Seminars in 2013


Ondrej Pejcha

The explosion mechanism of core-collapse supernovae and its observational signatures

Abstract: Many massive stars explode as core-collapse supernovae. Supernova simulations show that the shock wave accompanying formation of the proto-neutron star evolves into a quasi-static accretion shock and it proves difficult to revive its outward propagation. The stalled accretion shock turns into explosion when the neutrino luminosity from the collapsed core exceeds a critical value L_crit (the "neutrino mechanism"). I will show the connection between the steady-state isothermal accretion flows with bounding shocks and the neutrino mechanism: there is a maximum, critical sound speed above which it is impossible to maintain accretion with a standoff shock. I will derive the "antesonic" condition, which characterizes the transition to explosion over a broad range in accretion rate, PNS properties and microphysics. Additionally, I will characterize the effects of accretion luminosity and collective neutrino oscillations on L_crit. The physics of the explosion mechanism and the progenitor structure are imprinted in the observed distribution of neutron star masses. I will use Bayesian analysis to model the double neutron star mass distribution to infer the properties of the progenitor binary population, fallback during the explosion, and constrain the mass coordinate where the explosion develops. The Physics of the Neutrino Mechanism of Core-collapse Supernovae Effect of collective neutrino oscillations on the neutrino mechanism of core-collapse supernovae The observed neutron star mass distribution as a probe of the supernova explosion mechanism The progenitor dependence of the neutrino mechanism of core-collapse supernovae Pejcha & Thompson, in preparation


Bartosz Dabrowski

News from ALMA

The Atacama Large Millimeter/submillimeter Array (ALMA) is a major new facility for world astronomy. When completed in 2013, ALMA will consist of a giant array of 12-m antennas, with baselines up to 16 km, and an additional compact array of 7-m and 12-m antennas to greatly enhance ALMA's ability to image extended targets. Construction of ALMA started in 2003. The ALMA project is an international collaboration between Europe, East Asia and North America in cooperation with the Republic of Chile. ALMA Early Science Cycle 1 observations will start in January 2013 and span 10 months. It is anticipated that approximately 800 hours of array time will be available for Cycle 1 projects. I will present the Czech ALMA Node activity during the previous year.


Steven M. Kahn

The Large Synoptic Survey Telescope

The Large Synoptic Survey Telescope (LSST) will be a large-aperture, wide-field, ground-based telescope designed to provide deep images of half of the total sky in six optical colors every few nights. As such it will enable a wide variety of diverse astronomical investigations, ranging from making a census of small moving objects in the solar system, to mapping the outer regions of our Milky Way galaxy. Of particular interest for cosmology and fundamental physics, LSST will provide tight constraints on the nature of dark energy through a range of statistical analyses of the shapes and distributions of billions of galaxies out to moderate to high redshift. This project was recently ranked as the highest priority new ground-based astronomical facility by a committee of the US National Academy of Sciences. I will review the basic considerations that have led to the design of the LSST, and discuss a sampling of the exciting science opportunities that will be enabled by its construction.


Michal Bílek

Shell galaxy NGC 3923 and the missing mass problem

The missing mass problem is one of the main questions of modern physics. The two main candidates to its solution are the dark matter and the modified Newtonian dynamics (MOND). I will give a short introduction into MOND. Some elliptical galaxies are surrounded by faint arc-like features called the shells. The shells are remnants of minor galaxy mergers. I will describe the mechanism of the shell formation and say, how the shell distribution is connected with the host galaxy's potential. Using the laws of shell evolution, I tried to discern, whether the shells system surrounding NGC 3923 can be better reproduced in MOND or in classical dynamics with NFW dark haloes (resulting from LambdaCDM simulations) employing a new method. The currently available data do not allow to make a clear decision, because we were able to model the shell distribution equally well in the both theories. However, we made a prediction where yet unknown shells should be located. The prediction of MOND and LambdaCDM are different for them. Furthermore, the method employed also allowed to reveal the details of formation of the shell system surrounding NGC 3923 and, if LambdaCDM is correct, gave constraints on its dark halo.


Steven N. Shore

V407 Cygni 2010 - The answer is blowin' in the wind

The otherwise garden variety, almost boring symbiotic stay V407 Cygni turned into one of the most important novae of the last century without warning in Mar. 2010. It made astronomical history by being the first nova detected by the Fermi/LAT in high energy gamma-rays and was observed for more than a year in a multi-site panchromatic campaign in which Ondrejov played an important role. This nova explosion occurred in a wind-dominated binary system and the details of the event illuminate a wide range of previously unrelated and poorly understood astrophysical processes. This talk will highlight some of the physical insights gained from the study of this extraordinary event.


Grischa Karssen

Orbiting plasma cloud in strong gravity

Presentation of an ongoing PhD project. In this talk I will present some of the results of simulations of a plasma blob, assumed to be a synchrotron source, in Keplerian orbit around a black hole. This is a model which could explain the flares of SgrA* in the NIR.


Anthony Whitworth

The formation of low and intermediate stars

I will discuss how low and intermediate stars form, stressing why this is far from being a solved problem. My talk will address the origin and role of prestellar cores; the observed properties of cores and stars, and how these constrain theories of star formation; the statistics and the physics of how cores are converted into stars; and the role that may be played by discs in the formation of the lowest-mass stars. The talk will include movies of SPH simulations illustrating possible scenarios.


