University of Minnesota
University of Minnesota
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Astrophysics Colloquia

University of Minnesota

Minnesota Institute for Astrophysics

Astrophysics Colloquium


Spring 2013 - Coordinated by Paul Woodward
Held in Physics 210 at 2:30pm on Fridays
(unless otherwise noted)

January 25 No Colloquium
 
February 1 Dr. Paul Woodward, MIfA, LCSE
Title: Early Results from the NSF's Blue Waters Sustained Petascale Machine
Abstract: The NSF's largest computing system, Blue Waters, is now in operation. My team has used this machine when a sixth of it was made available last April and again over the holidays, when the full machine was available. I will review simulations we performed of the hydrogen ingestion flash in AGB stars and of the very late thermal pulse of Sakurai's object. Over the holidays, we used the machine to simulate turbulent mixing in the context of laser fusion using the same code and running on the full machine. This simulation was specially designed to exercise the entire machine; it had over a trillion cells and the machine ran well over its design specification of 1 Pflop/s (1 million Gflop/s). I will show some of those results also and point out their implications for astrophysical simulations on the machine in the coming year.
 
February 8 Dr. Damiano Caprioli, IAS, Princeton
Title: Supernova remnants and the origin of Galactic cosmic rays
Abstract: One century after the pioneering discovery of cosmic rays by V. Hess, the present generation of X- and gamma-ray telescopes is finally unravelling the origin of such an extraterrestrial radiation, at least for what concerns particles with energies below 10^8 GeV, which are thought to be accelerated in the forward shocks of Galactic supernova remnants (SNRs).
I discuss the present theoretical understanding of efficient particle acceleration at collisionless shocks, addressing with both analytical and numerical (particle-in-cell) techniques the crucial interplay between accelerated ions and magnetic turbulence. In SNRs, in fact, magnetic fields turn out to be a factor of 10-100 larger than in the interstellar medium because of plasma instabilities triggered by energetic particles.
Finally, I outline the observational counterparts of such a theory in terms of SNR multi-wavelength emission, with a special attention to Tycho's SNR, arguably the best laboratory where to test hadron acceleration at strong collisionless shocks.
Host: T. W. Jones
 
February 15 Dr. Brendan Miller, U Michigan
Title: Supermassive Black Hole Activity and Occupation Fraction in the Local Universe
Host: C. Scarlata
 
February 22 Dr. Terry Jones, MIfA
Title: High resolution imaging on the LBT and MMT
 
March 1 Dr. Jeff Grube, Adler Planetarium
Title: Gamma-ray Observations of Galactic Particle Accelerators
Abstract: Viewing the Universe in gamma-rays opens the window to non-thermal emission from a variety of extreme environments in which ultra-relativistic particles are accelerated to energies beyond the reach of man-made accelerators. Presented here are results from VERITAS observations of very high energy (> 0.1 TeV) gamma-rays within our Galaxy, including supernova remnants, pulsar wind nebula, and binary systems. In addition, I will present the prospects for greatly expanding Galactic VHE gamma-ray studies through high-sensitivity survey observations with the Cherenkov Telescope Array (CTA).
Host: L. Fortson
 
March 8 Dr. Peter Capak, Caltech
Title: What galaxy surveys tell us about galaxies, dark matter, and dark energy
Abstract: The field of extra-galactic astronomy is rapidly moving towards large multi-wavelength galaxy surveys such as DES, LSST, Hyper-Suprime-Cam, and Euclid which have the statistical power to measure the demographics of galaxy evolution, link galaxies to their underlying dark matter haloes, and act as probes of dark energy. Back in 2003, the COSMOS survey was one of the first such large scale projects. Its data have been used in over 500 publications, and are now a prototype for Euclid, and other dedicated surveys that combine ground and space based data. I will give a summary of what COSMOS has taught us about galaxy evolution, the link between galaxy evolution and dark matter, and cosmology. I will conclude with what we expect to learn from the current generation of surveys that grew out of COSMOS such as the Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH), and what we can expect from Euclid in a decade.
Host: C. Scarlata
 
March 15 Dr. Shea Brown, Univ. of Iowa
Title: Chasing the Largest Shocks in the Universe
Abstract: Some of the largest single objects in the local universe are shock-waves in the intergalactic medium (IGM) surrounding massive clusters of galaxies. These shocks are associated with cluster mergers, as well as steady-state accretion at the virial radii. I will outline the significance on these shocks and their turbulent wake for the physics of the IGM, and describe our attempts to detect these elusive and important features.
Host: L. Rudnick
 
