including the immersive Virtual reality and virtual worlds. The changing nature of next-decade astronomy also requires education of a new type of experts - the astronomical data scientist with deep knowledge of both astronomy and physics as well as modern computer science and software engineering, well prepared to analyze petabyte-scale data in multi-cloud environment. The need to address all such problems led to the emergence of a new astronomical discipline - Astroinformatics. We give a brief overview of its subjects and methodology and present some astroinformatics projects ongoing in the Astronomical Institute of CAS in collaboration with several faculties of information technology in CR.
01. 06. 2015
Multi-Generation Star Clusters
Abstract: Massive star clusters are essential components of galaxies. When they form and during their early times they influence the surrounding interstellar medium with radiation, winds and supernova ejecta. This stellar feedback removes the gas out of the parent molecular cloud, star cluster expands, and in many cases it dissolves completely. Remaining globular star clusters reside in galaxy haloes. The model of massive star cluster formation as the monolithic collapse of the parent molecular cloud needs to be revisited due to discovery of multiple stellar generations of long-lived stars in globular clusters. We propose a new "cooling winds model" explaning formation of the second stellar generation out of matter that is enriched by H-burning products of the first stellar generation. Above a critical mass of the star cluster, winds form thermally unstable clumps reducing locally the temperature and pressure of the hot 10^7 K cluster wind. The matter reinserted by stars, and mass loaded in interactions with pristine gas and with evaporating circumstellar disks, accumulates in clumps that are ionized with photons produced by massive stars. We shall discuss if they may become self-shielded when they reach the central part of the cluster, or even before it during their free fall to the cluster center. We shall explore the importance of early mass loading of the stellar winds, and we shall analyse the role of additional cooling by dust produced in supernova events.
01. 06. 2015
Non-Maxwellian distributions in the solar corona
Abstract: Observations of the solar wind show pervasive presence of the non-Maxwellian kappa-distributions of electron energies characterized by a power-law high-energy tail. Theory has shown that such distributions can arise in the solar corona during impulsive heating by magnetic reconnection and also wave-particle interactions. We explored the consequences of the presence of such kappa-distributions on the optically thin spectra of the solar corona emitted in the X-rays, UV, and visible wavelengths. In particular, several line combinations allow for diagnostics of the kappa-distributions. A diagnostic from the EUV line ratios observed by Hinode/EIS space-borne spectrometer is performed and it is shown that the electron distribution can be strongly non-Maxwellian.
01. 06. 2015
High-Mass X-ray Binaries in the Small Magellanic Cloud
Abstract: High-Mass X-ray Binaries (HMXBs) are a phase in the life of some binary stellar systems that consist of a compact object (black hole or neutron star) and a massive companion (an early OB-type star). Their X-ray emission is powered by the infall of matter, provided by the massive companion, into the strong gravitational field of the compact star. The Small Magellanic Cloud (SMC) is a powerhouse of HMXB production (almost 100 systems), and due to its proximity we are able to investigate individual sources. However, we haven't yet fully characterize its HMXB population. To address that we have initiated wide spectroscopic and Halpha imaging campaigns. I will discuss our results and how the SMC HMXB population compares with that of our Galaxy.
01. 06. 2015
Subcatastrophic collisions between asteroids
Abstract: Mutual collisions between asteroids affect their size distribution, spins and surface morphology. Subcatastrophic collisions may be responsible for an excited asteroid rotations. We created a numerical model to investigate this hypothesis and also other effects of small collisions between asteroids. Recently, we showed that erosion caused by collisions increases the elongation of asteroid shapes on a timescale much longer than their collisional lifetime.
23. 03. 2015
Solar spectropolarimetry from 0-D to 3-D geometry
Abstract: Scattering line polarization and the Hanle effect are among the most important mechanisms for diagnostics of the solar and stellar atmospheres. The fact that real stellar atmospheres are horizontally inhomogeneous makes the spectral synthesis and interpretation very challenging because the effect of thermodynamic fluctuations on spectral line polarization is entangled with the action of magnetic fields. This applies to the spatially resolved as well as to the averaged spectra. The necessary step towards the interpretation of such spectra is to study the line formation in sufficiently realistic 3D MHD models and to compare the synthetic spectra with observations. I'll present a brief overview of the relevant mechanisms involved in polarized spectral line formation and the recent progress in the field of 3D NLTE synthesis of polarized spectral lines.
23. 03. 2015
HI shells: why I find them interesting even after 20 years' acquaintance
Abstract: I will give an introduction to HI shells: what they are, where we can find them and why many members of our working group study them. I will focus on the connection between HI shells and other gaseous environment in the Galaxy and will deal with the importance of HI shells in the evolution of galaxies and the Galaxy.
23. 03. 2015
Exoplanets at Ondřejov
Abstract: Since 1995, when the first exoplanet was detected, this scientific field is rapidly developing. Nowadays, we are not only detecting the new worlds but we try to understand how did they evolve and how they do look like. The most recent topic is characterization of exoplanetary atmospheres. Current situation, advances and plans for the future development of the exoplantary research in Ondřejov will be presented.
23. 03. 2015
Bolide Žďár – First results of the recent precisely documented meteorite fall
Abstract: Instrumental observations of fireballs, especially those that can produce meteorites, are of great scientific interest and importance because meteorites provide us with a surviving physical record of the formation of our solar system, and a direct link to their parent asteroids. Last such event occurred over Czech Republic on December 9, 2014 at late dusk. It was exceptionally well documented because cameras of the Czech part of the European Fireball Network provided us with 10 all-sky photographic images and 21 radiometric records. It enabled us to get very reliable and complex information about position, dynamics, photometry and absolute timing of this extraordinary event. The first results based on the analysis of the best available records and describing fireball trajectory, luminosity, dynamics, fragmentation, orbit, and impact position will be presented. Recovery of meteorites and their analysis will be also shortly mentioned.
17. 12. 2014
Solar Flares and Superflares
Abstract: First, some examples of solar flares are presented and the standard model of solar flares is explained. Then, our recent results in observations and in the magnetohydrodynamic and particle-in-cell numerical simulations of solar flare processes are shown. We present the radio drifting pulsation structures, our new concept of fragmented magnetic reconnection, plasmoids and separation of the accelerated particles in magnetic reconnection. We also try to answer a question about possible solar superflares. Finally, we summarize a usage of results of solar flare studies.
