Archiv seminářů pro rok 2017
Dynamics of solar prominences
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.
Modelling of solar flare processes: comparison of two RHD codes Flarix and RADYN
The talk will introduce the topic of radiation hydrodynamic modelling (RHD) of solar flares and then focus on comparison of two autonomous, methodologically different RHD codes, Flarix and RADYN. RADYN code was developed by M. Carlsson at the University of Oslo for chromospheric modelling and has been extended and extensively used for the flare modelling. Flarix code is being developed at the Astronomical Institute with the primary purpose of the flare processes modelling. Both codes can compute the time evolution a 1D flare loop heated by the beam electrons propagating from the injection point in the corona down to the lower atmosphere. The codes are based on different numerical schemes and can treat various processes relevant for solar flares. I will present the results of testing the codes for a simplified case of the electron beam heating and compare the time evolution of the atmospheric structure (e.g. temperature, density) and the optically thick emission.
Breakfast of the beast - Flaring activity of Sagittarius A* as seen with ATCA
The supermassive black hole at the center of the Milky Way, associated with the compact radio source Sagittarius A*, is the closest galactic center to Earth, and a representative of low luminosity active galactic nuclei. Its proximity has allowed us to study the physics of the galactic nuclei with great detail. Sagittarius A* is currently classified as inactive, but it undergoes regular flaring events which arise from the innermost region of the accretion flow. These flares have been observed across different wavelengths, from NIR & X-ray to radio and submillimeter regimes. Multiwavelength observations have shown that the NIR & X-ray flares occur simultaneously, and are followed by the submm & radio flares. The study of the flaring activity gives us indirect constraints on the details of the emission mechanism and allows us to investigate the accretion around a low-luminosity Super-Massive Black Hole. Here, I will present the results of our observations of the Galactic Center at 3 mm wavelength using the Australia Telescope Compact Array, taken between 2010 and 2014. The flaring events in the lightcurves of Sagittarius A* were detected using the structure function analysis and Bayesian blocks representation methods. From this, six instances of flaring activity were detected, lasting 1.5 - 3 hours. I will present the results of the modelling of the flares using the adiabatically expanding plasmon model, and the subsequent physical properties of the source derived from it.
When Be stars talking
We are living in the era of the Big Data. Sky surveys like OGLE, MACHO, ASAS produce huge amount of the photometric data of the more the hundred million of the stars. It also means, that we need to change the way how to analyse such big amount of data and gather the most information. A global space astrometry mission, Gaia will make the largest, most precise three-dimensional map of our Galaxy by surveying more than a thousand million stars. Moreover, The Gaia Data Processing and Analysis Consortium (DPAC) process the raw data to be published in the largest stellar catalogue ever made. As the members of the DPAC consortium we are participate on this amazing space mission. In the CU7 unit our goal was to develop methods for detection and classification of the photometric variability of the Be stars. This is the very challenging task, due to the complexity of the Be stars variability (various type of variability ranging from short-term to long-term type variability) and amounts of the analyzed data (at the end of the mission we will have 1.5 billion stars observed by the GAIA). We developed an automatic, robust and very fast method for classification and statistical analysis of the light curves of Be stars suitable for the GAIA data processing using novel approach. Namely for classification of the light curves and detection of important irregular events like outburst we use symbolic aggregate approximation alias words representation of light curve. Result of the classification can be helpful for finding connections between dynamics of the Be star discs and their long term photometric variability, detect new type of the variability and their physical mechanism and for finding new candidates of the Be stars which exhibits photometric outbursts. Our developed code is the part of the whole package VariAlgol for photometric analysis of the GAIA data.
Properties of faint meteors studied by video observations
Parallel double-station video observations combined with spectroscopic video observations can be a good way to study millimeter-sized meteoroids. Almost two decades of video observations of meteors at the Ondřejov observatory give us broad database to study large quantities of meteoroids and their properties. We tried to combine spectral video observations and results of the modelling of the fragmentation of meteoroids. Along with complex information about meteoroid's trajectories and orbits, this can give us better understanding about internal structure of these millimeter-sized interplanetary bodies.
Orbital fragmentation of the September epsilon Perseid meteoroid
A bright fireball was observed by the cameras of the Czech fireball network on September 9, 2016 at 23:06:59 UT. Moreover the video cameras at two separated stations recorded eight fainter meteors flying on parallel atmospheric trajectories within less than 2 seconds. All the meteors belong to the September epsilon Perseid meteor shower. The measured proximity of all meteors during a very low activity meteor shower suggests that a cluster of meteors was observed. The atmospheric trajectories of the observed meteors, masses and mutual distances of individual particles were determined using a double station observation. According to the distances and masses of the particles the most probable distance and time of fragmentation was determined. The observed group of meteors was interpreted as the result of the orbital fragmentation of a bigger meteoroid. The fragmentation happened no earlier than 2 or 3 days before the encounter with the Earth at a distance smaller than ~0.08 AU from the Earth.
EST - European Solar Telescope
EST is a 4-meter class solar telescope to be installed in the Canary Islands with the first light expected in 2027. The EST design is optimised to observe the thermal, dynamic, and magnetic properties from the deep photosphere to the upper chromosphere simultaneously and thus enable to study the magnetic coupling between these layers. In this presentation we will describe the design baseline of EST, the current status of the EST project and the Czech participation in it.
