UNLV Department of Physics and Astronomy

Forum Schedule:  Fridays 3:45pm - 4:45pm

DateSpeakerTitle & Abstract
Aug 27
Sep 3
Sep 10Dr. Bob Noble
(host: Zhang)
"Feasibility of Radioisotope Electric Propulsion for Sample Return Missions"
Fast sample return from primitive objects in the outer Solar System would open an exciting new chapter in space science, but the vast distances make this a daunting task. A sample return mission involves several complicated steps to reach an object and obtain a sample, but only the interplanetary return phase of the mission is addressed in this presentation. Radioisotope electric propulsion (REP) is explored as a means to propel small, dedicated return vehicles for transferring sub-kilogram samples from deep space to Earth. Return times for both Earth orbital rendezvous and faster, direct atmospheric re-entry trajectories are calculated for objects as far away as 100 AU. A new option is also explored to bring the REP return vehicle to a Mars orbit, assuming that in the future there will already be robotic shuttles for transferring samples from Mars to Earth. This avoids the environmental issue of bringing a nuclear-fueled vehicle back to Earth and permits the re-use of the REP vehicle for a later mission.
Sep 17
Sep 24Dr. Feng Yuan
(Shanghai Astronomical Observatory)
(host: Zhang)
"AGN feedback at sub-pc scale"
The hot accretion flow is usually optically thin in the radial direction, therefore the photons produced at one radius can travel for a long distance without being absorbed. These photons can thus heat or cool electrons at other radii via Compton scattering. This effect has been ignored in most previous works. After taking into account this effect, we find that when the luminosity (or accretion rate) is higher than about 1% Eddington luminosity, we are not able to get the self-consistent accretion solution, because the equilibrium temperature of electrons at large radii is higher than the virial temperature as a result of strong Compton heating, so the accretion is suppressed. In this case the activity of the black hole will likely "oscillate" between an active and an inactive phase. Based on this mechanism we successfully explain the intermittent activity of some compact radio sources.
Oct 1Dr. Weihua Lei
(Huazhong University of Science and Technology, China)
(host: Zhang)
"The central engine of gamma-ray bursts: Blandford-Znajek Process and neutrino-antineutrino annihilation"
The nature of the central engine of gamma-ray bursts (GRBs) has been one of the main puzzles of astrophysics, which is widely believed to be related to the merger of two neutron stars, the merger of neutron star and black hole, or the collapse of a supermassive star. All of these scenarios involve a rotating black hole with a hyper accretion disk or torus, from which energy is extracted for powering GRBs by neutrino-antineutrino annihilation or by extracting rotational energy from the black hole or disk via large scale magnetic field. Here I will introduce a series of work I had done on these mechanisms, and give special emphasis to the Blandford-Znajek process. By using these mechanisms, we explained the basic properties of GRBs such as total energy, duration and lightcurves, as well as the production of X-ray flares after the GRB prompt emission.
Oct 8
Oct 15Prof. Mark Novotny
(Mississippi State University)
(host: Nagamine)
"Going Through Rough Times: Parallel Discrete Event Simulations - A Physicist's Perspective"
Discrete Event Simulations (DES) are used in a large number of scientific, engineering, military, and manufacturing applications. Our application is to use a type of DES known as kinetic Monte Carlo to study dynamics of nanoscale ferromagnets. I will introduce DES, and the idea of `virtual time' which is used in parallel DES (PDES) simulations. The importance of avoiding causality violations in PDES will be described, and the conservative approach to causality violations introduced. One important question to be addressed is the scalability of PDES algorithms, i.e. the computer simulation time required as the number of processing elements and the problem size grow. A connection [1] between non-equilibrium surface science and the PDES `virtual time' allows questions of scalability to be addressed. For example, we have proven in any dimension that ALL conservative short-ranged PDES are scalable for the computation portion of the algorithm [1]. This is independent of the specific PDES problem. Methods related to network theory to smooth rough virtual times [2,3] to make both the computation and the measurement portions of ALL short-ranged PDES scalable will be described. [1] G. Korniss, Z. Toroczkai, M.A. Novotny, and P.A. Rikvold, Physical Review Letters, vol. 84, pp. 1351 (2000). [2] G. Korniss, M.A. Novotny, Z. Toroczkai, and P.A. Rikvold, Science, vol. 299, p. 677 (2004). [3] H. Guclu, G. Korniss, M.A. Novotny, Z. Toroczkai, and Z. Racz, Phys. Rev. E, 73 066115 [20 pages] (2006).
Oct 22Dr. Judy Racusin
(host: Zhang)
"Fermi-LAT Gamma-ray Bursts and Insight from Swift"
A new revolution in Gamma-ray Burst (GRB) observations and theory has begun over the last two years since the launch of the Fermi Gamma-ray Space Telescope. The new window into high energy gamma-rays opened by the Fermi-Large Area Telescope (LAT) is providing insight into prompt emission mechanisms and possibly also afterglow physics. The LAT detected GRBs appear to be a new unique subset of extremely energetic and bright bursts compared to the large sample detected by Swift over the last 6 years. In this talk, I will discuss the context and recent discoveries from these LAT GRBs and the large database of broadband observations collected by the Swift X-ray Telescope (XRT) and UV/Optical Telescope (UVOT). Through comparisons between the GRBs detected by Swift-BAT, GBM, and LAT, we can learn about the unique characteristics, physical differences, and the relationships between each population. These population characteristics provide insight into the different physical parameters that contribute to the diversity of observational GRB properties.