17. 12. 2014
X-ray reverberation in Active Galactic Nuclei
Abstract: Active Galactic Nuclei (AGN) are characterised by very high energy output and large variability. This is caused by a supermassive black hole harbouring in their heart that significantly feeds on the nearby galactic matter. Recently a new technique has been developed to be used on latest as well as archival X-ray observations of AGN - reverberation mapping. It is hoped that with this method we will at last be able to uncover the geometry and physical properties of the AGN inner regions. In my talk I will explain basic principles of X-ray reverberation mapping and summarise our recent theoretical research in this domain.
17. 12. 2014
Pulsations as mass-loss trigger in blue supergiants
Abstract: Blue supergiant stars (BSGs) are known to display photometric and spectroscopic variability, which are suggested to be linked to stellar pulsations. On the other hand, pulsational activity in massive stars depends on the star’s evolutionary stage and is assumed to be connected with mass-loss episodes, the appearance of macroturbulent line broadening, and the formation of clumps in the wind. We initiated an observational campaign aimed at studying a large sample of Galactic BSGs. Based on high-quality spectroscopic data, we determine stellar and wind parameters from fitting spectral lines, and perform a period analysis based on established methods from asteroseismology. Our goal is to investigate a possible interplay between pulsations and mass-loss in these stars. First results will be presented.
17. 12. 2014
Photographic fireballs of 2013 September epsilon Perseids outburst
Abstract: Exceptional high activity of bright photographic fireballs belonging to September epsilon Perseid (SPE) meteor shower was observed on 9 September 2013. Photographic cameras of the Czech part of the European fireball network recorded 12 multi-station SPE meteors, and thus it was possible to determine their atmospheric trajectories and heliocentric orbits. Thanks to relatively large number of meteors it was also possible to describe reliably physical properties of unknown parent body of the shower and compare these properties with other meteor showers. Atmospheric ablation of SPE is very similar to that of Perseid and Orionid meteoroids.
16. 06. 2014
Search for faint meteors on the orbits of Pribram and Neuschwanstein meteorites
Abstract: We observed the faint meteors on the orbits close to the orbits of Pribram and Neuschwanstein meteorite falls and investigated the possibility that they belong to the stream. Several meteors with low orbital similarity criteria to Pribram and Neuschwanstein meteoroids were found. The atmospheric trajectories and heliocentric orbits of the detected meteors were analyzed to determine whether they are members of the same shower. An orbital evolution model was applied on a certain number of cloned particles to investigate their possible connection with the meteorite stream. Statistical tests were conducted to determine if such small sample of the orbits is similar by chance or if the stream is real. It was found that from the observational as well as the theoretical point of view it is impossible to prove the existence of faint meteor shower connected with the Pribram and Neuschwanstein meteorite stream.
13. 05. 2014
Science with the Arecibo Galaxy Environment Survey
Abstract: I present an overview of the Arecibo Galaxy Environment Survey, AGES. AGES is an extragalactic neutral hydrogen survey targeting a range of different environments, from the Local Void to rich clusters. When complete the survey will cover a total of 200 square degrees to an rms sensitivity of 0.7 mJy, equivalent to an HI mass of ∼ 10^7 M at the distance of the Virgo Cluster. I describe some of the results of the survey so far : 1) We have completed the observations for three isolated galaxies and find they have at most one companion each, far less than expected based on the HI mass function from the larger ALFALFA survey; 2) We found 8 HI detections within the Virgo cluster without obvious optical counterparts, some of which have velocity widths too large to fit the Tully-Fisher relation observed in the field (perhaps indicating they are non-primordial debris, but we do not rule out the prospect that they are so-called ”dark galaxies”); 3) Behind the galaxy group associated with NGC 7448, we find a dense filamentary structure of galaxies rich in HI streams, some of which are in excess of 800 kpc in length.
07. 04. 2014
Dynamical evolution of the disk of the Be star 60 Cygni
Abstract: Be stars are still very unknown in the respect of origin and geometry of circumstellar disk around a star. We present here a modified version of the program Shellspec, which is designed to solve simple radiative transfer along the line of sight in 3D moving media. We used this program for the model: a star with a disk (simple model for Be stars), and applied this model to study a certain star. We present here preliminary results for Be star 60 Cygni and its disk evolution (evolution of the inner and outer radius, opening angle, etc.) between years 2003 and 2011.
03. 03. 2014
Proton thermal energetics in the solar wind
Abstract: The proton temperature in the solar wind decreases slower than what is expected from adiabatic prediction, protons need to be heated. The proton thermal energetics is however more complicated because the solar wind protons are only weakly affected by Coulomb collisions and, consequently, proton parallel and perpendicular temperatures (with respect to the ambient magnetic field) need to be treated separately. Using Helios in situ observations between 0.3 and 1 AU proton parallel and perpendicular heating rates are estimated. Helios observations indicate that protons need to be heated in the perpendicular direction whereas in the parallel direction protons need to be cooled at 0.3 AU with a cooling rate comparable to the corresponding perpendicular heating rate; between 0.3 and 1 AU the required cooling rate decreases until a transition to heating occurs: by 1 AU the protons require parallel heating, with a heating rate comparable to that required to sustain the perpendicular temperature. The estimated heating and cooling rates are compared with estimations of the turbulent energy cascade rate at 1 AU and with results of numerical simulations of proton kinetic effects in the expanding solar wind.
09. 12. 2013
Recent results from binary and precessing asteroids modelling
Abstract: Determination of binary and precessing asteroid shapes and rotation parameters from inversion of their light curves have reached interesting applications in last years. Two recent results will be presented: 1. Photometric observations of binary Near-Earth asteroid 1996 FG3 taken from 1996 to 2013 lead to a solution for a quadratic drift of the mean anomaly of the mutual orbit close to zero value. This result confirms a recent theory of binary YORP (BYORP) effect where asymetric emission of thermal radiation and mutual tides between the two components are taken into account. 2. An inversion of a shape and a rotation state of a precessing asteroid (99942) Apophis will be presented. The outcome of Apophis modeling will be important for its Earth-impact probability refining in future, including the YORP and Yarkovski effects.
04. 11. 2013
Automatic Spectral Classification
Abstract: The ability to automatically find spectra of given properties in large data set could be vital for many reasons in astronomy. Discovering rare phenomena (like emission stars) or finding objects with extreme features as an example. Automation is also crucial because of dramatic increase of data volume produced by modern surveys such as SDSS and LAMOST. Tools and techniques of data acquisition, pre-processing and classification are discussed. Suggested methods are demonstrated on Ondrejov 2m telescope archive and Sloan Digital Sky Survey (SDSS).
