Forum Schedule: Fridays 3:45pm - 4:45pm
|Date||Speaker||Title & Abstract|
|Jan 15||Dr. Stuart Sim|
(Max Planck Institute for Astrophysics)
|"AGN outflows: interpreting signatures in X-ray spectra"|
Highly-ionized fast accretion-disk winds have been suggested as an explanation for a variety of observed absorption and emission features in the X-ray spectra of Active Galactic Nuclei. Simple estimates have suggested that these flows may be massive enough to be significant for the accretion process and might even be involved in creating the link between supermassive black holes and their host galaxies. However, testing these hypotheses, and quantifying the outflow signatures, is no easy task and requires high-quality theoretical model spectra for quantitative comparison. I will present our work on calculating synthetic spectra for simple AGN outflow models and demonstrate their ability to reproduce most of the hard X-ray spectroscopic features observed in real AGN spectra.
|Jan 22||Prof. Davide Lazzati|
(North Carolina State)
|"Relativistic outflows, their engines, and their progenitors: a new view on what shapes Gamma-Ray Burst light curves"|
Gamma-Ray Bursts are the most fascinating events in astrophysics. They are the brightest sources of radiation, they mysteriously appear and disappear in all directions, and they are seen from the largest distances and earlier epochs of the Universe. Gamma-Ray Bursts are also hard to understand and classify, owing to their extreme diversity of characteristics. After a historical introduction of GRBs, I will discuss their properties and diversity in the context of the collapsar model. I will show how the complex interaction between the engine, the massive star that hosts it, and the relativistic outflow can lead to a substantial diversity of light curves, even though each component is individually very similar from burst to burst. The discussion will be presented in simple intuitive form and supported by numerical simulations. I will finally outline a unified view of GRBs and the prospects for challenging this scenario with observations.
|Feb 12||Dr. Yosuke Mizuno|
(Marshall Space Flight Center, Huntsville)
|"Current-Driven Kink Instability in Relativistic Jets"|
We have investigated the development of current-driven (CD) kink instability in relativistic jets via 3D RMHD simulations. In this investigation a static force-free equilibrium helical magnetic configuration is considered in order to study the influence of the initial configuration on the linear and nonlinear evolution of the instability. We found that the initial configuration is strongly distorted but not disrupted by the CD kink instability. The linear growth and nonlinear evolution of the CD kink instability depend moderately on the radial density profile and strongly depend on the magnetic pitch profile. Kink amplitude growth in the nonlinear regime for decreasing magnetic pitch leads to a slender helically twisted column wrapped by magnetic field. On the other hand, kink amplitude growth in the nonlinear regime nearly ceases for increasing magnetic pitch. We also present preliminary results for the effect of velocity shear on the CD kink instability.
|Feb 19||Prof. Yuefang Wu|
|"Gravitational collapse in high-mass star formation region"|
The high-mass star formation mechanisms remain unclear. By calculation, when a forming star reaches 8 Msun, its radiation pressure can halt spherical infall, preventing its further growth. Two major theoretical models on the further growth of such stars were suggested. One model suggested the mergence of low mass stellar objects, and the other suggested a process through accretion-disk-outflow still. Both models need observational evidence. Gravitational collapse has been searched from the very beginning of star formation studies. Recent development in technology has boosted the search of the inflow motion. A number of high-mass collapse candidates were obtained. The collapse signatures seem more common in regions with developed radiation than in younger cores, which opposes the theoretical prediction and is also very different from that of low –mass star formation. Results seem to favor the accretion model. However, to solve the radiation problem, accretion should not be isotropic. The direct evidence of such infalling is still being searched. Future work is also needed to distinguish among infall, rotation and outflow, and to identify fast and slow inflows.
|Feb 26||Prof. Shigehiro Nagataki|
|"GRB-SN Connection: Central Engine of Long GRBs and Explosive Nucleosynthesis"|
I would like to present my recent study on collapsars and formation of jet with a General Relativistic MHD (GRMHD) code that I have developed (Nagataki ApJ 2009). Also, I would like to present the results of numerical simulations on collapsars using a newtonian MHD code with some microphysics such as a realistic equation of state and neutrino cooling/heating (Nagataki+ ApJ 2007). I would like to show our recent study on shock breakout of a relativistic jet from a progenitor and thermal radiation from it (Mizuta and Nagataki 2010, in prep). Finally, I would like to talk about explosive nucleosynthesis in a collapsar (Nagataki et al. ApJ 2006), which can give clues to understand the central engine of a long GRB.
|Mar 26||Prof. Gordon Richards|
|"Super-massive Black Holes: Rulers of the Universe?"|
Once the realm of philosophers, black holes have now been shown to exist. Indeed, black holes as massive as 1 million to 1 billion Suns populate the cores of essentially all massive galaxies. Contrary to popular thought, these super-massive black holes are messy eaters, spewing out nearly as much (in the form of mass and energy) as they consume. This "feedback" process has been postulated to be the valve that controls the growth and evolution of their host galaxies, shaping the very evolution of our Universe. I will discuss how statistical analyses of active galactic nuclei and quasars from the Sloan Digital Sky Survey (SDSS) can be used to test the hypothesis that super-massive black holes are so all-powerful. We'll find that the answer requires pushing even the currently most expansive dataset to its limits, providing an important argument for the next generation of astronomical surveys, including the LSST project.
