Archiv seminářů pro rok 2018
Discovery of a new branch of the Taurid meteoroid stream as a real source of potentially hazardous bodies
Taurid meteor shower produces prolonged but usually low activity every October and November. In some years, however, the activity is significantly enhanced. In our study we present discovery of a new branch of Southern Taurids which was evidently responsible for the enhanced activity occurred in 2015. Based on precise data on 144 Taurid fireballs observed by new digital cameras of the European Fireball Network we found that orbits of 113 fireballs show common characteristics and form together a well defined swarm of meteoroids trapped in the 7:2 resonance with Jupiter. The masses of the observed meteoroids were in a wide range from 0.1 g to more than 1000 kg. We found that all meteoroids larger than 300 g were very fragile (type IIIB), while those smaller than 30 g were much more compact (mostly of type II and some of them even type I). Based on orbital characteristics, we argue that asteroids 2015 TX24 and 2005 UR, both of diameters 200–300 m, are direct members of the new branch. It is therefore very likely that the new branch contains also numerous still not discovered objects of decameter or even larger size. Since asteroids of sizes of tens to hundreds meters pose a threat to the ground even if they are intrinsically weak, impact hazard increases significantly when the Earth encounters the Taurid new branch every few years. Further studies leading to better description of this real source of potentially hazardous objects, which can be large enough to cause significant regional or even continental damage on the Earth, are therefore extremely important.
Has the Milky Way collided with the Andromeda Galaxy in the past?
Modified Newtonian Dynamics (aka MOND) is the leading alternative to the dark matter hypothesis suggesting a modification of the laws of physics for low accelerations. It can explain many galaxy properties. When it was applied to the relative orbit of the Milky Way and its nearest big neighbor, the Andromeda Galaxy, it came out that the galaxies passed close to each other 7-11 Gyr ago. We conducted the first-ever N-body simulation of this encounter. Two galaxies similar to the Milky Way and the Andromeda Galaxy were set up so they eventually roughly reproduced their observed masses, sizes, disk inclinations, separation, and relative velocity. The simulated galaxies reached a minimal separation of only 24 kpc. The encounter induced in the galaxies several peculiar features which show an interesting similarity to the actually observed features. We identified, for example, the possible counterparts of the Milky Way disk warp, of the disk of satellite galaxies orbiting the Andromeda Galaxy, or of the stellar steams around the both galaxies.
Ghost hunting: Space weather effects in the Czech power grids
Solar eruptive events affect the close neighbourhood of the Earth. They also affect human infrastructure, power grids mainly, due to the induction of electrical currents. Only recently the attention was drawn not only to large flares, but also to estimate the long-term effects of increased solar activity by statistical methods. We analyse the failure rates in the main Czech power transmission system operated by the Czech national operator ČEPS. We show that the effects of solar activity on failure rate in the main Czech power grid cannot be excluded even for central-European country. This is a preliminary study.
Quest for the mass distribution of hot subdwarfs
A fraction of low and intermediate mass stars (0.8-2.3 Mʘ) suffer an extreme mass loss on the giant branch, before the helium core ignites. These stars end up in an intermediate evolutionary stage lasting for about 100 Myr as hot subdwarf stars (type sdB). The reason of mass loss is not yet known, although compelling theories have been proposed, several of which are able to reproduce the observed properties in detail. The past two decades have changed our view on hot subdwarfs. Non-radial pulsations had been proposed in 1997 and confirmed later that year by observations. Six years later radial pulsations were observed. Later it turned out that the observed population splits into two characteristic sequences according to their helium abundances. With larger samples these sequences break up to clumps of stars. A complex pattern emerges from the data and intangible correlations in the stellar parameter space seem to exist. A major step forward will be to find the precise mass and mass distribution of these stars. Such a task was not possible in the past, but now that Gaia parallaxes are around and important calibrators, such as eclipsing binaries and long period systems are identified we are able to take the next step in understanding hot subdwarfs. Preliminary data suggest that the currently assumed canonical sdB mass of 0.50 +- 0.04 solar mass may not hold for long.
Time variation of the Earth's lithospheric magnetic field
The lithospheric field that is one of the main objectives of the ESA's mission Swarm is slowly varying in time due to the induced component. This variation is known to be small (and it is usually neglected in the lithospheric modelling) but recent advances in processing strategies and still growing amount of satellite data (longer time series) raise questions whether such an effect should be considered in the development of the lithospheric models. We estimate this effect over the period of 17 years (since the launch of CHAMP). For this purpose, the magnetic field vector from CHAOS-6 was inverted into a vertically-integrated susceptibility map. With the susceptibility distribution, the core fields taken from CHAOS-6 can generate a time-varying lithospheric signal - a means for time referencing of satellite data.