23. 09. 2013
Structure of the NSC of the Milky Way Galaxy
Abstract: Nuclear star clusters are unambiguosly detected in about 50--70 % of spiral and spheroidal galaxies. They have typical half-light radii of 2--5 pc, dynamical mass ranging from 10^6--10^7 Msun, are brighter than globular clusters, and obey similar scaling relations with host galaxies as supermassive black holes. The Nuclear Stellar Cluster (NSC) which surrounds Sgr A*, the SMBH at the centre of our galaxy, is the nearest nuclear cluster to us, and can be resolved to scales of milliparsecs. The strong and highly variably extinction towards the Galactic centre makes it very hard to infer the intrinsic properties of the NSC (structure and size). We attempt a new way to infer its properties by using Spitzer MIR images in a wavelength regime (3--8 microns) where the extinction is at a minimum, and the NSC clearly stands out as a separate structure. We present results from our analysis, including extinction-corrected images and surface brightness profiles of the central few hundred parsecs of the Milky Way.
03. 06. 2013
Observations of solar prominences with ALMA
Abstract: During the last decades, several studies had focused on the determination of the prominence temperatures. However, practically all of them were based on the analysis of mainly optical spectral line shapes which brings several difficulties and uncertainties, namely in cases when the spectral lines are optically thick. At the same time, the need for a reliable determination of the temperature in central coolest parts of quiescent prominences has grown as a result of new modeling efforts trying to understand the energy-balance conditions. Using the microwave spectra with a high spatial resolution offers the new possibility. We demonstrate that the fine structures in solar quiescent prominences can be well detected with the ALMA interferometer (Atacama Large Millimeter Array in Chile, joint project of ESO, USA and Japan). ALMA is going to be fully operational and a special hardware setup will allow soon the solar observations. We simulate the visibility of prominence fine structures, their brightness temperatures at various wavelengths and demonstrate the feasibility and usefulness of ALMA observations of solar prominences. Our basic approach is the conversion of high-resolution H-alpha coronagraphic images into the microwave spectral images. Finally, we apply the ALMA image-processing software CASA to our simulated images to see what the ALMA would detect in reality. We discuss various ways how to determine the prominence kinetic temperature from ALMA spectral images.
13. 05. 2013
Mystery of the Benešov bolide revealed after 20 years; The Chelyabinsk superbolide – brief information about the first analyses of video records of this historic event
Abstract: The fall of the bolide Benešov occurred over Central Bohemia 22 years ago, on 7 May 1991. It became one of the most important bolide ever observed and from the very beginning it was clear that this very bright event terminated by a meteorite fall. However despite great efforts and many attempts no meteorite was found in the weeks and years after the fall. In spring 2011, just before 20th anniversary of this extraordinary case, all available all-sky records were re-measured and re-analyzed. Slightly different methods and new approaches which were gradually developed for analysis of several recent instrumentally observed meteorite falls resulted in finding of a new consistent picture of the Benešov event. It explained unsuccessful search in years after the fall and suggested new strategy which could lead to recovery of Benešov meteorites after 20 years. This new scenario was completely confirmed by find of for small meteorites of corresponding masses exactly in the predicted impact area. This result is in many aspects pioneering. It was proved that in some special cases it is still possible to predict and find meteorites long time after the fall. However the most important result is in heterogeneity of the recovered meteorites. Three different types of material discovered in these meteorites clearly show that larger meteoroids can be compositionally very complicated bodies. The Chelyabinsk superbolide of February 15, 2013 was the most energetic meteoric event globally since the Tunguska explosion in 1908. The blast wave caused moderate damage (broken windows etc.) in the city of Chelyabinsk and surrounding areas and injured about 1500 people. Thanks to numerous casual video records, the superbolide was well documented. Precise positional calibration of the videos represents, however, a challenge. We will report our work in progress of determining the bolide trajectory, velocity, fragmentation and dust trail formation.
04. 03. 2013
From Kepler to fine orbit tuning of GOCE
Abstract: Johanes Kepler, living in Prague, utilized observations carried out by Tycho Brahe also on the Czech territory to formulate the law known today as the “3rd Kepler law”. We recall it in its classical form and will derive it for relationship between the mean motion and selected mean semi-major axis of orbit of an artificial Earth satellite (AES) accounting for the polar flattening of the Earth. This simple tool has amazing applications in selection of orbits of AES to fulfill given tasks “at the best”. There is a close relation between density of ground tracks by satellite data, on which in turn precision and resolution of products derived from such measurements are dependent, and semi-major axis (or height of flight) of the satellite used. We tell the story of the GRACE satellites, which during their free decay in the atmosphere encountered important orbit resonances during those the density of ground tracks significantly (but temporarily) decreased. Consequently, also the quality of the monthly solutions for the variations of the gravity field has decreased. What we learned from GRACE, we applied for orbit choice of GOCE, the first satellite equipped by a gravity gradiometer. In this matter, we joined the manager of the GOCE project Dr. Rune Flobergahen, ESA. GOCE is also equipped by ultraprecise ion motor for orbit corrections so that the height of flight can be kept within unbelievably small range ±5 m. It is feasible to choose and keep the orbit at selected higher order resonance and by this way to reach the maximum quality of products derived from gradiometric measurements. This process is called „fine orbit tuning“. Orbit choice for remote sensing AES affects significantly the accuracy of the results derived from their measurements, thus our approach captured interest of ESA in the project of bistatic altimetric satellite GNSS-R. We also investigated the orbit choice for planetary orbiters of Mars, Venus and the Moon from viewpoint of possible most accurate determination of gravity field parameters of these bodies.
04. 02. 2013
Self-consistent polarized radiative transfer: Connecting theory and observations
Abstract: In the stellar atmospheres, state of matter at different points of the plasmas is non-linearly coupled by radiation transfer. Multiple processes leave their fingerprints in polarization state of light (scattering, collisions, magnetic fields, etc.). Polarization is thus a key observable providing invaluable information on the local physical conditions. Direct inversion of the physical quantities is often mathematically ill defined but a comparison of the observations with sufficiently realistic models provides suitable quantitative diagnostics and constrains of our models. I will briefly review the processes involved in the non-LTE polarized radiative transfer problem and I will show how is the ongoing research in this field related to the present and future space experiments in solar physics.
05. 11. 2012
Omicron Puppis: another Be + sdO binary?
Abstract: The spectrum of the Be star o Pup is shown to vary periodically with a period of 28.9 days. A radial-velocity variable He I 6678 A emission component suggests that the star is the same type as phi Per, 59 Cyg and FY CMa, i.e. binaries consisting of a Be star and a hot companion that illuminates the Be star disk (also called phi Per-type binaries or Be+sdO binaries). The range of radial-velocity variations of the stronger emission component in the helium line observed in o Pup is about 270 km/s, a value which is in good agreement with the range derived for phi Per-type binaries. The radial velocity curves defined by the prominent emission peak in He I 6678 A line and H alpha + Paschen emission move in anti-phase. We suggest that o Pup is generically similar to phi Per-type systems and may represent the fourth case of a Be star with a hot subdwarf companion.
