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Events During the Week of March 27th through April 3rd, 2016

Monday, March 28th, 2016

Plasma Physics (Physics/ECE/NE 922) Seminar
Magnetic Reconnection Onset
Time: 12:00 pm
Place: 1153 Mechanical Engineering
Speaker: Prof. Nuno Loureiro, MIT
Abstract: The recent realization that Sweet-Parker current sheets are violently unstable to the secondary tearing (plasmoid) instability implies that such current sheets cannot occur in real systems. This suggests that, in order to understand the onset of magnetic reconnection, one needs to consider the growth of the tearing instability in a current layer as it is being formed. We present such an analysis in the context of nonlinear resistive MHD for a generic time-dependent equilibrium representing a gradually forming current sheet. It is shown that two onset regimes, single-island and multi-island, are possible, depending on the rate of current sheet formation. A simple model is used to compute the criterion for transition between these two regimes, as well as the reconnection onset time and the current sheet parameters at that moment. For typical solar corona parameters this model yields results consistent with observations.
Host: UW Madison
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Tuesday, March 29th, 2016

Chaos & Complex Systems Seminar
Star-beings and stones: Origins and legends
Time: 12:05 pm
Place: 4274 Chamberlin Hall (refreshments will be served)
Speaker: Herman Bender, Freelance Geologist
Abstract: Native American myths, legends and oral traditions are rich with stories of giant beings existing in ancient times. They all talk of giant Thunderers or Thunder-beings, giant snakes and great Thunderbirds. Even the first humans were said to be giants, some half man, half animal. The Tsistsistas (Cheyenne) have a name for the giant beings their ancestors encountered during the early migration to the grasslands of the Great Plains. They called them haztova hotoxceo or “two-faced star people”. Other Plains tribes such as the Black Feet, Gros Ventres and Lakota have similar stories. These old stories may have real world counterparts. Discovered in a prehistoric effigy-mound group (the Kolterman Mounds) in southeastern Wisconsin (USA) is a human-like petroform effigy with a serpentine body and wing-like arms known as the ‘Star-Being’. Configured in stone, it is approximately 20 meters in length with a red colored, bison-shaped headstone aligned to face the summer solstice sunrise. However, it is not a lone or singular occurrence. The ‘Star-Being’ is but one of two human-like petroforms effigies discovered in southeastern Wisconsin. There is another of almost the same size called the Starman which also has a red-colored, bison-shaped headstone aligned to face the summer solstice sunrise. Both the Starman and Star-Being petroform complexes are codified by the State Historical Society of Wisconsin as archeological sites of Archaic age. Each giant effigy appears to be a reflection of certain constellations and stars, the ‘Star-Being’ a mirror-image of the (western) constellations of Scorpius and Libra (with Sagittarius); the Starman an almost exact representation of Taurus and the Pleiades. Both giant effigies are estimated to be 3500-6000 years old, embodiments of ancient legends and traditions writ large in stone and connected to ‘The People’ through ceremony and acts of cosmic renewal.
Host: Clint Sprott
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Theory Seminar (High Energy/Cosmology)
Type Ia Supernovae and Heavy Metals From Dark Matter
Time: 3:30 pm
Place: 5280 Chamberlin Hall
Speaker: Joseph Bramante, University of Notre Dame
Abstract: Recent studies of low-redshift type Ia supernovae indicate
that at least half explode from sub-Chandrasekhar mass progenitor
white dwarfs. This talk explains how asymmetric dark matter would
ignite white dwarfs, and form star-destroying black holes inside old
neutron stars inhabiting regions densely populated with dark matter.
Links between dark matter and the anomalously high heavy metal
abundance of Reticulum II will also be discussed.
Host: Yang Bai
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Wednesday, March 30th, 2016

R. G. Herb Condensed Matter Seminar
Cooling and stabilization of levitated graphite nanoplatelets
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Pavel Nagornykh, University of Maryland
Abstract: The discovery of graphene in 2004 led to a new spike of interest in 2D materials, with many new ones uncovered and studied in the last ten years. At the same time, it became apparent how important it is to minimize interaction between the 2D sample and the environment and, ideally, to separate them completely. One of the ways to solve this problem was proposed in 2010 by Bruce Kane in experiment, where the graphite microparticles were levitated in a quadrupole ion trap.

