Archiv seminářů pro rok 2014
Proton thermal energetics in the solar wind
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.
Dynamical evolution of the disk of the Be star 60 Cygni
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.
Science with the Arecibo Galaxy Environment Survey
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.
Search for faint meteors on the orbits of Pribram and Neuschwanstein meteorites
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.
Solar Flares and Superflares
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.
X-ray reverberation in Active Galactic Nuclei
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.
Pulsations as mass-loss trigger in blue supergiants
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.
Photographic fireballs of 2013 September epsilon Perseids outburst
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.