01. 10. 2012
Decelerations of radar meteors
Abstract: Deceleration of meteors is a common effect directly observable from their photographic or video records. This is due to the fact that these techniques are capable of covering their whole visible trajectory as a function of time. Contrary to photographic or video meteors their radar observations yield only time and velocity at selected points depending on the configuration of the observing system. Very important in this respect is the method of velocity determination. We compare two of them and discuss their possibilities. The knowledge of sufficiently reliable radar meteor velocities at different points of trajectory enables to determine meteoroid deceleration and, consequently, to compute its heliocentric orbit. The results of application of such approach to the data from CMOR are presented.
04. 06. 2012
Transition from regular to chaotic motion in black hole magnetospheres
Abstract: We study an astrophysical model consisting of the rotating black hole immersed into the asymptotically uniform magnetic field. Structure of the electromagnetic field emerging in the generalized setup involving the oblique background field (misaligned with the rotation axis) and translational motion of the black hole is explored in detail. For a sufficiently rapid drift we localise separator null points of both electric and magnetic fields. The latter are of an utmost interest since they prove that the interplay of the rotation and the drift of the compact body may act as a trigger for magnetic reconnection. Subsequently we study the motion of charged particles in the simplified setup of aligned field. We concern ourselves mainly with the dynamic properties of the trajectories bound in the off-equatorial potential lobes which are relevant for a description of diluted astrophysical coronae. Employing the recurrence analysis we locate transitions between regular and chaotic regimes of motion. Besides the bound off-equatorial orbits we also study trajectories escaping from the equatorial plane in the collimated jet-like structures.
( seminární místnost za 2-metrem)
16. 05. 2012
Massive star magnetospheres: from X-ray to radio
Abstract: Magnetic massive stars experience confinement of stellar wind plasma by their strong, large-scale magnetic fields. This magnetospheric material can produce observable spectral and photometric effects in wavelengths from X-rays to radio. I will present a summary of the observational and theoretical efforts to study massive star magnetospheres, as well as discuss future possibilities for these stars in the infrared.
02. 04. 2012
Fundamental magnetic field properties of sunspots
Abstract: The magnetic nature of sunspots was discovered by G.E. Hale in 1908. Since then, the magnetic field properties were thoroughly studied. I will present the summary of magnetic field properties in sunspots along with brief explanation how to determine the vector of magnetic field in solar photosphere. The aim of our analysis was to determine the magnetic field properties on penumbral boundaries, which were not yet studied explicitly. We tried to determine if the magnetic field strength or inclination is constant on these boundaries and if these values are dependent on sunspot size. The most important result is that the inner penumbral boundaries are defined by the critical value of the vertical component of the magnetic field. This implies that the penumbral filaments have a convective origin.
12. 03. 2012
Anizotropní rozložení pólů binárních asteroidů, revize albed asteroidů z infračervených pozorování družicí WISE a vyhlídky na detekci binárů metodou oscilace fotocentra z astrometrických měření družicí Gaia. Anizotropní rozložení pólů binárních asteroid
Abstract: Ve své přednášce stručně shrnu naše nové výsledky, které jsme letos opublikovali, resp. máme zaslány k publikaci, případně jsou v revizi. V práci  jsme studovali rozložení pólů drah binárních asteroidů na vzorku 18 z nich, které jsme pozorovali z Ondřejova a spolupracujících stanic. Zjistili jsme, že jejich póly jsou rozloženy značně nerovnoměrně, koncentrují se v okolí do 30 stupňů od pólů ekliptiky. Navrhli jsme, že jde o důsledek působení YORP efektu, který "narovnává" rotační osy jejich mateřských těles (pre-formation mechanism), případně hlavních složek -primárů- binárních systémů (post-formation mechanism). V práci  jsme prozkoumali zkreslení v odhadech albed asteroidů, která získal tým Mainzer et al. (Astrophys. J. 741, 90, 2011) modelováním infračervených měření z družice WISE. Analýzou srovnávacího vzorku 583 asteroidů, jejichž absolutní magnitudy jsme změřili z Ondřejova a z Table Mountain Observatory v Kalifornii, jsme zjistili, že jejich odhadovaná albeda jsou systematicky přeceněná a revidovali jsme je. Odvodili jsme, že střední geometrické V albedo asteroidů S typu je 0.197 ± 0.006 a že se s velikostí asteroidu (v intervalu průměrů 0.6-200 km) významně nemění; tím jsme opravili předchozí omyl týmu Mainzer et al.. V práci  jsme pak analyzovali možnost detekce binárních asteroidů metodou oscilace fotocentra z ultra-přesných astrometrických měření družicí Gaia. Ukázali jsme, že při dosažení plánované astrometrické přesnosti 2*10^(-5) až 1*10^(-3) obloukové vteřiny (v závislosti na jasnosti objektu) bude možno v jejích datech detekovat periodickou oscilaci fotocentra způsobenou oběhem složek bináru pro středně vzdálené satelity s periodami v řádu dní. Populace těchto středně širokých binárů je dosud do značné míry nepopsaná, protože dosavadní observační techniky (fotometrie, adaptivní optika) mají v jejich oblasti parametrů nízkou či nulovou účinnost. (Literatura:  Pravec P., Scheirich P., Vokrouhlicky D., Harris A.W., et al., Binary Asteroid Population. 2. Anisotropic distribution of orbit poles of small, inner main-belt binaries. Icarus 218 (2012) 125-143,  Pravec P., Harris A.W., Kusnirak P., Galad A., Hornoch K., Absolute magnitudes of asteroids and a revision of asteroid albedo estimates from WISE thermal observations. Icarus, submitted.  Pravec P., Scheirich P., Small binary asteroids and prospects for their observations with Gaia, Planet. Space Sci., in revision)
13. 02. 2012
Formation of the AGN torus in a nuclear starburst environment.
Abstract: I will present a hydrodynamical model for the formation of the gaseous torus in active galactic nuclei from the matter reinserted by a rotating young nuclear star cluster (NSC). In the model, a NSC evolving in a catastrophic cooling regime is a necessary condition for the formation of the torus. By using typical parameters of NSCs (mass, size, etc,... ) and black holes (BHs) in Seyfert galaxies, the model predicts sizes and thickness for the tori as those inferred from observations. An analytic formula to estimate the radius at which mass accumulates and forms the torus will be presented as well.