Michal Šmíd

X-ray Spectroscopy of Laboratory Laser-produced Plasmas: Experiments and Models

The irradiation of solid targets with available lasers can produce high-density high-temperature plasmas, in which we are able construct different interaction scenarios and directly study various processes. I will present current experiments and results from the Prague PALS laser, with emphasis on measurement and modeling of the x-ray spectra in the range of thousands eV. Mentioned experiments are primary targeted at the inertial confinement fusion research, especially dealing with such effects as the impact of the plasma on solid wall, jet production, or generation of suprathermal electrons and consequent florescence emission from cold plasma. All the phenomena are extensively modeled by using hydrodynamic, atomic, and collisional-radiative codes that will be presented. These plasma codes works on the same principles as models used for astrophysics. Possibilities of 'laboratory astrophysics' with high-power lasers will be briefly discussed.


Pavel Jachym

ALMA News and the User's guide

I will talk about the current status of the Atacama Large Millimeter/submillimeter Array (ALMA), the role of the Czech ALMA regional center node, the up to date ALMA science highlights, and the forthcoming call for proposals (Cycle 2).


Dong Lai

Merging Compact Binaries

The merger of binary systems containing neutron stars, black holes or white dwarfs can lead to various extreme phenomena that are observable throughout the universe. I will discuss recent works on merging neutron star/black hole binaries and white dwarf binaries, focusing on dynamical processes in the pre-merger phase, gravitational waves and potential constraint on nuclear matter. Time permitting, I will also give a brief review on the physics of matter and radiation in strong magnetic fields and how it affects the observational manifestations of various isolated neutron stars.


Michaela Brchnelova

X-ray dynamics measurements of the Tycho’s supernova remnant G120.1+01.4

Intel International Science and Engineering Fair 2013, international pre-college science competition. In the following work we present X-ray dynamics measurements of the Tycho’s supernova remnant G120.1+01.4. We compare observations and spectra from 2005 and 2009 archived in XMM-Newton Science Archive in order to determine differences caused by collision with surrounding interstellar medium (ISM) as well as by remnant’s own expansion. We have calculated the azimuthal expansion of remnant’s edges to vary from 0.194 arcsec/yr to 0.438 arcsec/yr, while the highest values are found to have the azimuth of about 60° in the south-east and the lowest expansion overall is estimated on the north. Comparison of fluxes has shown that the highest estimated energy gain of 3.1 times was measured in reverse shock region around the azimuth of 300° in the energy range from 6.1 keV to 8 keV, whereas the highest energy loss was found to be in the same energy range in forward shock region with the azimuth of approximately 70° reaching 2.4 times lower energy compared with the values from 2005. We have also defined the most abundant heavy elements within energy scale from 200 eV to 8 keV, which are identified through spectral lines to be Fe XVIII (0.849 keV), Mg XI and XII (1.34 keV, 1.46 keV), Si XIII (1.83 keV), S XV (2.41 keV, 2.86 keV) and Ca XIX (3.84 keV).


Devaky Kunneriath

Structure of the NSC of the Milky Way Galaxy

Nuclear star clusters are unambiguosly detected in about 50&ndash;70% of spiral and spheroidal galaxies. They have typical half-light radii of 2&ndash;5 pc, dynamical mass ranging from 10<sup>6</sup>&ndash;10<sup>7</sup> M<sub>sun</sub>, 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 variable 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&ndash;8 micron) 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.


Thomas Henning

Physics of star formation

Star formation is a fundamental process in the universe, shaping the structure of our and other galaxies. The birth of stars is triggered by the gravitational collapse and fragmentation of cold molecular clouds. The talk will summarize the general physical principles of the star formation process. It will discuss global properties such as the star formation efficiency and the initial mass function. In addition, it will also demonstrate the power of adaptive optics in revealing the structure of star-forming regions and will show new exciting results obtained with the Herschel Space Observatory.


Jiri Svoboda

Spectral states of active galactic nuclei from simultaneous X-ray and UV measurements by XMM-Newton

It is generally believed that the black holes in active galactic nuclei (AGNs) and X-ray binaries (XRBs) work in a similar way. While XRBs evolve rapidly and several sources have undergone a few complete cycles from quiescence to an outburst and back, AGNs remain in the same state due to their larger characteristic time-scale, proportional to their size. However, the study of AGN spectral states is still possible with a large sample of sources. Multiwavelength observations are needed for this purpose since the AGN thermal disc-emission dominates in the ultraviolet energy range while the up-scattered hot-corona emission is detected in X-rays. Based on ROSAT All-Sky Survey and SDSS database, Koerding et al. (2006) constructed the disc-fraction/luminosity diagrams of AGNs that revealed the analogy with the state diagrams of XRBs. I would like to present our results that are based on the comparison of strictly simultaneous UV and X-ray measurements of AGNs obtained with the XMM-Newton satellite.


Ronaldo S. S. Vieira

Vertical stability of equatorial circular orbits in Newtonian razor-thin disks

In this talk we present a definite vertical stability criterion for equatorial circular orbits in Newtonian razor-thin disks, which takes into account the discontinuity in the gravitational field. This criterion allows us to obtain a simple analytical formula describing an approximate third integral of motion for nearly equatorial orbits in these systems. The formula is extended to 3D disks in order to describe motion of disk-crossing stars in regions where the usual adiabatic approximation gives poor results.

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