Oct 26
(Special Forum: Tue, 11:45a-1pm)
Prof. Zong-Hong Zhu
(Beijing Normal University, China)
(host: Zhang)
"Testing the coincidence problem of dark energy"
We first place observational constraints on the parameters of a phenomenological model of Dalal et al.2001 , by using the Constitution Set, the CMB shift parameter from the 5-year WMAP and the SDSS baryon acoustic peak. The result shows that the $\Lambda$CDM model still remains a good fit to the recent observational data, as well as, the coincidence problem indeed exists and is quite severe, in the framework of this simple phenomenological model. We further constrain the model with the transition redshift (deceleration / acceleration). It shows that if the transition from deceleration to acceleration happens at the redshit $z > 0.73$, within the framework of this model, we can conclude that the interaction between DE and DM is necessary. We further suggest a quantitative criteria for judging the severity of the coincidence problem. Applying this criteria to three different interacting models, including the interacting quintessence, interacting phantom, and interacting Chaplygin gas models, we find that the interacting Chaplygin gas model has a better chance to solve the coincidence problem. Quantitatively, we find that the coincidence index C for the interacting Chaplygin gas model is smaller than that for the interacting quintessence and phantom models by six orders of magnitude.
Oct 29NO FORUMNevada Day Recess
Nov 5Prof. Fabio Governato
(Univ. of Washington)
(host: Nagamine)
"Outflows and Wandering Black Holes in cosmological simulations of galaxy formation"
I will discuss recent results that show how the interplay between baryonic outflows and dark matter shapes the properties of the central regions of galaxies. I will then show the predictions for a new population of 'wandering BHs' as the result of tidal destruction of galaxy satellites infalling onto the halos of L* galaxies.
Nov 12(better to avoid -- near NSF deadline)
Nov 19Dr. Evan Kirby
(host: Rhee)
"The Chemical Evolution of Dwarf Satellite Galaxies from Keck/DEIMOS Multi-Element Abundance Measurements"
From Keck/DEIMOS spectra, we derive the star formation histories of eight dwarf spheroidal (dSph) Milky Way satellite galaxies from their alpha element abundance patterns. Nearly 3000 stars comprise our data set. The average [alpha/Fe] ratio for all dSphs follows the same path with increasing [Fe/H]. We do not observe the predicted knees in the [alpha/Fe] vs. [Fe/H] diagram, corresponding to the metallicity at which Type Ia supernovae begin to explode. Instead, we find that Type Ia supernovae ejecta contribute to the abundances of all but the most metal-poor ([Fe/H] < -2.5) stars. We have also developed a chemical evolution model that tracks the star formation rate, Types II and Ia supernova explosions, and supernova feedback. Massive amounts of gas loss define the history of all dSphs except Fornax, the most luminous in our sample. All six of the best-fit model parameters correlate with dSph luminosity but not with velocity dispersion, half-light radius, or Galactocentric distance.
Nov 26NO FORUMThanksgiving Break
Dec 3Dr. T. J. Cox
(Carnegie Observatories)
(host: Nagamine)
"Using Numerical Simulations to Study the Formation and Evolution of Galaxies"
The past decade has produced an amazingly robust picture for the universe we live in. This picture predicts that structure forms hierarchically, i.e., small objects collapse at early times and grow via mergers and gravity. The prevailing idea for the formation of galaxies is that the morphology and structure that we observe is a direct byproduct of this hierarchical merger history; however, a detailed mapping between specific merger histories, and the wide variety of galaxy types, is still uncertain. By using a comprehensive set of state-of-the-art numerical simulations, we show how this process is being studied, and what some of the common scenarios might be. For example, we show that a single disk-disk merger, such as that which will occur in 5 Gyr between our own Milky Way and our nearest neighbor Andromeda, is a plausible mechanism to form many elliptical galaxies provided that dissipation is involved. We also show were this picture fails, and outline how current and future work will address these shortcomings and yield testable predictions of the model.
Dec 10Prof. Shuang-Nan Zhang
(Institute of High Energy Physics, Beijing, China)
(host: Zhang)
"Two topics on accreting high energy phenomena"
(1) An essential element in the unification scheme of active galactic nuclei (AGNs) is the existence of dusty tori, which have been found in a substantial fraction of all AGNs. However the physical process for the formation of these dusty tori and their roles in the evolution of AGNs are still unclear. I will present a model of the formation and evolution of dusty tori in AGNs, involving the anisotropic radiation from their accretion disks. (2) Two classes of weakly magnetized neutron star X-ray binaries exist, i.e., atoll and Z sources, according to the shapes of their X-ray color-color diagrams. However the common and different physical properties of these two classes of sources are still not clear. I will address this problem with data analysis and interpretation using disk-magnetosphere interactions of several atoll and Z sources. I will also compare them with accretion disk evolutions in black hole binaries. He will also discuss the "current status and future programs of space science and space astronomy in China" during the morning astro coffee time.

Past forums: Fall '08   Spring '09   Fall '09   Spring '10