16. 01. 2012
3D radiative transfer in clumped hot star winds
05. 12. 2011
Validated helioseismic inversions for 3-D vector flows: Applications to SDO data
Abstract: Helioseismic tomography is currently the only one method, which allows as to "see" beneath the solar photosphere. Various methods utilising the analysis of the surface waves carrying the information about the subphotospheric plasma were developed in the last few decades. We improved and validated travel-time inversions for 3-D vector flows. The main improvement comes from the explicit minimization of the cross-talk between the three flow components. The validation was performed using synthetic travel-time maps obtained by convolving a known flow field from a realistic numerical simulation of solar convection with travel-time sensitivity kernels computed in the Born approximation. Realistic noise is added to the travel times. The inversion of synthetic travel-times returns unbiased answers for horizontal flow velocities in layers shallower than 4 Mm. We also show that the vertical component of velocity near the surface can only be inferred under the explicit condition that the cross-talk between the flow components is minimized. A preliminary application to SDO/HMI quiet-Sun data is presented.
07. 11. 2011
Tepelná napětí v meteoroidech
Abstract: Tepelná napětí vznikají v pevném tělese vlivem nehomogenních a časově proměnných teplotních polí. Na pozemských pouštích, ale i na Měsíci, Merkuru a blízkozemních asteroidech dochází k tak intenzivnímu ohřevu úlomků hornin vlivem slunečního záření, že odpovídající tepelná napětí mohou překročit pevnost těchto hornin a způsobit jejich rozpad. V přednášce se pokusím ukázat, že tepelná napětí jsou velmi důležitá pro erozi a rozpad meteoroidů o velikostech 1 mm – 10 cm v meziplanetárním prostoru. Budu probírat několik scénářů působení tepelných napětí: (i) celková destrukce meteoroidů, (ii) tvorba rozpraskané vrstvy na povrchu, (iii) zmenšování velikosti meteoroidů vlivem tepelné eroze povrchu. Zvláštní pozornost budu věnovat poslednímu případu, neboť tento jev má za následek přednostní destrukci velkých a křehkých těles v rojích s malou heliocentrickou vzdáleností perihelu. Výsledkem je odlišné rozdělení pevností pro malá a velká tělesa, což lze testovat pomocí pozorování.
(místo konání: seminární místnost budovy KL v Ondřejově )
03. 10. 2011
Spot/flare models of black-hole accretion disc variability
Abstract: Observed X-ray fluxes from galactic black-hole candidates and active galactic nuclei are remarkably variable. A large class of models in the literature is based on assumption that this variability is driven by magnetic flares or hot spots residing on the surface of an accretion disc. In my talk I will discuss plausibility and limitations of these models. I will present a general framework for modeling of signals, produced by many randomly generated spots, taking into account the influence of general relativistic effects. Finally, I will show a new results concerning the characteristic time-scales of the variability.
06. 06. 2011
A pan-spectral method of abundance determination
(takes place at the seminar room behind the 2-meter telescope)
02. 05. 2011
Projekt Evropského slunečního dalekohledu a naše účast v něm
Abstract: Projektu Evropského slunečního dalekohledu (EST) se účastní devět evropských zemí včetně České republiky. EST bude zrcadlový dalekohled o průměru 4 metry, umístěný na Kanárských ostrovech. Jeho hlavním úkolem bude zobrazování a spektroskopie sluneční fotosféry a chromosféry současně v několika oborech viditelného a infračerveného záření. V polovině letošního roku končí fáze koncepčního návrhu dalekohledu. Náš ústav spolu s Ústavem fyziky plazmatu AV ČR rozpracoval návrh pomocného celodiskového dalekohledu. Ten bude sloužit k výběru pozorovacích objeků a k navádění hlavního dalekohledu, ale také jako samostatný, plně automatický synoptický dalekohled, který bude zaznamenávat jak dlouhodobý vývoj sluneční aktivity, tak i rychle se měnící aktivní jevy, například erupce.
11. 04. 2011
Fyzikální vlastnosti meteoroidů
Abstract: Meteoroidy jsou drobná tělesa sluneční soustavy (od zlomku milimetru po několik metrů). Jsou to většinou úlomky planetek a komet a můžeme je studovat, když krátce září v zemské atmosféře jako meteory. Světelné křivky, brždění a spektra meteorů vypovídají o fyzikálních vlastnostech, struktuře a složení meteoroidů a tím i o vlastnostech planetek a komet. Odvození struktury meteoroidů z pozorovaní meteorů však není jednoduchá úloha a výsledek může záviset na použitém modelu. Přednáška shrne naše snahy v této oblasti.
07. 03. 2011
Rozmotávání spekter - teorie, praxe a výsledky metody
Abstract: Disentangling of spectra – theory, practice, and results of the method. The method of disentagling is a tool enabling determination of free parameters of a theoretical model to fit directly the observed data. Originally it was developed to solve for orbital parameters of multiple stellar systems and simultaneously to separate spectra of their components from a time series of spectrograms. New generalizations of the method enable to fit also other physical parameters of the observed objects depending on the models built in the corresponding code.
(Nejlepší vědecký výsledek oddělení GPS za rok 2010)
14. 02. 2011
Chaotic pulsations: model, observation and prediction
Abstract: Dynamical pulsations, radial or non-radial, may be one of the main sources of intrinsic variability in many type of stars. In our work we focus primarily on pulsating stars, which exhibit some irregularities in their light curves. Such complex behaviour may be explain in terms of chaotic dynamics. We explained how we can model chaotic pulsation, how we can compare it with observations and how we can predict light curve with using nonlinear time series analysis.
10. 01. 2011
Role of plasmoids in theory and diagnostics of solar flare processes
Abstract: Our recent studies show that plasmoids are natural parts of the magnetic field reconnection and their observations can be used for diagnostics of flare processes. In the present talk we start with simulations of a formation and ejection of the plasmoids. Then we show an effect of the interaction of the plasmoids with an arcade of flare loops. Model of the drifting pulsating structures (DPSs) and radio spectra generated during coalescence processes in a flare current sheet are described. Furthermore, the fractal reconnection, driven coalescence processes, narrowband dm-spikes, and the above-the-loop-top hard X-ray sources are mentioned and their mutual relations explained. Finally, we show an example of the flare indicating the reconnection of a kinking flux rope triggering the plasmoid ejection.