In this talk, I'm going to discuss our progress on learning how to control these levitated particles in high vacuum conditions, where a feedback cooling is required. I will present our results, which include cooling down the translational motion of the particles down to 20K and study of the effect of stray electric fields on the efficiency of the feedback cooling. Finally, I'm going to talk about our most recent work on locking nanoplatelets' rotation to an external RF fields, which allows us to tune their spinning frequency as well as the spinning axis orientation. This shows that we are close to having full control over both translational and rotational degrees of freedom for the levitated graphite flakes, which is a crucial condition for further research on levitated 2D materials.
Host: McDermott
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Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Accelerating the Search for Cosmic Accelerators
Time: 2:00 pm
Place: 5280 Chamberlin Hall
Speaker: Amanda Weinstein, Iowa State University
Abstract: Over the past 15 years, gamma-ray observations have become a cornerstone of the rapidly expanding field of multi-messenger astronomy. Gamma-ray astronomy has helped revolutionize our view of the non-thermal universe and galvanized our search for the powerful natural particle accelerators found throughout the cosmos. These advances have relied on the capabilities of both the Fermi Gamma-ray Space Telescope (sensitive to energies between 30 MeV and 1 TeV) and the ground-based very high energy (VHE; E>85 GeV) gamma-ray observatories VERITAS, H.E.S.S., MAGIC, and HAWC.

Our progress has left us with as many new questions as we have answers. In the long term, these fresh questions may be addressed by next-generation observatories using new technology. However, much can be done now by expanding the capabilities of existing instruments and by more effectively combining data from the different observatories. I will outline a promising new approach that extends the reach of the gamma-ray observatory VERITAS while providing a natural context for combining data from multiple instruments. I will also consider the application of this approach to several of the field's outstanding questions and goals.
Host: Albrecht Karle
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Thursday, March 31st, 2016

Cosmology Journal Club
An Informal discussion about a broad variety of arXiv papers related to Cosmology
Time: 12:15 pm
Place: 5242 Chamberlin Hall
Abstract: Please visit the following link for more details:
http://cmb.physics.wisc.edu/journal/index.html
Please feel free to bring your lunch!
If you have questions or comments about this journal club, would like to propose a topic or volunteer to introduce a paper, please email Amol Upadhye (aupadhye@wisc.edu).
Host: Amol Upadhye
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Astronomy Colloquium
Supernovae as Drivers of Dust Evolution in Galaxies
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and cookies 3:30 pm, Talk 3:45 PM
Speaker: Tea Temim, NASA
Abstract: The presence of dust in galaxies has a profound effect on the physical, chemical, and thermal state of their interstellar media, but despite its significant role in many astrophysical processes, the nature, origin, and evolution of dust are still not well understood. Dust grains are formed in the ejecta of core collapse supernovae (SNe) and mass outflows of evolved stars, and then subsequently destroyed by SN shocks that expand into the surrounding interstellar medium. The amount of destruction determines whether the galaxy’s dust budget can be balanced by dust formation in stellar sources, or if an additional source is required. I will summarize the recent progress on the study of dust formation and processing in supernova remnants (SNRs), including observations of dust heated by pulsar winds that reveal important information about the properties of pristine SN-condensed grains. I will also discuss the balance between dust formation and destruction by SNe and its implications for dust evolution models and our understanding of the origin of interstellar dust in galaxies.
Host: Prof Snezana Stanirmirovic
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Friday, April 1st, 2016

Physics Department Colloquium
New Satellite Galaxies of Our Milky Way
Time: 3:30 pm
Place: 2241 Chamberlin Hall
Speaker: Keith Bechtol, UW - Madison, Wisconsin IceCube Particle Astrophysics Center
Abstract: The population of satellite galaxies orbiting the Milky Way includes the most ancient, most chemically pristine, and most dark matter dominated galaxies in the known universe. These objects have reshaped how we define a “galaxy” and provide a unique testing ground both for galaxy formation models and the cold dark matter paradigm. The current census of roughly thirty galaxies surrounding the Milky Way is almost certainly incomplete. Ongoing and near-future wide-field optical imaging surveys are anticipated to find hundreds of ultra-faint Milky Way companions at lower luminosities, greater distances, and in less explored regions of the sky. Over twenty new low-luminosity stellar systems were discovered in 2015, and several of these have now been dynamically and/or chemically confirmed as galaxies. I will place these recent results in context and consider what the emergent population of Milky Way satellites can teach us about the first stars and galaxies, as well as the particle nature of dark matter. Finally, I will discuss the exciting prospects for near-field cosmology in the coming decade.
Host: Jim Lawler
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