Univerza v Ljubljani, Fakulteta za matematiko in fiziko

torek, 27. 5. 2008, ob 15h v F6

The dark matter is on of the great puzzles of contemporary physics. Only recently have the experiments that probe the microscopic properties been proposed. Until then only distribution and mass measurements existed. In 1990 a preposition to detect scintilations of dark matter particles colliding in highly shielded detector was made.

Among all experiments one is chosen for presentation: DAMA/LIBRA. It differs from other experiments by scintillator size, threshold of detection, operating time and a positive detection of dark matter collisions with 8.2 confidence level. Scientific community did not accept the results as unambiguous since one experiment contradicts the results of DAMA/LIBRA.

When accepting the results limitations for the ratio of cross section for low energy elastic scattering of dark matter particles on nuclei and their mass follows. The approach unifying spins and charges offers a dark matter realisation. From DAMA/LIBRA an interval for mass of stable new quarks of fifth family follows.

Max Planck Institute for Radio Astronomy

torek, 20. 5. 2008, ob 15h v F6

Magnetic fields play a key role in the formation of jets. Very high magnetic fields at the surface of neutron stars or in the accretion disk of black holes inhibit the production of jets. We have quantified the upper limit value of the magnetic field strength that allow the jet formation in X-ray binaries and AGNs. By using the Alfven Radius, , we have studied what we called the basic condition for jet formation, or (LSO, last stable orbit). The basic condition has a direct dependency with the magnetic field strength and the mass accretion rate. We have analysed these results in 3-D and 2-D plots in the case of neutron star and black hole accretor systems, respectively. For this purpose, we did a systematic search of all available observational data for magnetic field strength and mass accretion rate. We studied all the possible compact object accreting scenarios and discern when the formation is possible and under which magnetic field conditions: classical X-ray pulsars, low-mass X-ray binaries with neutron stars (Z- and Atoll-sources), millisecond X-ray pulsars, black-hole X-ray binaries, and AGNs. The theoretical results that we obtain are in complete agreement with available observational data.

INAF – Trieste Astronomical Observatory

torek, 22. 4. 2008, ob 15h v F6

I will review the role played by clusters of galaxies in the study of the formation of large-scale cosmic structures. I will first discuss how the evolution of the cluster population can be used to plae tight constraints on the Dark Matter and Dark Energy content of the Universe. I will then critically assess the reliability of these constraints and how they depend on the detailed knowledge of the properties of the hot baryons permetating clusters. In this context I will show how modern cosmological numerical simulations can be used to improve our knowledge of the physics of galaxy clusters and which are the direction of improvement of these simulations in the future.

INAF – Torino Astronomical Observatory

torek, 15. 4. 2008, ob 15h v F6

The present-day sample of ~ 270 extrasolar planets allows one to appreciate how close a relationship exists between characteristics and frequencies of planetary systems and the physical properties of the host stars. For example, more or less well-established trends of planetary frequency with stellar metallicity and mass have been uncovered in the recent past. For the small (but quickly growing) sample of transiting systems, additional, and somewhat tentative, connections have been made, such as that relating the inferred planetary core masses and the metal content of the hosts. In presence of small-number statistics and/or large uncertainties in the determination of both planetary and stellar properties, significant ambiguities still affect our theoretical understanding of such trends.

I will review the observational evidence for various aspects of the planet-host star connection, and then describe novel methods and experiments which can improve our knowledge of stellar and planetary parameters. I will focus in particular on the sample of transiting systems, currently the only ones amenable for a large amount of follow-up studies which can usefully inform structural and evolutionary models of extrasolar planets.

Univerza v Ljubljani, Fakulteta za matematiko in fiziko

torek, 1. 4. 2008, ob 15h v F6

We present the second data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities and stellar atmosphere parameters (temperature, metallicity, surface gravity, and rotational velocity) of up to one million stars using the 6 degree-field multi-object spectrograph on the UK Schmidt Telescope of the Anglo-Australian Observatory. We will highlight some scientific uses of this data set, which include the identification and study of the current structure of the Galaxy and of remnants of its formation, recent accretion events, as well as discovery of individual peculiar objects and spectroscopic binary stars.