06. 12. 2010
Fyzikální ústav AV ČR
Metody detekce temné hmoty
Abstract: I když téměř čtvrtina energetické bilance vesmíru má být ukryta v tzv. temné či skryté hmotě, zatím se stále nedaří odhalit její podstatu. Jak hledají a poznávají temnou hmotu astronomové? Jaké experimenty provádějí částicoví fyzici ve svých podzemních laboratořích? A jak mohou pomoci data z umělých družic? V přednášce bude podán přehled současných přímých i nepřímých metod detekce temné hmoty a budou shrnuty dosavadní výsledky nejvýznamnějších experimentů, zabývajících se jejím hledáním.
01. 11. 2010
Asteroidální páry a jejich formování rotačním štěpením
Abstract: A population of small main-belt asteroid pairs residing on very similar heliocentric orbits has been found and studied recently (Vokrouhlický and Nesvorný, Astron. J. 136, 280, 2008; Pravec and Vokrouhlický, Icarus 204, 580, 2009; Vokrouhlický, Astron. J. 137, 111 2009). These asteroid pairs show some common properties: they are ubiquitous with pairs found throughout the asteroid population, pair members are separated with low relative velocities (as low as 0.17~m/s), they are young with most pairs separated less than 1 Myr ago, and their sizes and mass ratios overlap with the population of orbiting, bound binary systems. Previous investigations of binary asteroids suggest that they were formed from parent bodies spinning at a critical rate by some sort of fission or mass shedding process (Pravec and Harris, Icarus 190, 250, 2007; Scheeres, Icarus 189, 370, 2007; Walsh et al., Nature 454, 188, 2008), however the possibility that these two populations of asteroid pairs and binaries were related was intriguing but lacked of bservational data. In our recent paper (Pravec et al., Nature 466, 1085, 2010), we reported on a systematic observation campaign of spin rates, relative sizes and shapes of paired asteroids which enables this population to be analyzed. Two key characteristics of the asteroid pairs population appear: the primary spin rate is correlated with the mass ratio between a pair's components, and there is a cut-off in mass ratios of asteroid pairs above a value of 0.2. Both of these results are predicted by the rotational fission process hypothesized in Scheeres (Icarus 189, 370, 2007), and suggests this or a similar process as the genesis of the asteroid pairs and by implication as a fundamental process in the formation of asteroid binary systems. This formation mechanism is distinct from previous hypotheses to explain the population of orbiting, bound binary asteroid systems (Walsh et al., Nature 454, 188, 2008).
11. 10. 2010
Gravitational fragmentation of expanding shells and triggered star formation
Abstract: I will discuss the work I have been doing here in Prague in collaboration with Richard Wunsch, Jan Palous and also with Ant Whitworth at Cardiff University in the UK. I will present results from our detailed 3D AMR and SPH numerical study of the fragmentation of expanding gaseous shells. I will explore the limitations of the thin-shell model traditionally used to model these systems and describe a new and superior model which takes into account the boundary conditions on the shell surfaces. In particular, I will explain how the pressure felt by the shell influences the masses of fragments produced. Finally, I will show how the process of accretion in shells can lead to a very top-heavy fragment mass function and discuss the implications this has for the triggering of star formation.
07. 06. 2010
Demise of ancient planetary systems: the circumstellar environment of white dwarf stars
Abstract: I will discuss some of the evidence for the presence of debris material in the immediate vicinity of white dwarf stars (< 1 Rsun). Chemical analysis of the material that presumably accreted onto the surface of these white dwarfs often suggests a link with the tidal disruption of asteroids and minor planets and the formation of a debris disc. In addition, infrared observations do indicate the presence of warm discs associated with these white dwarfs. Because this phenomenon is apparent in a substantial fraction of white dwarfs, it must be representative of the likely fate of the Solar system.
03. 05. 2010
Observations and modeling of Ellerman bombs
Abstract: Observations of Ellerman bombs (EBs) show them as short-lived, compact and well spatially localized emissions that are well observable in the wings of the H-alpha hydrogen line. The H-alpha line profiles of EBs are characterized by deep absorption at the line center and enhanced emission in the wings with maximum at ± 0.1 nm from the line centre, fading around ± 0.5 nm. EBs may be also observed in the chromospheric Ca II lines and in UV as a bright points often located within active regions. In the analysis we used observations of EBs obtained in the H-alpha line and in UV range at 160 nm. This one--hour long simultaneous sequence, obtained with high temporal and spatial resolution, was used for the analysis of the correlations between H-alpha and UV emission. We also made some theoretical simulations of the EBs emission using NLTE codes with model atmosphere. The comparison between observations and the theoretical calculations allowed us to conclude about the structure of EBs.
12. 04. 2010
Pád meteoritu Bunburra Rockhole – unikátní výsledek systematického sledování bolidů v poušti Nullarbor v JZ Austrálii (+ bolidové aktuality ze Střední Evropy)
Abstract: Ve své prezentaci budu hovořit především o nálezu velmi vzácného meteoritu Bunburra Rockhole v rámci projektu Pouštní bolidové sítě v JZ Austrálii a našem podílu na tomto unikátním výsledku. Předcházel mu poměrně nenápadný bolid, který krátce před svítáním v sobotu 21. července 2007 začal svítit nad rozsáhlými oblastmi Nullarborské planiny v Jižní Austrálii. V maximu dosáhl -9. magnitudu a po téměř 5 sekund takto osvětloval pustou pouštní krajinu. Tento průlet zaznamenaly dvě východní automatické stanice Pouštní bolidové sítě a to jak na fotografický film, tak citlivými fotometry. Bez těchto záznamů by tento úkaz zůstal zcela nepovšimnut, protože byl příliš slabý na to, aby ho zachytily různé globální systémy a příliš daleko od civilizace, aby byl pozorován náhodnými svědky. Díky záznamům z těchto dvou stanic se ale tento případ stal jedním z nejpřesněji dokumentovaných pádů meteoritu v historii. Z fotografických snímků jsme určili nejen světelnou dráhu tělesa v atmosféře a jeho původní dráhu ve Sluneční soustavě, ale také pádovou oblast, kde měly ležet jeho zbytky ve formě malých meteoritů. To byla v tomto případě velmi složitá úloha, protože těleso pohaslo relativně vysoko, necelých 30 km nad zemským povrchem a let po temné dráze trval pro nejpravděpodobnější hmotnost meteoritu kolem 100 gramů dlouhých 6 minut. Během této doby byly jednotlivé meteority zaneseny výškovým prouděním v atmosféře o mnoho kilometrů. Velkým úspěchem tedy je, že všechny 3 dosud nalezené meteority o hmotnostech 174, 150 a 15 gramů byly ve vzdálenosti pouze do 100 metrů od vypočtené polohy pro odpovídající hmotnosti. Hlavní výjimečnost tohoto případu ale spočívá ve složení meteoritu a jeho původní dráze. Těleso se totiž před srážkou se Zemí pohybovalo na velmi vzácné dráze typu Aten, kdy naprostá většina jeho dráhy ležela uvnitř dráhy zemské. Ještě vzácnější se však ukázal být sám meteorit – jedná se o tzv. achondrit a je klasifikován jako vzácný typ bazaltické vyvřelé horniny což znamená, že pochází z relativně velkého tělesa. Bylo to vůbec poprvé v historii, kdy jsme měli v rukou achondrit s tzv. rodokmenem. Navíc na základě podrobných analýz bylo objeveno, že se jedná o anomální typ achondritu, který se sice nejvíce podobá tzv. eukritům, nicméně v některých parametrech se od nich natolik významně liší, že je brán jako dosud neklasifikovaný typ této skupiny meteoritů. Výjimečnost tohoto případu podtrhuje fakt, že hlavní výsledky naší práce byly publikovány loni v září v prestižním časopise Science. Na závěr přednášky se v krátkosti ještě zmíním o nejvýznamnějších aktuálních výsledcích z našich pozorování ve Střední Evropě.