|Apr 2||NO FORUM||Spring Break|
|Apr 9||Prof. Rachid Ouyed|
(University of Calgary)
|"Astrophysical signatures of quark-matter phase transitions (the Quark-Nova)"|
Supernova explosions of massive stars are generally thought to leave behind either a black hole or a neutron star. However, allowing for a quark star phase (via a Quark-Nova explosion) leads to a dual-shock phenomena (the supernova shock followed by the Quark-Nova shock) with unique astrophysical implications. In this talk I will make a case for: (i) Quark-Novae having manifested themselves as extremely Superluminous Supernovae (e.g. 2006gy, SN2005gj, SN2005ap, SN2008fz, SN2003ma). For Quark-Novae that arise days after the SNe, I will show that the collision between the Quark-Nova ejecta and the supernova ejecta leads to photometry and spectroscopy with encouraging fits to superluminous Supernovae features (lightcurves and emission/absoprtion lines). (ii) Quark-Novae as a possible inner engine of the still elusive Gamma Ray Bursters (GRBs). In particular, here I will argue that allowing for a quark star stage/phase (between a neutron star and a black hole phase) offers natural explanations for some intriguing photometric and spectroscopic features of GRBs (such as the extended emission in short hard GRBs, the Band spectrum ect...). I will finish with some philosophical notes arguing on how remarkable it would be if the solution to some of longstanding problems in astrophysics (including the enigmatic Ultra-High Cosmic Rays) could find answers in the discovery of stable quark matter in the universe via a Quark-Nova.
|Apr 16||Prof. Kin-Wang Ng|
(Academia Sinica / SLAC)
|"Probing the early Universe with cosmic microwave background"|
The 2.7K cosmic microwave background (CMB), the relic photons from the hot big bang, is a powerful probe for the initial conditions of the early Universe. Since the detection of the CMB temperature anisotropy by the COBE satellite in 1992, cosmology has been in fast development and recently turned into a precision science. We will review the development and current status of the CMB research, to see how we use the CMB to measure the cosmological parameters such as the age and the matter content of the Universe, and further to probe the inflationary epoch.
|Prof. Anatoly Klypin|
(New Mexico State)
|"Abundance of dwarf galaxies: is there a problem for the standard cosmological model"|
I will discuss current status of theoretical predictions for properties and statistics of galaxies. I will show results from recent simulation named Bolshoi which in a large cosmological volume resolves the dark matter halos from small dwarfs of size of Magellanic Clouds to Clusters of galaxies comparable to the Coma Cluster. We find that a simple and reasonable corrections for baryonic content of dark matter halos produce remarkably accurate catalogs of "galaxies", which reproduce the luminosity function, the Tully-Fisher relation of observed galaxies, and the correlation function. However, there is a substantial disagreement in the abundance of dwarf galaxies.
|Apr 30||Ms. Margherita Giustini|
(Bologna University, Italy)
|"An X-ray View through AGN Winds"|
Accretion disk winds/outflows are probably a key ingredient for the full comprehension of the structure of Active Galactic Nuclei. Furthermore, they may play a significant role in providing the feedback between AGN and their surrounding (and feeding) media, in regulating the central supermassive black hole-bulge coevolution in galaxies and, on larger scales, in shaping the growth of cosmic structures. X-ray observations can help getting precious insights into the physical mechanism(s) responsible for launching and accelerating such outflows. I will briefly review the results of over one decade of X-ray observations of AGN with powerful ultraviolet winds, and then present the newest results obtained with the last generation X-ray observatories on such systems, discussing their characteristics in the context of accretion disk winds scenarios.
|May 7||Matt Turk|
(UC San Diego)
|"The Formation of The First Stars: Binaries, Chemistry and the Initial Mass Function"|
Studies of the formation of the first stars in the Universe seem to be converging on a central thesis: these stars were massive, formed in isolation, and were but a brief moment in the history of the universe. We discuss a cosmological simulation in which the central 50 solar-mass clump breaks up into two cores, having a mass ratio of two to one, with one fragment collapsing to densities of 10^-8 gm/cm^3. The second fragment, at a distance of ~ 800 AU, is also optically thick to its own cooling radiation from molecular hydrogen lines, but is still able to cool via collision-induced emission. The two dense peaks will continue to accrete from the surrounding cold gas reservoir over a period of ~ 10^5 years and will likely form a binary star system. Furthermore, we discuss results of a parameter study comparing different molecular hydrogen formation rates on the collapse of primordial clouds, and the implications this may have on the initial mass function of Population III stars. Finally, we will discuss ongoing work to sample a number of primordial star-forming halos, to constrain the initial mass function of these first stars.