Max Planck Institute for Astronomy

torek, 25. 3. 2008, ob 14h v F6

In recent years, a general consensus has been reached on the cosmological model describing the properties of the Universe as a whole. However, while the initial conditions are specified and the basic physical laws are known, the formation of galaxies is still an open problem, due to the high level of non-linearity of the processes involved. I will compare the predictions from the MOdel for the Rise of GAlaxies aNd Active nuclei (MORGANA, Monaco, Fontanot & Taffoni, 2007) with a number of observational evidencies, in order to shed light on the complex interplay between the relevant physical processes responsible for the formation and evolution of structures in the Universe. I will discuss the main improvements we introduce in MORGANA, with respect to alternative semi-analytical models. I will then present the main successes of the model. MORGANA is able to reproduce both the downsizing of AGNs (thanks to the treatment of the stellar kinetic feedback that arises in star-forming bulges as a consequence of the high level of turbulence); and the early assembly and late, almost-passive evolution of massive galaxies (thanks to the improved modeling of gas cooling and infall in Dark Matter Halos). Despite these successes, I will show in the last part of the talk that the ‘downsizing’ trend (i.e. the modest evolution of the most massive galaxies accompanied by a build-up of small galaxies at z < 1) is still not fully reproduce in MORGANA: the most significant discrepancy refers to the overprediction of the space density of low-mass galaxies at z<2.

Astrophysics Research Institute, Liverpool John Moores University

torek, 18. 3. 2008, ob 14h v F6

A few times a day our sky flashes with a powerful pulse of gamma-rays. A single gamma-ray burst (GRB) releases in seconds the same amount of energy Sun will emit over its 10 billion-year lifetime. Recent observations revolutionized our understanding of GRBs, and it suggests that GRBs involve the formation of a black hole via a catastrophic stellar collapse event or possibly a neutron star merger. Paying attention to interesting relativistic effects, I briefly explain the standard gamma-ray burst model, and I summarize several open issues which are not explained in the standard framework. Magnetized outflow model is recently attracting attentions of researchers, because it could circumvent the problems in the standard model. I will discuss how polarization measurements can give constraints to the magnetized model.

INAF – Brera Astronomical Observatory

torek, 4. 3. 2008, ob 14h v F6

The study of polarisation from astrophysical sources is an important tool to derive fundamental information about emission processes, source geometry, interstellar medium, etc. However, it is also well known that optical polarimetry is a rather difficult technique, requiring high S/N, large telescopes and delicate data analysis to give meaningful results. In spite of that, optical polarimetry in the context of GRB afterglow
studies, have always been considered one of the best available probes to discriminate among competitor models, to derive information about the source evolution, etc. In this talk I will review the best results obtained so far with optical polarimetry of GRB afterglows, and discuss in some detail the most promising advancements for the future.

Trieste Astronomical Observatory

torek, 26. 2. 2008, ob 14h v F6

Astronomical observations can effectively probe in space-time the variability of the physical dimensionless constants such as the fine structure constant and proton-to-electron mass ratio which are related to fundamental forces of nature. Current astronomical observations and constraints on the possible variability of the fine structure and of the electron-to-mass ratio will be reviewed. Emphasis will be given on the different methodologies followed by the different groups trying to understand the controversial results obtained. The proposal for the ESPRESSO (Echelle Spectrograph for Precision Super Stable Observations) spectrograph at the combined incoherent focus of the 4 ESO-VLT units (a potential 16 m equivalent telescope) will be briefly described together with its follow up for the European ELT CODEX (COsmic Dynamics EXperiment). These instruments, if realized, will make possible a significant improvement, in the range of one and two orders of magnitude respectively, thus solving the present controversy and opening a new field of research in case positive results are provided.

University of Washington, Department of Astronomy

petek, 22. 2. 2008, ob 12h v F3

Traditional theories of disk galaxy formation successfully predict that the light distribution in stellar disks should roughly follow an exponential radial profile. However, an abundance of recent evidence suggests that this is an oversimplification – many galaxies exhibit profiles, which deviate from a single exponential in the outer parts. We investigate this phenomenon with an N-body/SPH approach. First, we simulate a suite of idealized models representative of galaxy formation through dissipational collapse after the last major merger. Our approach is unique in that we allow the disk to form spontaneously from the collapse of the halo, without any a priori assumptions about the disk’s structure. Our method therefore allows us to make detailed predictions regarding stellar populations across the break region. We find that the break is seeded by a drop in star formation density, while the outer exponential is populated almost exclusively by stars that migrated there from the interior. The large extent of radial migration has far-reaching implications for studies ranging from the solar neighborhood to extragalactic stellar populations. I will discuss preliminary results from cosmological simulations as well as observational evidence that lends support to this theory.