23. 03. 2010
Pozorování s dánským 1.5-m na La Silla
Abstract: Na podzim loňského roku jsme dělali zkušební pozorování s dánským 1.54-m dalekohledem na La Silla. Popovídám o tom, jak naše pozorování na La Silla probíhala a o podmínkách, zkušenostech a zážitcích, které jsme tam ziskali.
(Mimořádný seminář, začátek ve 14:30, budova kosmické laboratoře)
08. 03. 2010
Spin measurements of accreting black holes with relativistic iron lines
Abstract: Spin of an accreting black hole can be determined by spectroscopy of the emission and absorption features produced in the inner regions of an accretion disc. We will discuss the method employing the relativistic line profiles of iron in the X-ray domain, where the emergent spectrum is blurred by general relativistic effects. Precision of spectra fitting procedure could be compromised by inappropriate account of the angular distribution of the disc emission. Often a unique profile is assumed, invariable over the entire range of radii in the disc and energy in the spectral band. An isotropic distribution or a particular limb-darkening law have been frequently assumed, although some radiation transfer computations exhibit an emission excess towards the grazing angles (i.e., the limb brightening). By assuming a rotating black hole in the centre of an accretion disc, we perform radiation transfer computations of an X-ray irradiated disc atmosphere (NOAR code) to determine the directionality of outgoing X-rays in the 2-10 keV energy band. Based on these computations we produce a new extension to the KY software package for X-ray spectra fitting of relativistic accretion discs. We study how sensitive the spin determination is to the assumptions about the intrinsic angular distribution of the emitted photons. We find that on the whole the isotropic directionality reproduces the simulated data to the best precision. Our results also suggest that an improper usage of limb darkening can partly mimic a steeper profile of radial emissivity. We demonstrate these results on the case of MCG-6-30-15 observed with XMM-Newton, for which we construct confidence levels of chi-squared statistics, and on the simulated data for the future X-ray IXO mission.
01. 02. 2010
B[e] supergiants - a special evolutionary phase or the result from massive binary mergers?
Abstract: Stars with the B[e] phenomenon are a very heterogeneous group of emission line stars with strong near- and mid-infrared excess due to hot and warm circumstellar dust. Based on these characteristics, the group of B[e] stars was found to consist of stars in quite different evolutionary phases, ranging from pre-main sequence (Herbig stars) to post-main sequence phases (compact planetary nebulae and supergiants). Especially the B[e] supergiants, though studied in great detail, are far from being understood. About 15 B[e] supergiants are known to exist in the Magellanic Clouds, and I will first describe the difficulties in finding B[e] supergiants in our Galaxy. Then I will discuss the problematics in determining their proper place in the evolutionary path of massive stars. Finally, I will discuss the rather speculative but nevertheless interesting idea whether B[e] supergiants might result from massive binary mergers.
04. 01. 2010
Deciphering Chromospheric Magnetism via the Hydrogen Spectropolarimetry
Abstract: The quiet chromosphere overlies most of the solar surface. It is in this region, where the dominant physics changes from hydrodynamic to magnetic forces. In contrast to the underlying photosphere, this layer remains rather enigmatic. The only way to obtain reliable empirical information on the magnetic fields is via the measurement and physical nterpretation of polarization signals in spectral lines. Since the Zeeman effect is useless in the quiet chromosphere, we turn our attention to the scattering polarization and the Hanle effect. I will show that the spectropolarimetric observations and the detailed radiative transfer modeling of the hydrogen H-alpha line allow us to determine not only a lower limit of the magnetic field strength but they also reveal an existence of a significant gradient of the field strength right below the chromosphere-corona transition region.
07. 12. 2009
Bolidy meteorických rojů
Abstract: Během posledních několika let byly z Evropské a Australské bolidové sítě vyfotografovány bolidy příslušející několika hlavním meteorickým rojům. U jednotlivých rojů byly na základě různých metod studovány jejich fyzikální vlastnosti, světelné křivky meteorů a jejich heliocentrické dráhy. Téměř sto zařazených meteorů přísluší k šesti rojům, kterými jsou Orionidy, Geminidy, alfa-Capricornidy, Jižní-delta Aquaridy, Leonidy a Perseidy a jejich fyzikální vlastnosti byly studovány na základě výšek začátků a konců bolidů, zdánlivých ablačních koeficientů a dynamických tlaků ve výškách fragmentace či koncovém výbuchu. Provedeno bylo také srovnání základních vlastností rojových bolidů s bolidy, u kterých je pravděpodobnost pádu meteoritu. Z tohoto srovnání vyplývá, že existuje možnost pádu malého meteoritu z roje Geminid.
02. 11. 2009
Smoothed Particle Hydrodynamics simulations of expanding HII regions.
Abstract: The interaction between the ionizing radiation emitted by massive stars and the interstellar medium plays a crucial role in star formation at all epochs. We have implemented a new 3D algorithm into the Smoothed Particle Hydrodynamics code SEREN, for including the dynamical effects of this interaction. We will present numerical simulations and we will show several examples of expanding HII regions. In particular we will describe the evolution of the HII region with the exciting star placed i) at the centre of a spherical cloud, ii) at the edge of a spherical cloud, and iii) outside and far away of a spherical clump. Using these examples we will discuss conditions under which the ionizing radiation triggers star formation.