Nebulae around evolved massive stars
Massive stars (with masses higher than 30 Mʘ) undergo the mass loss during their whole lifes. During hydrogen burning in their cores massive stars demonstrate fast winds with velocity about V∞=2500 km/s and relatively low mass-loss rate about 10-7-10-6 Mʘ/year. At the same time, after the end of core hydrogen burning stars become less stable and mass-loss rate increases, stars move to the stage of Luminous Blue variable (LBV). LBVs are relatively short evolutionary stage in life of massive stars, during which they lose significant amount of mass through strong stellar winds and occasional giant eruptive events. As a result, they shed their outer layers and eventually become hydrogen-deficient Wolf–Rayet stars. Massive stars with 25-40 Mʘ reach LBV stage after being red supergiants i.e reaching cool end on Hertzsprung–Russell diagram, more massive stars became LBV right after blue supergiants phase. Consideration of the initial mass function leads to conclusion that LBV stars are extremely rare and there should be no more than a few dozens such objects in the Galaxy. Detection of LBV-like shells may be considered an indication of that their associated stars are massive and evolved. Searches for such shells using the infrared surveys resulted in the discovery of many dozens of such shells, while follow-up spectroscopy of their central stars led to the discovery of dozens of new candidates to LBVs. In the talk I will tell about properties of nebula around post-LBV star GR290 and about estimated parameters of couple of stars with LBV-like shells, and discuss that at least in some cases the envelope loss occurs before LBV stage.
Ablation of iron meteoroids
A population of faint meteors with low velocities, low beginning heights, and only iron lines in their spectra have been described by Borovička et al. (2005). Most of them have unusual light curves that can not be explained by classical single body ablation theory or by fragmentation models. We developed a model which considers iron meteoroids and several different processes of the ablation. The comparison of theoretical and observed meteor heights and light curves leads to the conclusions that fast removal of the liquid from the meteoroid's surface and fast release of its kinetic energy is the most probable process leading to the observed meteors.
Spectroscopic diagnostics of the non-Maxwellian κ-distributions using SDO/EVE flare spectra
X-ray and also EUV emission spectrum during solar flares are affected by the presence of the high-energy electrons accelerated by magnetic reconnection. Recent theoretical papers and RHESSI observations of coronal X-ray sources suggested that the electron distribution function could have a form of kappa-distributions. Using KAPPA package based on the CHIANTI, we have calculated synthetic SDO/EVE spectra for different temperatures, electron densities and values of parameter kappa to propose diagnostics of the plasma parameters. These diagnostics have been applied on the Fe flare line intensities in the SDO/EVE spectra observed during the 2012 March 7 X-class flare. We have found that kappa-distributions have only a small effect on the diagnosed electron densities but they strongly influence temperature diagnosed from the line ratios of ions in a different degree of ionization. Diagnostics of the distribution function demonstrated the presence of the strongly non-thermal distribution with kappa » 2 during the impulsive phase of the flare and later gradual thermalization of the electron distribution. DEM analysis showed broadly multithermal flare plasma, but the effect of multithermality on the diagnostics of distribution is relatively small.
Constraint on AGN corona size with fully relativistic modelling of 3D corona
The “lamp-post” scenario was usually used to describe AGN corona, in which the corona is assumed to be an infinitesimal point source on the symmetry axis. In some AGNs, especially NLS1s, the broad iron line profile requires extremely compact corona. On the other hand, the corona must be large enough to intercept enough seed photons to be able to produce the observed X-ray continuum and reflection spectrum. One attempt to constrain AGN corona size was made by Dovciak & Done 2016, in which the corona size was estimated by conservation of photons during Comptonization process. They found out that for one NLS1 1H 0707-495, the size of the corona must be substantially larger than 1 GM/c2. In that work, the Comptonized spectrum was evaluated assuming lamp-post geometry and the corona size was estimated under the assumption that the emission of the corona is homogeneous. To calculate the spectrum of 3D corona (i.e., the corona is extended and with finite height) self-consistently, we perform a fully relativistic Monte Carlo calculation of energy and polarization spectrum of AGNs of disk-corona geometry, taking into account propagation of null geodesic in Kerr space-time and Compton scattering process.
