Archiv seminářů pro rok 2016
On the direct detection of gravitational waves
Current information about recent announcement of the first in history detection of gravitational waves.
The past activity of the Galactic centre
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.
Evolution of binary near-Earth asteroids
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'.
High-resolution fine-structure synthetic imaging of an entire prominence using 3D whole-prominence fine structure modelling
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.
Study of Gamma-Ray Bursts With Robotic Telescopes
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.
Long-term activity of soft X-ray transients
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.
Spectroscopy and modeling of Ellerman bursts
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.
New regime of fragmentation of gaseous layers
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.
Stubenberg meteorite fall - demonstration of capabilities of the Czech fireball network
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.
Radioastronomy in the Czech Republic: Tradition, current state and perspectives
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.
The structure of large meteoroids and small asteroids from observations of superbolides
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.
The evolution of low-metallicity massive stars
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.
Dust in the wind
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.
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.
3-D Radiative Transfer in inhomogeneous winds of hot massive stars
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.
CME plasma diagnostics using space coronagraphs
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).
Very young asteroid pairs candidates
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.