05. 10. 2009
Exploring the white dwarf population
Abstract: White dwarfs are the primary end products of stellar evolution. Studies of their mass, radius, core and surface compositions as well as their magnetic properties help establish physical links between main-sequence progenitors and white dwarf stars. We have used the telescopes at the ESO (NTT and VLT), KPNO, and CTIO to explore the population of high proper motion white dwarfs. These observations have revealed a number interesting stars, in particular white dwarfs with extremely low mass (M < 0.2 M_solar), which could have only formed through binary star evolution. I will talk about our studies of the white dwarf population including our recent discoveries of these extremely low mass white dwarfs. Our kinematical study of these objects uncovered the tip of a population of very old low mass white dwarfs.
01. 06. 2009
Determination of dynamics of solar prominence from the Ondrejov optical spectra and filtergrams
Abstract: Optical spectra of prominences from the HSFA-2 obtained with high spectral and temporal resolution have been processed in order to determine parameters derivable from the line profiles (like integrated intensity, Doppler velocity, etc.). In the first part of my talk I will describe a method how to obtain calibrated spectra from HSFA-2. In the second part I will talk about a substantial problem of elimination of the scattered light. Preliminary results and future prospects will be described in the last part.
01. 06. 2009
Determination of dynamics of solar prominence from the Ondrejov optical spectra and filtergrams.
Abstract: Optical spectra of prominences from the HSFA-2 obtained with high spectral and temporal resolution have been processed in order to determine parameters derivable from the line profiles (like integrated intensity, Doppler velocity, etc.). In the first part of my talk I will describe a method how to obtain calibrated spectra from HSFA-2. In the second part I will talk about a substantial problem of elimination of the scattered light. Preliminary results and future prospects will be described in the last part.
06. 04. 2009
Kvantitativní model úniku sodíku z meteoroidů v blízkosti Slunce
Abstract: Pozorování spekter meteoroidů z roje Geminid ukázalo, že tato tělesa jsou silně ochuzena o sodík. Abychom vysvětlili zmíněnou skutečnost, vyvinuli jsme kvantitativní model úniku sodíku z meteoroidů v důsledku jejich zahřívání v blízkosti Slunce. Zjistili jsme, že Geminidy mohou ztratit téměř všechen sodík za několik tisíc let, pokud jsou složeny ze zrn o velikostech do ~100 um. Pozorované variace obsahu sodíku ve spektrech Geminid vysvětlujeme rozdílnou zrnitostí jednotlivých meteoroidů. Ochuzení o sodík rovněž očekáváme u dalších rojů, které mají vzdálenost perihelu menší než ~0.2 AU. V našem modelu předpokládáme, že meteoroidy jsou tzv. „prachové koule“ složené z dílčích zrn a obsahující systém vzájemně propojených pórů. Zrna pak mají nulovou porositu a obsahují běžné minerály, známé z meteoritů a meziplanetárních prachových částic, včetně malého množství minerálů obsahujících sodík. Únik sodíku probíhá podle našeho modelu ve třech krocích: (i) difuze atomů Na z příslušných minerálů k povrchu zrn, (ii) tepelná desorpce z povrchů zrn do pórového systému, (iii) difuze systémem pórů k povrchu meteoroidu a únik do prostoru.
02. 03. 2009
Measuring black-hole spins
Abstract: Black holes are extremely simple astrophysical objects. They are fully characterized by two parameters only: their mass and spin. Mass simply sets the system scale, while spin changes the geometry and fundamentally conditions the ways in which black holes interact with their environments. The determination of black-hole masses has been possible for some time already by dynamical measurements. However, inferring spins is much harder task. Only recently, accretion disk theory has reached the state, where it can be used to probe the strong-gravity region close to the hole and to make specific spin estimates. The talk will review three basic methods that can be used to estimate black-hole spins, their principles and limitations. I will also present the current best model for spectral fitting (SLIMBB) that is developed in a Prague-Warsaw-Harvard collaboration.
09. 02. 2009
Dlouhodobá aktivita binárních rentgenových zdrojů a důležitost jejího monitorování
Abstract: Binární rentgenové zdroje jsou velice aktivní systémy, v nichž probíhá akrece hmoty na kompaktní objekt, jímž je bílý trpaslík, neutronová hvězda nebo černá díra. Tyto objekty představují důležité laboratoře pro studium akrečních procesů na kompaktní objekt a jejich výzkum je významnou součástí moderní astrofyziky. Během této akrece dochází k emisi záření prostřednictvím různých mechanismů, a to ve velice širokém spektrálním pásmu. Binární rentgenové zdroje jsou díky působení rozličných vlivů (fyzikálních i geometrických) aktivní na velice široké časové škále, od zlomku sekundy až po desetiletí, nemluvě o vývojových změnách. Právě dlouhodobá aktivita těchto systémů na škálách let a desetiletí (tedy kam až sahají dostupná pozorování) je dosud jen málo prozkoumána, přestože má často největší amplitudu a je velmi důležitá pro porozumění v nich probíhajícím fyzikálním procesům. V této přednášce uvedeme přehled toho, co nám umožňují dosavadní rentgenové monitory. Rovněž uvedeme základní fyzikální procesy, které dle dosavadních poznatků řídí dlouhodobou aktivitu binárních rentgenových zdrojů. Jedná se zejména o termální nestabilitu akrečního disku, změny množství přenášené hmoty a termonukleární reakci na povrchu bílého trpaslíka. Uvedeme také, jakou roli tyto mechanismy hrají při interpretaci pozorované aktivity systémů.
05. 01. 2009
Highlights of the Hinode satellite
Abstract: I spent the last two years in Japan working with the Hinode satellite. In the first part of the talk, I will introduce the scientific instruments on board and show some examples of the observed data. In the second part, I will talk about my own research that is focused on the penumbral fine structure. It is based on the analysis of spectropolarietric measurements using the inversion codes.
05. 05. 2008
Studium meteorických rojů pomocí videotechniky
Abstract: Dvojstaniční pozorování slabých meteorů pomocí videokamer se zesilovači obrazu umožňuje získat heliocentrické dráhy i atmosférická data o jednotlivých meteorech nebo profily aktivity rojů. Na základě velkého vzorku dat můžeme usuzovat na vlastnosti konkrétního meteorického roje a srovnávat je mezi sebou navzájem. V přednášce budou představeny nejzajímavější výsledky získané v uplynulých letech. Studium vlastností Kvadrantid nám umožnilo vyvodit důsledky pro mateřské těleso tohoto meteorického roje. Návrat Leonid v roce 2006 proběhl přesně podle předpovězeného scénáře. Jev zvaný sputtering může vysvětlit záření meteorů ve vysokých výškách. A konečně neobvyklé chování Drakonid otevřelo cestu k detailnějšímu studiu struktury meteoroidů.