Exoplanets in Ondřejov, ground based support of space missions - first results
Exoplanet group in Ondrejov, CZ was founded officially in 2018. In the seminar an overview about the potential of our ground based support program with OES for exoplanetary missions will be presented along with first results from 2017/2018 campaign. Furthermore, exoplanetary group in cooperation with Tautenburg Observatory and Universidad Catolica de Chile plans to build and operate a new spectrograph for 1.52m telescope at ESO La Silla observatory, Chile which should contribute to candidate vetting process for PLATO in the future and most certainly also for TESS.
Age estimation of asteroid pairs and clusters
We will briefly present a method for identification of young (age ≲ 2 Myr) genetically related asteroids and estimation of the time of their separation. Assuming a gentle break up event (e.g., by rotational fission), the studied paired or clustered asteroids had very low relative velocities at the time of separation, on an order of 1 m/s, and their distances in the space of mean orbital elements are still rather low at present. For selected candidates we performed backwards integrations of their geometric and Yarkovsky clones and we were searching for close and slow encounters between them. In the end we will overview several interesting results.
Solar Patrol Service of Ondřejov Observatory, Astronomical Institute CAS
There is a long tradition of the Solar Patrol Service at the Ondřejov Observatory for more than 40 years. This service daily produces solar astronomical data like sunspot drawings, white light and H-alpha images, depending just on weather and technical conditions. Since 1978, solar activity forecast is issued weekly and distributed to international solar databases and several public institutions within the Czech Republic. Recently also the daily forecasts were established, which are now broadcasted by the national radios and TVs. The huge data archive contains over 80 years of photospheric or chromospheric observations. Nowadays the older data are digitised to build a fully digital version of archive. The long term data-sets can be used for studies of solar variability.
The Renchen meteorite fall – recovery of meteorites exactly according to our prediction
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 rare event occurred over the most western part of Germany around the middle Rhine near the borders of Germany and France on July 10, 2018 shortly before half past eleven local time (CEST). In spite of a quite bad weather over whole Central Europe this bolide was recorded by instruments of the Czech and German parts of the European Fireball Network, from sites where it was at least partly clear at the time of its passage. Thanks to these records, this bolide could be reliably and accurately described. Among others, we were able to predict that it ended with a multiple meteorite fall and determine the area where meteorites fell and how large they should be. The great success is that shortly after sending the information about the impact area to the German colleagues, the meteorites were recovered by a German searching team exactly in the predicted locations for given meteorite mass. This event therefore fall into the rare category of meteorites “with pedigree”, i.e. meteorites with known trajectory and pre-impact heliocentric orbit. The basic results based on the analysis of the available records will be presented. Recovery of meteorites and their analysis will be also mentioned.
Sparsity regularization and 3D NLTE inversion problem
In the last decade, we have witnessed a great progress in the mathematical theory of sparsity regularization. The breakthrough discoveries in this and related fields led to many important applications in science and technology (compressed sensing, matrix completion, computer tomography, geosciences, deep learning, ...). I will show how the concept of sparsity regularization can be used to solve, for the first time, the notoriously difficult 3D NLTE inversion problem.
Truncation of AGN jets by their interaction with stellar clusters
We study the effects of interaction of jets in Active Galactic Nuclei when they encounter various obstacles, namely, stars in Nuclear Star Cluster surrounding the nucleus and passing across the inner jet. The interaction provides a scenario to address non–thermal processes and jet mass loading. In jet–star interactions a double bow–shock structure is formed where particles get accelerated via diffusive mechanism. Individual encounters have a limited effect, however, dense clusters of massive stars can truncate the jet as the cluster crosses the jet line near the jet launching region. Much of the jet kinetic energy density is transferred to the shock and it becomes available to accelerate particles. We conclude that the interaction of jets with clusters of massive stars is a promising way to explain detectable levels of gamma rays from Fanaroff–Riley class I of edge–brightened radio galaxies.
A New Observing Instrument For Transient Phenomena in Ondřejov: The Small Binocular Telescope
There is a new observational device in Ondřejov - a double 20 cm wide field camera, designed primarily to search for transient astrophysical phenomena as supernovae, GRB afterglows, flare stars etc., and to permit local (i.e. within Ondřejov) triggering of other telescopes. I will show the motivation for this work and discuss various scientific cases.