Behaviour of matter close to the black hole simulated with HARM
I will talk about the general relativistic magnetohydrodynamical simulations of accreting gas in a close vicinity of a black hole in different situations. The main accent of the talk will be put on recent results of the studies of low angular momentum accretion of matter onto Kerr hole, which may contain standing or oscillating shocks. The research has observable consequences on black holes on the whole mass scale, in particular, it could be related to the time-scale and shape of luminosity flares in Sgr A* or to the evolution of QPO frequency during outbursts of microquasars. I will also briefly mention the other interesting topic - the stability in 2D and 3D of stationary magnetized torus described by Komissarov (2006) and Montero et al (2007), which is a good starting model for further studies (e.g. the case of a disrupted torus by a stellar fly-by).
Extreme luminous stars in the Local Group
The upper part of the Hertzsprung-Russell diagram is home to the most enigmatic types of massive stars, which serve as progenitors of various subclasses of core collapse supernovae.Obstacle in inderstanding the physics of such stars is the lack of observational data due to the rarity of these objects and, moreover, their short lifetime. Fortunately, these stars are frequently enshrouded behind their own dust, which renders them bright targets in the infrared. I aim to explore the extreme stellar content of other star-forming Local Group galaxies, by exploiting Group galaxies, by exploiting the wealth of archival data in the infrared and complementing with optical and near-infrared counterparts from the publicly available databases of 2MASS, SDSS, Hubble, Gaia. The upcoming time-series photometry from Pan-STARRS will further provide insight on the variable behaviour of these stars. Selection candidates will follow spectroscopic investigation essential for their proper classification and for the study of their circumstellar environment. Up to now my work on the post-RSG evolution in the M33 galaxy has successfully yielded a newly discovered Yellow Hypergiant candidate and several dusty yellow supergiants ideal for future investigation. By increasing the number of extreme, luminous objects in the Local Group galaxies, I aim towards a better understanding of the stellar evolution in diverse environments.
Asteroid clusters similar to asteroid pairs
We studied the membership and properties of 13 asteroid clusters consisting of between 3 and 19 known members that are on similar heliocentric orbits. By backward orbital integrations of their orbits, we estimated their ages between 105 and a few 106 years. From our photometric observations for all the cluster primaries and a sample of secondaries, we derived their absolute magnitudes and rotation periods. We found that 11 of the 13 clusters follow the same trend of primary rotation period vs mass ratio as asteroid pairs that was revealed by Pravec et al. (Nature 266, 1085--1088, 2010). We generalized the model of the post-fission system for asteroid pairs by Pravec et al. (2010) to a system of N components formed by rotational fission and we found excellent agreement between the data for the 11 asteroid clusters and the prediction from the theory of their formation by rotational fission. The two exceptions are the high-mass ratio clusters of (18777) Hobson and (22280) Mandragora for which a different formation mechanism is needed. Two candidate mechanisms for formation of more than one secondary by rotational fission were published: the secondary fission process proposed by Jacobson and Scheeres (Icarus 214, 161-178, 2011) and a cratering collision event onto a nearly critically rotating primary proposed by Vokrouhlicky et al. (Astron. Astrophys. 598, A91, 2017). It will have to be revealed from future studies which of the clusters were formed by one or the other process. To that point, we found certain further interesting properties and features of the asteroid clusters that place constraints on the theories of their formation, among them the most intriguing being the possibility of a cascade disruption for some of the clusters.
Superflares on Cool Stars
Solar flares are often observed in the visible light as a continuum enhancement lasting several minutes. Quite recently, much stronger enhancements have been detected on many cool stars by Kepler satellite. Kepler is primarily the NASA mission to discover exoplanets and such strong flares have been detected as a by=product, namely on solar-type stars (G-stars) and on red M dwarfs. Enhancement of the stellar flux during flares is much larger as compared to that produced by strongest solar flares and this gives up to four orders of magnitude larger total energy. We will discuss the nature of these so-called superflares and possible radiation mechanisms which produce the visible-light enhancement. There are also interesting questions concerning the impact of such flares on exoplanets, the issue to be dealt with new space missions like TESS (NASA) and PLATO (ESA).
Still mysterious RR Lyrae stars
The space telescopes together with large-scale sky surveys revealed the true complexity of pulsation of RR Lyrae stars that have been considered as boring radial pulsators. Additional (non)radial modes, Blazhko effect and binarity are the current most puzzling problems. I will introduce myself and discuss the open questions during my talk.
ALMA observations of ram pressure stripped galaxies
With the millimeter telescopes APEX and IRAM 30m we have discovered for the first time that ram pressure stripped gas tails of galaxies in clusters can be abundant in molecular gas out to large distances of several tens of kpc from the galaxy. The molecular gas may form a substantial fraction of the total mass of the stripped gas, or in some cases even dominate it. While good correlation is found between the cold molecular and warm ionized components, the efficiency of star formation is in the ram pressure stripped gas tails very low. With the ALMA interferometer we have currently obtained the first ever map of the distribution of cold molecular gas in the Southern sky Norma cluster galaxy ESO137-001, one of the best examples of strong ram pressure stripping in action. The galaxy has an 80 kpc long, double structure tail that is also bright in X-rays and Halpha emission. Our ALMA observations have revealed a surprisingly rich distribution of mostly compact molecular clumps that allow us to study in unprecedented detail the evolution of the stripped interstellar gas, its mixing with the surrounding hot intra-cluster medium, and formation of new stars in the intra-cluster space, outside of galaxies.