March 22 No Colloquium — Spring Break
 
March 29 Dr. Falk Herwig, U of Victoria, Canada
Title: Hydrodynamics of nuclear combustion for stars from the early Universe
Abstract: The origin of the elements in stars and stellar explosions in the early Universe is increasingly revealed by observations of stars of very low metal content, both in our galaxy and elsewhere. Observational evidence indicates modes of stellar evolution and nucleosynthesis that present models can not explain. Stellar evolution of the first generations of stars features convective mixing of proton-rich material into convective 12C-rich He-burning regions. During such events nuclear and convective mixing time scales are similar and substantial nuclear energy release into the hydrodynamic flow implies a combustion regime that can not be described with spherically symmetric stellar evolution assumptions. I will report on 3D stellar hydrodynamics simulations of stellar combustion, including their verification and validation. The implications of our simulation results for the origin of the elements in the first generations of stars and galactic chemical evolution will be presented.
Host: P. Woodward
 
Monday,
April 1		 Dr. Katherine Blundell, University of Oxford
(Cosmology Seminar: 12:15 in Physics 435)
Host: L. Rudnick
 
April 5 No Colloquium
 
April 12 Dr. Dawn Erb, U Wisconsin, Milwaukee
Title: Feedback in Faint Galaxies at the Peak Epoch of Star Formation
Abstract: Because faint, low mass galaxies are numerous at high redshifts, their impact on the Universe is expected to be significant. They may host a substantial fraction of global star formation, provide many of the energetic photons needed to reionize the universe, and contribute to the enrichment of the intergalactic medium through the expulsion of metals in galactic outflows. Because of their faintness, however, the properties of these galaxies are difficult to determine. I will discuss a variety of observations aimed at characterizing the physical conditions in low mass galaxies at redshifts z~2-3, the peak epoch of star formation in the Universe, with particular emphasis on the study of galactic outflows in faint galaxies.
Host: T. J. Jones
 
April 19 Dr. Justin Crepp, Notre Dame
Title: The TRENDS High-Contrast Imaging Program
Abstract: I will describe a new interdisciplinary program that uses ultra-precise radial velocity measurements to identify promising targets for follow-up high-contrast imaging observations. The goal of TRENDS is to directly detect and study the companions responsible for accelerating their host star. Using a priori knowledge gained from years of Doppler monitoring, we take a much-needed short-cut for selecting stars to observe with "extreme" AO, coronagraphy, aggressive point-spread function subtraction, and integral field spectroscopy. In addition to a high detection efficiency, TRENDS enables the study of low-mass companions with unprecedented detail, permitting an independent estimate of their mass, age, and metallicity. In this talk, I will announce the discovery of two benchmark brown dwarfs. These objects are old and cold and would rarely, if ever, make the target lists of traditional high-contrast imaging programs. Finally, the most interesting aspect of TRENDS may be our non-detections.
Host: C. Woodward
 
April 26 Dr. Richard Pogge, Ohio State University
Title: First Science Results from the MODS1 Spectrograph on the LBT
Abstract: MODS1, the first of two facility Multi-Object Double Spectrographs on the Large Binocular Telescope, is now coming to the end of its second year of science operations. I will review the on-sky performance of MODS1, present examples of recent science results from the instrument, and give a brief update on MODS2 which will be shipped to the LBT this summer.
Host: E. Skillman
 
May 3 Dr. Larry Lyons, UCLA
Title: Connections Between Large- and Meso-Scale Plasma Sheet Transport, the Ionosphere, and Substorm Dynamics
Abstract: Space Weather refers to the changing structure and dynamics of the near-Earth space environment, including the energetic charged particles and fields within the magnetosphere and their coupling with the ionosphere. Many Space Weather disturbances can be visually identified via production of bright and dynamic aurora. A recent significant change in our understanding is that Space Weather dynamics associated with the plasma sheet can potentially be viewed as an interplay between a slowly-changing large-scale structure and much more dynamic meso-scale structures associated with flow channels. On the large scale, plasma moves earthward (equatorward in the ionosphere) after enhancements in convection to reach the near-Earth plasma sheet, leading to the enhancements in plasma sheet pressure. This transport is controlled by electric and magnetic drift. The energy-dependent magnetic drift leads to a divergence of particles, particle energy flux, and perpendicular current that give rise to significant violation of entropy conservation along the direction of the plasma bulk velocity and to the Region 2 field-aligned current system. Region 2 currents couple to the ionosphere, which leads to substantial modification in electric fields and to strong modification of plasma transport. Meso-scale flow channels are superimposed upon this large-scale system, and lead to most disturbances. Of particular interest are recent studies showing a close connection between the large-scale system, meso-scale flow channels, and substorm dynamics.
Host: R. Lysak